Basic Usage

The spack command has many subcommands. You’ll only need a small subset of them for typical usage.

Note that Spack colorizes output. less -R should be used with Spack to maintain this colorization. E.g.:

$ spack find | less -R

It is recommended that the following be put in your .bashrc file:

alias less='less -R'

If you do not see colorized output when using less -R it is because color is being disabled in the piped output. In this case, tell spack to force colorized output with a flag

$ spack --color always find | less -R

or an environment variable

$ SPACK_COLOR=always spack find | less -R

Listing available packages

To install software with Spack, you need to know what software is available. You can see a list of available package names at the website, or using the spack list command.

spack list

The spack list command prints out a list of all of the packages Spack can install:

$ spack list
3dtk                                      py-gosam
3proxy                                    py-gpaw
7zip                                      py-gpustat
abacus                                    py-gputil
abduco                                    py-gpy
abi-compliance-checker                    py-gpyopt
abi-dumper                                py-gpytorch
abinit                                    py-gql
abseil-cpp                                py-gradio
abyss                                     py-gradio-client

There are thousands of them, so we’ve truncated the output above, but you can find a full list here. Packages are listed by name in alphabetical order. A pattern to match with no wildcards, * or ?, will be treated as though it started and ended with *, so util is equivalent to *util*. All patterns will be treated as case-insensitive. You can also add the -d to search the description of the package in addition to the name. Some examples:

All packages whose names contain “sql”:

$ spack list sql
mysql              py-agate-sql                     py-mysqlclient  py-sqlalchemy-migrate  r-rpostgresql  sqlitebrowser
mysql-connector-c  py-aiosqlite                     py-mysqldb1     py-sqlalchemy-stubs    r-rsqlite
mysqlpp            py-azure-mgmt-sql                py-pygresql     py-sqlalchemy-utils    r-sqldf
perl-dbd-mysql     py-azure-mgmt-sqlvirtualmachine  py-pymysql      py-sqlitedict          sqlcipher
perl-dbd-sqlite    py-flask-sqlalchemy              py-pysqlite3    py-sqlparse            sqlite
postgresql         py-mysql-connector-python        py-sqlalchemy   r-rmysql               sqlite-jdbc

All packages whose names or descriptions contain documentation:

$ spack list --search-description documentation
asciidoc-py3       perl-bioperl          py-interface-meta       py-sphinx-tabs               r-spam
byacc              perl-db-file          py-markdown             py-sphinxautomodapi          r-stanheaders
compositeproto     perl-io-prompt        py-mkdocs               py-sphinxcontrib-websupport  r-units
damageproto        py-alabaster          py-mkdocs-material      r-downlit                    r-uwot
double-conversion  py-astropy-helpers    py-mkdocstrings         r-lifecycle                  sowing
doxygen            py-dask-sphinx-theme  py-myst-parser          r-modeltools                 texinfo
gflags             py-docutils           py-param                r-pkgdown                    totalview
gtk-doc            py-epydoc             py-pdoc3                r-quadprog                   xorg-docs
libxfixes          py-exhale             py-python-docs-theme    r-rcpp                       xorg-sgml-doctools
libxpresent        py-fastai             py-recommonmark         r-rdpack
man-db             py-ford               py-sphinx               r-rinside
ntl                py-furo               py-sphinx-immaterial    r-roxygen2
oommf              py-griffe             py-sphinx-multiversion  r-satellite

spack info

To get more information on a particular package from spack list, use spack info. Just supply the name of a package:

$ spack info --all mpich
AutotoolsPackage:   mpich

    MPICH is a high performance and widely portable implementation of the
    Message Passing Interface (MPI) standard.


Maintainers: @raffenet @yfguo

Externally Detectable: 
    True (version, variants)

    detectable  e4s

Preferred version:  

Safe versions:  
    develop    [git]

Deprecated versions:  

    Name [Default]              When                             Allowed values          Description
    ========================    =============================    ====================    ==============================================================

    amdgpu_target [none]        [+rocm]                          none, gfx1100,          AMD GPU architecture
                                                                 gfx1011, gfx900,        
                                                                 gfx904, gfx1101,        
                                                                 gfx1035, gfx1030,       
                                                                 gfx906, gfx909,         
                                                                 gfx801, gfx1012,        
                                                                 gfx1036, gfx1013,       
                                                                 gfx1102, gfx802,        
                                                                 gfx803, gfx902,         
                                                                 gfx1034, gfx1032,       
                                                                 gfx908, gfx90a,         
                                                                 gfx90c, gfx1010,        
                                                                 gfx1031, gfx1103,       
                                                                 gfx701, gfx940          
    argobots [off]              --                               on, off                 Enable Argobots support
    build_system [autotools]    --                               autotools               Build systems supported by the package
    cuda [off]                  --                               on, off                 Build with CUDA
    cuda_arch [none]            [+cuda]                          none, 11, 72, 12,       CUDA architecture
                                                                 35, 60, 61, 32, 10,     
                                                                 80, 86, 52, 37, 70,     
                                                                 89, 53, 87, 90, 30,     
                                                                 75, 62, 21, 20, 13,     
    datatype-engine [auto]      [@3.4:]                          dataloop, yaksa,        controls the datatype engine to use
    device [ch4]                --                               ch3, ch4                Abstract Device Interface (ADI)
                                                                                         implementation. The ch4 device is in experimental state for
                                                                                         before 3.4.
    fortran [on]                --                               on, off                 Enable Fortran support
    hcoll [off]                 [@3.3: device=ch4 netmod=ucx]    on, off                 Enable support for Mellanox HCOLL accelerated collective
                                                                                         operations library
    hwloc [on]                  --                               on, off                 Use external hwloc package
    hydra [on]                  --                               on, off                 Build the hydra process manager
    libxml2 [on]                --                               on, off                 Use libxml2 for XML support instead of the custom minimalistic
    netmod [ofi]                --                               tcp, mxm, ofi, ucx      Network module. Only single netmod builds are
                                                                                         supported. For ch3 device configurations, this presumes the
                                                                                         ch3:nemesis communication channel. ch3:sock is not supported
                                                                                         by this
                                                                                         spack package at this time.
    pci [on]                    --                               on, off                 Support analyzing devices on PCI bus
    pmi [pmi]                   --                               off, pmi, pmi2,         PMI interface.
                                                                 pmix, cray              
    rocm [off]                  --                               on, off                 Enable ROCm support
    romio [on]                  --                               on, off                 Enable ROMIO MPI I/O implementation
    slurm [off]                 --                               on, off                 Enable SLURM support
    vci [off]                   [@4: device=ch4]                 on, off                 Enable multiple VCI (virtual communication interface) critical
                                                                                         sections to improve performance of applications that do heavy
                                                                                         concurrent MPIcommunications. Set MPIR_CVAR_CH4_NUM_VCIS=<N>
                                                                                         to enable multiple vcis at runtime.
    verbs [off]                 --                               on, off                 Build support for OpenFabrics verbs.
    wrapperrpath [on]           --                               on, off                 Enable wrapper rpath

Installation Phases:
    autoreconf    configure    build    install

Build Dependencies:
    argobots  cray-pmi   gnuconfig  hsa-rocr-dev  libpciaccess  llvm-amdgpu  pkgconfig  slurm
    autoconf  cuda       hcoll      hwloc         libtool       m4           pmix       ucx
    automake  findutils  hip        libfabric     libxml2       mxm          python     yaksa

Link Dependencies:
    argobots  cuda   hip           hwloc      libpciaccess  llvm-amdgpu  pmix   ucx
    cray-pmi  hcoll  hsa-rocr-dev  libfabric  libxml2       mxm          slurm  yaksa

Run Dependencies:

Virtual Packages: 
    mpich provides mpi@:4.0
    mpich@:3.2 provides mpi@:3.1
    mpich@:3.1 provides mpi@:3.0
    mpich@:1.2 provides mpi@:2.2
    mpich@:1.1 provides mpi@:2.1
    mpich@:1.0 provides mpi@:2.0

Available Build Phase Test Methods:

Available Install Phase Test Methods:

Stand-Alone/Smoke Test Methods:
    Mpi.test_mpi_hello  Mpich.test_cpi  Mpich.test_finalized  Mpich.test_manyrma  Mpich.test_sendrecv

Most of the information is self-explanatory. The safe versions are versions that Spack knows the checksum for, and it will use the checksum to verify that these versions download without errors or viruses.

Dependencies and virtual dependencies are described in more detail later.

spack versions

To see more available versions of a package, run spack versions. For example:

$ spack versions libelf

There are two sections in the output. Safe versions are versions for which Spack has a checksum on file. It can verify that these versions are downloaded correctly.

In many cases, Spack can also show you what versions are available out on the web—these are remote versions. Spack gets this information by scraping it directly from package web pages. Depending on the package and how its releases are organized, Spack may or may not be able to find remote versions.

Installing and uninstalling

spack install

spack install will install any package shown by spack list. For example, To install the latest version of the mpileaks package, you might type this:

$ spack install mpileaks

If mpileaks depends on other packages, Spack will install the dependencies first. It then fetches the mpileaks tarball, expands it, verifies that it was downloaded without errors, builds it, and installs it in its own directory under $SPACK_ROOT/opt. You’ll see a number of messages from Spack, a lot of build output, and a message that the package is installed.

$ spack install mpileaks
... dependency build output ...
==> Installing mpileaks-1.0-ph7pbnhl334wuhogmugriohcwempqry2
==> No binary for mpileaks-1.0-ph7pbnhl334wuhogmugriohcwempqry2 found: installing from source
==> mpileaks: Executing phase: 'autoreconf'
==> mpileaks: Executing phase: 'configure'
==> mpileaks: Executing phase: 'build'
==> mpileaks: Executing phase: 'install'
[+] ~/spack/opt/linux-rhel7-broadwell/gcc-8.1.0/mpileaks-1.0-ph7pbnhl334wuhogmugriohcwempqry2

The last line, with the [+], indicates where the package is installed.

Add the Spack debug option (one or more times) – spack -d install mpileaks – to get additional (and even more verbose) output.

Building a specific version

Spack can also build specific versions of a package. To do this, just add @ after the package name, followed by a version:

$ spack install mpich@3.0.4

Any number of versions of the same package can be installed at once without interfering with each other. This is good for multi-user sites, as installing a version that one user needs will not disrupt existing installations for other users.

In addition to different versions, Spack can customize the compiler, compile-time options (variants), compiler flags, and platform (for cross compiles) of an installation. Spack is unique in that it can also configure the dependencies a package is built with. For example, two configurations of the same version of a package, one built with boost 1.39.0, and the other version built with version 1.43.0, can coexist.

This can all be done on the command line using the spec syntax. Spack calls the descriptor used to refer to a particular package configuration a spec. In the commands above, mpileaks and mpileaks@3.0.4 are both valid specs. We’ll talk more about how you can use them to customize an installation in Specs & dependencies.

Reusing installed dependencies

By default, when you run spack install, Spack tries hard to reuse existing installations as dependencies, either from a local store or from remote buildcaches if configured. This minimizes unwanted rebuilds of common dependencies, in particular if you update Spack frequently.

In case you want the latest versions and configurations to be installed instead, you can add the --fresh option:

$ spack install --fresh mpich

Reusing installations in this mode is “accidental”, and happening only if there’s a match between existing installations and what Spack would have installed anyhow.

You can use the spack spec -I mpich command to see what will be reused and what will be built before you install.

You can configure Spack to use the --fresh behavior by default in concretizer.yaml:

  reuse: false

spack uninstall

To uninstall a package, type spack uninstall <package>. This will ask the user for confirmation before completely removing the directory in which the package was installed.

$ spack uninstall mpich

If there are still installed packages that depend on the package to be uninstalled, spack will refuse to uninstall it.

To uninstall a package and every package that depends on it, you may give the --dependents option.

$ spack uninstall --dependents mpich

will display a list of all the packages that depend on mpich and, upon confirmation, will uninstall them in the right order.

A command like

$ spack uninstall mpich

may be ambiguous if multiple mpich configurations are installed. For example, if both mpich@3.0.2 and mpich@3.1 are installed, mpich could refer to either one. Because it cannot determine which one to uninstall, Spack will ask you either to provide a version number to remove the ambiguity or use the --all option to uninstall all of the matching packages.

You may force uninstall a package with the --force option

$ spack uninstall --force mpich

but you risk breaking other installed packages. In general, it is safer to remove dependent packages before removing their dependencies or use the --dependents option.

Garbage collection

When Spack builds software from sources, if often installs tools that are needed just to build or test other software. These are not necessary at runtime. To support cases where removing these tools can be a benefit Spack provides the spack gc (“garbage collector”) command, which will uninstall all unneeded packages:

$ spack find
==> 24 installed packages
-- linux-ubuntu18.04-broadwell / gcc@9.0.1 ----------------------
autoconf@2.69    findutils@4.6.0  libiconv@1.16        libszip@2.1.1  m4@1.4.18    openjpeg@2.3.1  pkgconf@1.6.3  util-macros@1.19.1
automake@1.16.1  gdbm@1.18.1      libpciaccess@0.13.5  libtool@2.4.6  mpich@3.3.2  openssl@1.1.1d  readline@8.0   xz@5.2.4
cmake@3.16.1     hdf5@1.10.5      libsigsegv@2.12      libxml2@2.9.9  ncurses@6.1  perl@5.30.0     texinfo@6.5    zlib@1.2.11

$ spack gc
==> The following packages will be uninstalled:

    -- linux-ubuntu18.04-broadwell / gcc@9.0.1 ----------------------
    vn47edz autoconf@2.69    6m3f2qn findutils@4.6.0  ubl6bgk libtool@2.4.6  pksawhz openssl@1.1.1d  urdw22a readline@8.0
    ki6nfw5 automake@1.16.1  fklde6b gdbm@1.18.1      b6pswuo m4@1.4.18      k3s2csy perl@5.30.0     lp5ya3t texinfo@6.5
    ylvgsov cmake@3.16.1     5omotir libsigsegv@2.12  leuzbbh ncurses@6.1    5vmfbrq pkgconf@1.6.3   5bmv4tg util-macros@1.19.1

==> Do you want to proceed? [y/N] y

[ ... ]

$ spack find
==> 9 installed packages
-- linux-ubuntu18.04-broadwell / gcc@9.0.1 ----------------------
hdf5@1.10.5  libiconv@1.16  libpciaccess@0.13.5  libszip@2.1.1  libxml2@2.9.9  mpich@3.3.2  openjpeg@2.3.1  xz@5.2.4  zlib@1.2.11

In the example above Spack went through all the packages in the package database and removed everything that is not either:

  1. A package installed upon explicit request of the user

  2. A link or run dependency, even transitive, of one of the packages at point 1.

You can check Viewing more metadata to see how to query for explicitly installed packages or Dependency types for a more thorough treatment of dependency types.

Marking packages explicit or implicit

By default, Spack will mark packages a user installs as explicitly installed, while all of its dependencies will be marked as implicitly installed. Packages can be marked manually as explicitly or implicitly installed by using spack mark. This can be used in combination with spack gc to clean up packages that are no longer required.

$ spack install m4
==> 29005: Installing libsigsegv
==> 29005: Installing m4

$ spack install m4 ^libsigsegv@2.11
==> 39798: Installing libsigsegv
==> 39798: Installing m4

$ spack find -d
==> 4 installed packages
-- linux-fedora32-haswell / gcc@10.1.1 --------------------------




$ spack gc
==> There are no unused specs. Spack's store is clean.

$ spack mark -i m4 ^libsigsegv@2.11
==> m4@1.4.18 : marking the package implicit

$ spack gc
==> The following packages will be uninstalled:

    -- linux-fedora32-haswell / gcc@10.1.1 --------------------------
    5fj7p2o libsigsegv@2.11  c6ensc6 m4@1.4.18

==> Do you want to proceed? [y/N]

In the example above, we ended up with two versions of m4 since they depend on different versions of libsigsegv. spack gc will not remove any of the packages since both versions of m4 have been installed explicitly and both versions of libsigsegv are required by the m4 packages.

spack mark can also be used to implement upgrade workflows. The following example demonstrates how the spack mark and spack gc can be used to only keep the current version of a package installed.

When updating Spack via git pull, new versions for either libsigsegv or m4 might be introduced. This will cause Spack to install duplicates. Since we only want to keep one version, we mark everything as implicitly installed before updating Spack. If there is no new version for either of the packages, spack install will simply mark them as explicitly installed and spack gc will not remove them.

$ spack install m4
==> 62843: Installing libsigsegv
==> 62843: Installing m4

$ spack mark -i -a
==> m4@1.4.18 : marking the package implicit

$ git pull

$ spack install m4
==> m4@1.4.18 : marking the package explicit

$ spack gc
==> There are no unused specs. Spack's store is clean.

When using this workflow for installations that contain more packages, care has to be taken to either only mark selected packages or issue spack install for all packages that should be kept.

You can check Viewing more metadata to see how to query for explicitly or implicitly installed packages.

Non-Downloadable Tarballs

The tarballs for some packages cannot be automatically downloaded by Spack. This could be for a number of reasons:

  1. The author requires users to manually accept a license agreement before downloading (jdk and galahad).

  2. The software is proprietary and cannot be downloaded on the open Internet.

To install these packages, one must create a mirror and manually add the tarballs in question to it (see Mirrors (mirrors.yaml)):

  1. Create a directory for the mirror. You can create this directory anywhere you like, it does not have to be inside ~/.spack:

    $ mkdir ~/.spack/manual_mirror
  2. Register the mirror with Spack by creating ~/.spack/mirrors.yaml:

      manual: file://~/.spack/manual_mirror
  3. Put your tarballs in it. Tarballs should be named <package>/<package>-<version>.tar.gz. For example:

    $ ls -l manual_mirror/galahad
    -rw-------. 1 me me 11657206 Jun 21 19:25 galahad-2.60003.tar.gz
  4. Install as usual:

    $ spack install galahad

Seeing installed packages

We know that spack list shows you the names of available packages, but how do you figure out which are already installed?

spack find

spack find shows the specs of installed packages. A spec is like a name, but it has a version, compiler, architecture, and build options associated with it. In spack, you can have many installations of the same package with different specs.

Running spack find with no arguments lists installed packages:

$ spack find
==> 74 installed packages.
-- linux-debian7-x86_64 / gcc@4.4.7 --------------------------------
ImageMagick@6.8.9-10  libdwarf@20130729  py-dateutil@2.4.0
adept-utils@1.0       libdwarf@20130729  py-ipython@2.3.1
atk@2.14.0            libelf@0.8.12      py-matplotlib@1.4.2
boost@1.55.0          libelf@0.8.13      py-nose@1.3.4
bzip2@1.0.6           libffi@3.1         py-numpy@1.9.1
cairo@1.14.0          libmng@2.0.2       py-pygments@2.0.1
callpath@1.0.2        libpng@1.6.16      py-pyparsing@2.0.3
cmake@3.0.2           libtiff@4.0.3      py-pyside@1.2.2
dbus@1.8.6            libtool@2.4.2      py-pytz@2014.10
dbus@1.9.0            libxcb@1.11        py-setuptools@11.3.1
dyninst@8.1.2         libxml2@2.9.2      py-six@1.9.0
fontconfig@2.11.1     libxml2@2.9.2      python@2.7.8
freetype@2.5.3        llvm@3.0           qhull@1.0
gdk-pixbuf@2.31.2     memaxes@0.5        qt@4.8.6
glib@2.42.1           mesa@8.0.5         qt@5.4.0
graphlib@2.0.0        mpich@3.0.4        readline@6.3
gtkplus@2.24.25       mpileaks@1.0       sqlite@3.8.5
harfbuzz@0.9.37       mrnet@4.1.0        stat@2.1.0
hdf5@1.8.13           ncurses@5.9        tcl@8.6.3
icu@54.1              netcdf@4.3.3       tk@src
jpeg@9a               openssl@1.0.1h     vtk@6.1.0
launchmon@1.0.1       pango@1.36.8       xcb-proto@1.11
lcms@2.6              pixman@0.32.6      xz@5.2.0
libdrm@2.4.33         py-dateutil@2.4.0  zlib@1.2.8

-- linux-debian7-x86_64 / gcc@4.9.2 --------------------------------
libelf@0.8.10  mpich@3.0.4

Packages are divided into groups according to their architecture and compiler. Within each group, Spack tries to keep the view simple, and only shows the version of installed packages.

Viewing more metadata

spack find can filter the package list based on the package name, spec, or a number of properties of their installation status. For example, missing dependencies of a spec can be shown with --missing, deprecated packages can be included with --deprecated, packages which were explicitly installed with spack install <package> can be singled out with --explicit and those which have been pulled in only as dependencies with --implicit.

In some cases, there may be different configurations of the same version of a package installed. For example, there are two installations of libdwarf@20130729 above. We can look at them in more detail using spack find --deps, and by asking only to show libdwarf packages:

$ spack find --deps libdwarf
==> 2 installed packages.
-- linux-debian7-x86_64 / gcc@4.4.7 --------------------------------

Now we see that the two instances of libdwarf depend on different versions of libelf: 0.8.12 and 0.8.13. This view can become complicated for packages with many dependencies. If you just want to know whether two packages’ dependencies differ, you can use spack find --long:

$ spack find --long libdwarf
==> 2 installed packages.
-- linux-debian7-x86_64 / gcc@4.4.7 --------------------------------
libdwarf@20130729-d9b90962  libdwarf@20130729-b52fac98

Now the libdwarf installs have hashes after their names. These are hashes over all of the dependencies of each package. If the hashes are the same, then the packages have the same dependency configuration.

If you want to know the path where each package is installed, you can use spack find --paths:

$ spack find --paths
==> 74 installed packages.
-- linux-debian7-x86_64 / gcc@4.4.7 --------------------------------
    ImageMagick@6.8.9-10  ~/spack/opt/linux-debian7-x86_64/gcc@4.4.7/ImageMagick@6.8.9-10-4df950dd
    adept-utils@1.0       ~/spack/opt/linux-debian7-x86_64/gcc@4.4.7/adept-utils@1.0-5adef8da
    atk@2.14.0            ~/spack/opt/linux-debian7-x86_64/gcc@4.4.7/atk@2.14.0-3d09ac09
    boost@1.55.0          ~/spack/opt/linux-debian7-x86_64/gcc@4.4.7/boost@1.55.0
    bzip2@1.0.6           ~/spack/opt/linux-debian7-x86_64/gcc@4.4.7/bzip2@1.0.6
    cairo@1.14.0          ~/spack/opt/linux-debian7-x86_64/gcc@4.4.7/cairo@1.14.0-fcc2ab44
    callpath@1.0.2        ~/spack/opt/linux-debian7-x86_64/gcc@4.4.7/callpath@1.0.2-5dce4318

You can restrict your search to a particular package by supplying its name:

$ spack find --paths libelf
-- linux-debian7-x86_64 / gcc@4.4.7 --------------------------------
    libelf@0.8.11  ~/spack/opt/linux-debian7-x86_64/gcc@4.4.7/libelf@0.8.11
    libelf@0.8.12  ~/spack/opt/linux-debian7-x86_64/gcc@4.4.7/libelf@0.8.12
    libelf@0.8.13  ~/spack/opt/linux-debian7-x86_64/gcc@4.4.7/libelf@0.8.13

Spec queries

spack find actually does a lot more than this. You can use specs to query for specific configurations and builds of each package. If you want to find only libelf versions greater than version 0.8.12, you could say:

$ spack find libelf@0.8.12:
-- linux-debian7-x86_64 / gcc@4.4.7 --------------------------------
    libelf@0.8.12  libelf@0.8.13

Finding just the versions of libdwarf built with a particular version of libelf would look like this:

$ spack find --long libdwarf ^libelf@0.8.12
==> 1 installed packages.
-- linux-debian7-x86_64 / gcc@4.4.7 --------------------------------

We can also search for packages that have a certain attribute. For example, spack find libdwarf +debug will show only installations of libdwarf with the ‘debug’ compile-time option enabled.

The full spec syntax is discussed in detail in Specs & dependencies.

Machine-readable output

If you only want to see very specific things about installed packages, Spack has some options for you. spack find --format can be used to output only specific fields:

$ spack find --format "{name}-{version}-{hash}"


$ spack find --format "{hash:7}"

This uses the same syntax as described in documentation for format() – you can use any of the options there. This is useful for passing metadata about packages to other command-line tools.

Alternately, if you want something even more machine readable, you can output each spec as JSON records using spack find --json. This will output metadata on specs and all dependencies as json:

$ spack find --json sqlite@3.28.0
  "name": "sqlite",
  "hash": "3ws7bsihwbn44ghf6ep4s6h4y2o6eznv",
  "version": "3.28.0",
  "arch": {
   "platform": "darwin",
   "platform_os": "mojave",
   "target": "x86_64"
  "compiler": {
   "name": "apple-clang",
   "version": "10.0.0"
  "namespace": "builtin",
  "parameters": {
   "fts": true,
   "functions": false,
   "cflags": [],
   "cppflags": [],
   "cxxflags": [],
   "fflags": [],
   "ldflags": [],
   "ldlibs": []
  "dependencies": {
   "readline": {
    "hash": "722dzmgymxyxd6ovjvh4742kcetkqtfs",
    "type": [

You can use this with tools like jq to quickly create JSON records structured the way you want:

$ spack find --json sqlite@3.28.0 | jq -C '.[] | { name, version, hash }'
  "name": "sqlite",
  "version": "3.28.0",
  "hash": "3ws7bsihwbn44ghf6ep4s6h4y2o6eznv"
  "name": "readline",
  "version": "7.0",
  "hash": "722dzmgymxyxd6ovjvh4742kcetkqtfs"
  "name": "ncurses",
  "version": "6.1",
  "hash": "zvaa4lhlhilypw5quj3akyd3apbq5gap"

spack diff

It’s often the case that you have two versions of a spec that you need to disambiguate. Let’s say that we’ve installed two variants of zlib, one with and one without the optimize variant:

$ spack install zlib
$ spack install zlib -optimize

When we do spack find we see the two versions.

$ spack find zlib
==> 2 installed packages
-- linux-ubuntu20.04-skylake / gcc@9.3.0 ------------------------
zlib@1.2.11  zlib@1.2.11

Let’s now say that we want to uninstall zlib. We run the command, and hit a problem real quickly since we have two!

$ spack uninstall zlib
==> Error: zlib matches multiple packages:

    -- linux-ubuntu20.04-skylake / gcc@9.3.0 ------------------------
    efzjziy zlib@1.2.11  sl7m27m zlib@1.2.11

==> Error: You can either:
    a) use a more specific spec, or
    b) specify the spec by its hash (e.g. `spack uninstall /hash`), or
    c) use `spack uninstall --all` to uninstall ALL matching specs.

Oh no! We can see from the above that we have two different versions of zlib installed, and the only difference between the two is the hash. This is a good use case for spack diff, which can easily show us the “diff” or set difference between properties for two packages. Let’s try it out. Since the only difference we see in the spack find view is the hash, let’s use spack diff to look for more detail. We will provide the two hashes:

$ spack diff /efzjziy /sl7m27m
==> Warning: This interface is subject to change.

--- zlib@1.2.11efzjziyc3dmb5h5u5azsthgbgog5mj7g
+++ zlib@1.2.11sl7m27mzkbejtkrajigj3a3m37ygv4u2
@@ variant_value @@
-  zlib optimize False
+  zlib optimize True

The output is colored, and written in the style of a git diff. This means that you can copy and paste it into a GitHub markdown as a code block with language “diff” and it will render nicely! Here is an example:

--- zlib@1.2.11/efzjziyc3dmb5h5u5azsthgbgog5mj7g
+++ zlib@1.2.11/sl7m27mzkbejtkrajigj3a3m37ygv4u2
@@ variant_value @@
-  zlib optimize False
+  zlib optimize True

Awesome! Now let’s read the diff. It tells us that our first zlib was built with ~optimize (False) and the second was built with +optimize (True). You can’t see it in the docs here, but the output above is also colored based on the content being an addition (+) or subtraction (-).

This is a small example, but you will be able to see differences for any attributes on the installation spec. Running spack diff A B means we’ll see which spec attributes are on B but not on A (green) and which are on A but not on B (red). Here is another example with an additional difference type, version:

$ spack diff python@2.7.8 python@3.8.11
==> Warning: This interface is subject to change.

--- python@2.7.8/tsxdi6gl4lihp25qrm4d6nys3nypufbf
+++ python@3.8.11/yjtseru4nbpllbaxb46q7wfkyxbuvzxx
@@ variant_value @@
-  python patches a8c52415a8b03c0e5f28b5d52ae498f7a7e602007db2b9554df28cd5685839b8
+  python patches 0d98e93189bc278fbc37a50ed7f183bd8aaf249a8e1670a465f0db6bb4f8cf87
@@ version @@
-  openssl 1.0.2u
+  openssl 1.1.1k
-  python 2.7.8
+  python 3.8.11

Let’s say that we were only interested in one kind of attribute above, version. We can ask the command to only output this attribute. To do this, you’d add the --attribute for attribute parameter, which defaults to all. Here is how you would filter to show just versions:

$ spack diff --attribute version python@2.7.8 python@3.8.11
==> Warning: This interface is subject to change.

--- python@2.7.8/tsxdi6gl4lihp25qrm4d6nys3nypufbf
+++ python@3.8.11/yjtseru4nbpllbaxb46q7wfkyxbuvzxx
@@ version @@
-  openssl 1.0.2u
+  openssl 1.1.1k
-  python 2.7.8
+  python 3.8.11

And you can add as many attributes as you’d like with multiple –attribute arguments (for lots of attributes, you can use -a for short). Finally, if you want to view the data as json (and possibly pipe into an output file) just add --json:

$ spack diff --json python@2.7.8 python@3.8.11

This data will be much longer because along with the differences for A vs. B and B vs. A, the JSON output also showsthe intersection.

Using installed packages

There are several different ways to use Spack packages once you have installed them. As you’ve seen, spack packages are installed into long paths with hashes, and you need a way to get them into your path. The easiest way is to use spack load, which is described in the next section.

Some more advanced ways to use Spack packages include:

  • environments, which you can use to bundle a number of related packages to “activate” all at once, and

  • environment modules, which are commonly used on supercomputing clusters. Spack generates module files for every installation automatically, and you can customize how this is done.

spack load / unload

If you have shell support enabled you can use the spack load command to quickly get a package on your PATH.

For example this will add the mpich package built with gcc to your path:

$ spack install mpich %gcc@4.4.7

# ... wait for install ...

$ spack load mpich %gcc@4.4.7
$ which mpicc

These commands will add appropriate directories to your PATH and MANPATH according to the prefix inspections defined in your modules configuration. When you no longer want to use a package, you can type unload or unuse similarly:

$ spack unload mpich %gcc@4.4.7

Ambiguous specs

If a spec used with load/unload or is ambiguous (i.e. more than one installed package matches it), then Spack will warn you:

$ spack load libelf
==> Error: libelf matches multiple packages.
Matching packages:
  qmm4kso libelf@0.8.13%gcc@4.4.7 arch=linux-debian7-x86_64
  cd2u6jt libelf@0.8.13%intel@15.0.0 arch=linux-debian7-x86_64
Use a more specific spec

You can either type the spack load command again with a fully qualified argument, or you can add just enough extra constraints to identify one package. For example, above, the key differentiator is that one libelf is built with the Intel compiler, while the other used gcc. You could therefore just type:

$ spack load libelf %intel

To identify just the one built with the Intel compiler. If you want to be very specific, you can load it by its hash. For example, to load the first libelf above, you would run:

$ spack load /qmm4kso

To see which packages that you have loaded to your environment you would use spack find --loaded.

$ spack find --loaded
==> 2 installed packages
-- linux-debian7 / gcc@4.4.7 ------------------------------------

-- linux-debian7 / intel@15.0.0 ---------------------------------

You can also use spack load --list to get the same output, but it does not have the full set of query options that spack find offers.

We’ll learn more about Spack’s spec syntax in the next section.

Specs & dependencies

We know that spack install, spack uninstall, and other commands take a package name with an optional version specifier. In Spack, that descriptor is called a spec. Spack uses specs to refer to a particular build configuration (or configurations) of a package. Specs are more than a package name and a version; you can use them to specify the compiler, compiler version, architecture, compile options, and dependency options for a build. In this section, we’ll go over the full syntax of specs.

Here is an example of a much longer spec than we’ve seen thus far:

mpileaks @1.2:1.4 %gcc@4.7.5 +debug -qt target=x86_64 ^callpath @1.1 %gcc@4.7.2

If provided to spack install, this will install the mpileaks library at some version between 1.2 and 1.4 (inclusive), built using gcc at version 4.7.5 for a generic x86_64 architecture, with debug options enabled, and without Qt support. Additionally, it says to link it with the callpath library (which it depends on), and to build callpath with gcc 4.7.2. Most specs will not be as complicated as this one, but this is a good example of what is possible with specs.

More formally, a spec consists of the following pieces:

  • Package name identifier (mpileaks above)

  • @ Optional version specifier (@1.2:1.4)

  • % Optional compiler specifier, with an optional compiler version (gcc or gcc@4.7.3)

  • + or - or ~ Optional variant specifiers (+debug, -qt, or ~qt) for boolean variants. Use ++ or -- or ~~ to propagate variants through the dependencies (++debug, --qt, or ~~qt).

  • name=<value> Optional variant specifiers that are not restricted to boolean variants. Use name==<value> to propagate variant through the dependencies.

  • name=<value> Optional compiler flag specifiers. Valid flag names are cflags, cxxflags, fflags, cppflags, ldflags, and ldlibs. Use name==<value> to propagate compiler flags through the dependencies.

  • target=<value> os=<value> Optional architecture specifier (target=haswell os=CNL10)

  • ^ Dependency specs (^callpath@1.1)

There are two things to notice here. The first is that specs are recursively defined. That is, each dependency after ^ is a spec itself. The second is that everything is optional except for the initial package name identifier. Users can be as vague or as specific as they want about the details of building packages, and this makes spack good for beginners and experts alike.

To really understand what’s going on above, we need to think about how software is structured. An executable or a library (these are generally the artifacts produced by building software) depends on other libraries in order to run. We can represent the relationship between a package and its dependencies as a graph. Here is the full dependency graph for mpileaks:

digraph { mpileaks -> mpich mpileaks -> callpath -> mpich callpath -> dyninst dyninst -> libdwarf -> libelf dyninst -> libelf }

Each box above is a package and each arrow represents a dependency on some other package. For example, we say that the package mpileaks depends on callpath and mpich. mpileaks also depends indirectly on dyninst, libdwarf, and libelf, in that these libraries are dependencies of callpath. To install mpileaks, Spack has to build all of these packages. Dependency graphs in Spack have to be acyclic, and the depends on relationship is directional, so this is a directed, acyclic graph or DAG.

The package name identifier in the spec is the root of some dependency DAG, and the DAG itself is implicit. Spack knows the precise dependencies among packages, but users do not need to know the full DAG structure. Each ^ in the full spec refers to some dependency of the root package. Spack will raise an error if you supply a name after ^ that the root does not actually depend on (e.g. mpileaks ^emacs@23.3).

Spack further simplifies things by only allowing one configuration of each package within any single build. Above, both mpileaks and callpath depend on mpich, but mpich appears only once in the DAG. You cannot build an mpileaks version that depends on one version of mpich and on a callpath version that depends on some other version of mpich. In general, such a configuration would likely behave unexpectedly at runtime, and Spack enforces this to ensure a consistent runtime environment.

The point of specs is to abstract this full DAG from Spack users. If a user does not care about the DAG at all, she can refer to mpileaks by simply writing mpileaks. If she knows that mpileaks indirectly uses dyninst and she wants a particular version of dyninst, then she can refer to mpileaks ^dyninst@8.1. Spack will fill in the rest when it parses the spec; the user only needs to know package names and minimal details about their relationship.

When spack prints out specs, it sorts package names alphabetically to normalize the way they are displayed, but users do not need to worry about this when they write specs. The only restriction on the order of dependencies within a spec is that they appear after the root package. For example, these two specs represent exactly the same configuration:

mpileaks ^callpath@1.0 ^libelf@0.8.3
mpileaks ^libelf@0.8.3 ^callpath@1.0

You can put all the same modifiers on dependency specs that you would put on the root spec. That is, you can specify their versions, compilers, variants, and architectures just like any other spec. Specifiers are associated with the nearest package name to their left. For example, above, @1.1 and %gcc@4.7.2 associates with the callpath package, while @1.2:1.4, %gcc@4.7.5, +debug, -qt, and target=haswell os=CNL10 all associate with the mpileaks package.

In the diagram above, mpileaks depends on mpich with an unspecified version, but packages can depend on other packages with constraints by adding more specifiers. For example, mpileaks could depend on mpich@1.2: if it can only build with version 1.2 or higher of mpich.

Below are more details about the specifiers that you can add to specs.

Version specifier

A version specifier pkg@<specifier> comes after a package name and starts with @. It can be something abstract that matches multiple known versions, or a specific version. During concretization, Spack will pick the optimal version within the spec’s constraints according to policies set for the particular Spack installation.

The version specifier can be a specific version, such as @=1.0.0 or @=1.2a7. Or, it can be a range of versions, such as @1.0:1.5. Version ranges are inclusive, so this example includes both 1.0 and any 1.5.x version. Version ranges can be unbounded, e.g. @:3 means any version up to and including 3. This would include 3.4 and 3.4.2. Similarly, @4.2: means any version above and including 4.2. As a short-hand, @3 is equivalent to the range @3:3 and includes any version with major version 3.

Notice that you can distinguish between the specific version @=3.2 and the range @3.2. This is useful for packages that follow a versioning scheme that omits the zero patch version number: 3.2, 3.2.1, 3.2.2, etc. In general it is preferable to use the range syntax @3.2, since ranges also match versions with one-off suffixes, such as 3.2-custom.

A version specifier can also be a list of ranges and specific versions, separated by commas. For example, @1.0:1.5,=1.7.1 matches any version in the range 1.0:1.5 and the specific version 1.7.1.

For packages with a git attribute, git references may be specified instead of a numerical version i.e. branches, tags and commits. Spack will stage and build based off the git reference provided. Acceptable syntaxes for this are:

 # commit hashes
foo@abcdef1234abcdef1234abcdef1234abcdef1234    # 40 character hashes are automatically treated as git commits

 # branches and tags
foo@git.develop # use the develop branch
foo@git.0.19 # use the 0.19 tag

Spack always needs to associate a Spack version with the git reference, which is used for version comparison. This Spack version is heuristically taken from the closest valid git tag among ancestors of the git ref.

Once a Spack version is associated with a git ref, it always printed with the git ref. For example, if the commit @git.abcdefg is tagged 0.19, then the spec will be shown as @git.abcdefg=0.19.

If the git ref is not exactly a tag, then the distance to the nearest tag is also part of the resolved version. @git.abcdefg=0.19.git.8 means that the commit is 8 commits away from the 0.19 tag.

In cases where Spack cannot resolve a sensible version from a git ref, users can specify the Spack version to use for the git ref. This is done by appending = and the Spack version to the git ref. For example:

foo@git.my_ref=3.2 # use the my_ref tag or branch, but treat it as version 3.2 for version comparisons
foo@git.abcdef1234abcdef1234abcdef1234abcdef1234=develop # use the given commit, but treat it as develop for version comparisons

Details about how versions are compared and how Spack determines if one version is less than another are discussed in the developer guide.

Compiler specifier

A compiler specifier comes somewhere after a package name and starts with %. It tells Spack what compiler(s) a particular package should be built with. After the % should come the name of some registered Spack compiler. This might include gcc, or intel, but the specific compilers available depend on the site. You can run spack compilers to get a list; more on this below.

The compiler spec can be followed by an optional compiler version. A compiler version specifier looks exactly like a package version specifier. Version specifiers will associate with the nearest package name or compiler specifier to their left in the spec.

If the compiler spec is omitted, Spack will choose a default compiler based on site policies.


Variants are named options associated with a particular package. They are optional, as each package must provide default values for each variant it makes available. Variants can be specified using a flexible parameter syntax name=<value>. For example, spack install mercury debug=True will install mercury built with debug flags. The names of particular variants available for a package depend on what was provided by the package author. spack info <package> will provide information on what build variants are available.

For compatibility with earlier versions, variants which happen to be boolean in nature can be specified by a syntax that represents turning options on and off. For example, in the previous spec we could have supplied mercury +debug with the same effect of enabling the debug compile time option for the libelf package.

Depending on the package a variant may have any default value. For mercury here, debug is False by default, and we turned it on with debug=True or +debug. If a variant is True by default you can turn it off by either adding -name or ~name to the spec.

There are two syntaxes here because, depending on context, ~ and - may mean different things. In most shells, the following will result in the shell performing home directory substitution:

mpileaks ~debug   # shell may try to substitute this!
mpileaks~debug    # use this instead

If there is a user called debug, the ~ will be incorrectly expanded. In this situation, you would want to write libelf -debug. However, - can be ambiguous when included after a package name without spaces:

mpileaks-debug     # wrong!
mpileaks -debug    # right

Spack allows the - character to be part of package names, so the above will be interpreted as a request for the mpileaks-debug package, not a request for mpileaks built without debug options. In this scenario, you should write mpileaks~debug to avoid ambiguity.

When spack normalizes specs, it prints them out with no spaces boolean variants using the backwards compatibility syntax and uses only ~ for disabled boolean variants. The - and spaces on the command line are provided for convenience and legibility.

Spack allows variants to propagate their value to the package’s dependency by using ++, --, and ~~ for boolean variants. For example, for a debug variant:

mpileaks ++debug   # enabled debug will be propagated to dependencies
mpileaks +debug    # only mpileaks will have debug enabled

To propagate the value of non-boolean variants Spack uses name==value. For example, for the stackstart variant:

mpileaks stackstart==4   # variant will be propagated to dependencies
mpileaks stackstart=4    # only mpileaks will have this variant value

Compiler Flags

Compiler flags are specified using the same syntax as non-boolean variants, but fulfill a different purpose. While the function of a variant is set by the package, compiler flags are used by the compiler wrappers to inject flags into the compile line of the build. Additionally, compiler flags can be inherited by dependencies by using ==. spack install libdwarf cppflags=="-g" will install both libdwarf and libelf with the -g flag injected into their compile line.


versions of spack prior to 0.19.0 will propagate compiler flags using the = syntax.

Notice that the value of the compiler flags must be quoted if it contains any spaces. Any of cppflags=-O3, cppflags="-O3", cppflags='-O3', and cppflags="-O3 -fPIC" are acceptable, but cppflags=-O3 -fPIC is not. Additionally, if the value of the compiler flags is not the last thing on the line, it must be followed by a space. The command spack install libelf cppflags="-O3"%intel will be interpreted as an attempt to set cppflags="-O3%intel".

The six compiler flags are injected in the order of implicit make commands in GNU Autotools. If all flags are set, the order is $cppflags $cflags|$cxxflags $ldflags <command> $ldlibs for C and C++ and $fflags $cppflags $ldflags <command> $ldlibs for Fortran.

Compiler environment variables and additional RPATHs

Sometimes compilers require setting special environment variables to operate correctly. Spack handles these cases by allowing custom environment modifications in the environment attribute of the compiler configuration section. See also the Environment Modifications section of the configuration files docs for more information.

It is also possible to specify additional RPATHs that the compiler will add to all executables generated by that compiler. This is useful for forcing certain compilers to RPATH their own runtime libraries, so that executables will run without the need to set LD_LIBRARY_PATH.

  - compiler:
      spec: gcc@4.9.3
        cc: /opt/gcc/bin/gcc
        c++: /opt/gcc/bin/g++
        f77: /opt/gcc/bin/gfortran
        fc: /opt/gcc/bin/gfortran
          - BAD_VARIABLE
          GOOD_VARIABLE_NUM: 1
          GOOD_VARIABLE_STR: good
          PATH: /path/to/binutils
          LD_LIBRARY_PATH: /opt/gcc/lib
      - /path/to/some/compiler/runtime/directory
      - /path/to/some/other/compiler/runtime/directory

Architecture specifiers

Each node in the dependency graph of a spec has an architecture attribute. This attribute is a triplet of platform, operating system and processor. You can specify the elements either separately, by using the reserved keywords platform, os and target:

$ spack install libelf platform=linux
$ spack install libelf os=ubuntu18.04
$ spack install libelf target=broadwell

or together by using the reserved keyword arch:

$ spack install libelf arch=cray-CNL10-haswell

Normally users don’t have to bother specifying the architecture if they are installing software for their current host, as in that case the values will be detected automatically. If you need fine-grained control over which packages use which targets (or over all packages’ default target), see Package Preferences.

Cray machines

The situation is a little bit different for Cray machines and a detailed explanation on how the architecture can be set on them can be found at Spack on Cray

Support for specific microarchitectures

Spack knows how to detect and optimize for many specific microarchitectures (including recent Intel, AMD and IBM chips) and encodes this information in the target portion of the architecture specification. A complete list of the microarchitectures known to Spack can be obtained in the following way:

$ spack arch --known-targets
Generic architectures (families)
    aarch64  armv8.1a  armv8.3a  armv8.5a  ppc64    ppcle    sparc    x86     x86_64_v2  x86_64_v4
    arm      armv8.2a  armv8.4a  ppc       ppc64le  riscv64  sparc64  x86_64  x86_64_v3

GenuineIntel - x86
    i686  pentium2  pentium3  pentium4  prescott

GenuineIntel - x86_64
    nocona  nehalem   sandybridge  haswell    skylake  cannonlake      cascadelake  sapphirerapids
    core2   westmere  ivybridge    broadwell  mic_knl  skylake_avx512  icelake

AuthenticAMD - x86_64
    k10  bulldozer  piledriver  zen  steamroller  zen2  zen3  excavator  zen4

IBM - ppc64
    power7  power8  power9

IBM - ppc64le
    power8le  power9le

Cavium - aarch64

Fujitsu - aarch64

ARM - aarch64
    cortex_a72  neoverse_n1  neoverse_v1

Apple - aarch64
    m1  m2

SiFive - riscv64

When a spec is installed Spack matches the compiler being used with the microarchitecture being targeted to inject appropriate optimization flags at compile time. Giving a command such as the following:

$ spack install zlib%gcc@9.0.1 target=icelake

will produce compilation lines similar to:

$ /usr/bin/gcc-9 -march=icelake-client -mtune=icelake-client -c ztest10532.c
$ /usr/bin/gcc-9 -march=icelake-client -mtune=icelake-client -c -fPIC -O2 ztest10532.

where the flags -march=icelake-client -mtune=icelake-client are injected by Spack based on the requested target and compiler.

If Spack knows that the requested compiler can’t optimize for the current target or can’t build binaries for that target at all, it will exit with a meaningful error message:

$ spack install zlib%gcc@5.5.0 target=icelake
==> Error: cannot produce optimized binary for micro-architecture "icelake" with gcc@5.5.0 [supported compiler versions are 8:]

When instead an old compiler is selected on a recent enough microarchitecture but there is no explicit target specification, Spack will optimize for the best match it can find instead of failing:

$ spack arch

$ spack spec zlib%gcc@4.8
Input spec

zlib@1.2.11%gcc@4.8+optimize+pic+shared arch=linux-ubuntu18.04-haswell

$ spack spec zlib%gcc@9.0.1
Input spec

zlib@1.2.11%gcc@9.0.1+optimize+pic+shared arch=linux-ubuntu18.04-broadwell

In the snippet above, for instance, the microarchitecture was demoted to haswell when compiling with gcc@4.8 since support to optimize for broadwell starts from gcc@4.9:.

Finally, if Spack has no information to match compiler and target, it will proceed with the installation but avoid injecting any microarchitecture specific flags.


Currently, Spack doesn’t print any warning to the user if it has no information on which optimization flags should be used for a given compiler. This behavior might change in the future.

Virtual dependencies

The dependency graph for mpileaks we saw above wasn’t quite accurate. mpileaks uses MPI, which is an interface that has many different implementations. Above, we showed mpileaks and callpath depending on mpich, which is one particular implementation of MPI. However, we could build either with another implementation, such as openmpi or mvapich.

Spack represents interfaces like this using virtual dependencies. The real dependency DAG for mpileaks looks like this:

digraph { mpi [color=red] mpileaks -> mpi mpileaks -> callpath -> mpi callpath -> dyninst dyninst -> libdwarf -> libelf dyninst -> libelf }

Notice that mpich has now been replaced with mpi. There is no real MPI package, but some packages provide the MPI interface, and these packages can be substituted in for mpi when mpileaks is built.

You can see what virtual packages a particular package provides by getting info on it:

$ spack info --virtuals mpich
AutotoolsPackage:   mpich

    MPICH is a high performance and widely portable implementation of the
    Message Passing Interface (MPI) standard.


Preferred version:  

Safe versions:  
    develop    [git]

Deprecated versions:  

    Name [Default]              When                             Allowed values          Description
    ========================    =============================    ====================    ==============================================================

    amdgpu_target [none]        [+rocm]                          none, gfx1034,          AMD GPU architecture
                                                                 gfx906, gfx1103,        
                                                                 gfx904, gfx1030,        
                                                                 gfx802, gfx701,         
                                                                 gfx1101, gfx1032,       
                                                                 gfx902, gfx1011,        
                                                                 gfx801, gfx803,         
                                                                 gfx1100, gfx1031,       
                                                                 gfx1102, gfx90a,        
                                                                 gfx909, gfx90c,         
                                                                 gfx908, gfx1010,        
                                                                 gfx1033, gfx900,        
                                                                 gfx1013, gfx1035,       
    argobots [off]              --                               on, off                 Enable Argobots support
    build_system [autotools]    --                               autotools               Build systems supported by the package
    cuda [off]                  --                               on, off                 Build with CUDA
    cuda_arch [none]            [+cuda]                          none, 62, 11, 72,       CUDA architecture
                                                                 50, 12, 75, 90, 61,     
                                                                 20, 89, 80, 87, 35,     
                                                                 52, 30, 32, 37, 21,     
                                                                 86, 10, 13, 70, 53,     
    datatype-engine [auto]      [@3.4:]                          dataloop, yaksa,        controls the datatype engine to use
    device [ch4]                --                               ch3, ch4                Abstract Device Interface (ADI)
                                                                                         implementation. The ch4 device is in experimental state for
                                                                                         before 3.4.
    fortran [on]                --                               on, off                 Enable Fortran support
    hcoll [off]                 [@3.3: device=ch4 netmod=ucx]    on, off                 Enable support for Mellanox HCOLL accelerated collective
                                                                                         operations library
    hwloc [on]                  --                               on, off                 Use external hwloc package
    hydra [on]                  --                               on, off                 Build the hydra process manager
    libxml2 [on]                --                               on, off                 Use libxml2 for XML support instead of the custom minimalistic
    netmod [ofi]                --                               tcp, mxm, ofi, ucx      Network module. Only single netmod builds are
                                                                                         supported. For ch3 device configurations, this presumes the
                                                                                         ch3:nemesis communication channel. ch3:sock is not supported
                                                                                         by this
                                                                                         spack package at this time.
    pci [on]                    --                               on, off                 Support analyzing devices on PCI bus
    pmi [pmi]                   --                               off, pmi, pmi2,         PMI interface.
                                                                 pmix, cray              
    rocm [off]                  --                               on, off                 Enable ROCm support
    romio [on]                  --                               on, off                 Enable ROMIO MPI I/O implementation
    slurm [off]                 --                               on, off                 Enable SLURM support
    vci [off]                   [@4: device=ch4]                 on, off                 Enable multiple VCI (virtual communication interface) critical
                                                                                         sections to improve performance of applications that do heavy
                                                                                         concurrent MPIcommunications. Set MPIR_CVAR_CH4_NUM_VCIS=<N>
                                                                                         to enable multiple vcis at runtime.
    verbs [off]                 --                               on, off                 Build support for OpenFabrics verbs.
    wrapperrpath [on]           --                               on, off                 Enable wrapper rpath

Build Dependencies:
    argobots  cray-pmi   gnuconfig  hsa-rocr-dev  libpciaccess  llvm-amdgpu  pkgconfig  slurm
    autoconf  cuda       hcoll      hwloc         libtool       m4           pmix       ucx
    automake  findutils  hip        libfabric     libxml2       mxm          python     yaksa

Link Dependencies:
    argobots  cuda   hip           hwloc      libpciaccess  llvm-amdgpu  pmix   ucx
    cray-pmi  hcoll  hsa-rocr-dev  libfabric  libxml2       mxm          slurm  yaksa

Run Dependencies:

Virtual Packages: 
    mpich provides mpi@:4.0
    mpich@:3.2 provides mpi@:3.1
    mpich@:3.1 provides mpi@:3.0
    mpich@:1.2 provides mpi@:2.2
    mpich@:1.1 provides mpi@:2.1
    mpich@:1.0 provides mpi@:2.0

Spack is unique in that its virtual packages can be versioned, just like regular packages. A particular version of a package may provide a particular version of a virtual package, and we can see above that mpich versions 1 and above provide all mpi interface versions up to 1, and mpich versions 3 and above provide mpi versions up to 3. A package can depend on a particular version of a virtual package, e.g. if an application needs MPI-2 functions, it can depend on mpi@2: to indicate that it needs some implementation that provides MPI-2 functions.

Constraining virtual packages

When installing a package that depends on a virtual package, you can opt to specify the particular provider you want to use, or you can let Spack pick. For example, if you just type this:

$ spack install mpileaks

Then spack will pick a provider for you according to site policies. If you really want a particular version, say mpich, then you could run this instead:

$ spack install mpileaks ^mpich

This forces spack to use some version of mpich for its implementation. As always, you can be even more specific and require a particular mpich version:

$ spack install mpileaks ^mpich@3

The mpileaks package in particular only needs MPI-1 commands, so any MPI implementation will do. If another package depends on mpi@2 and you try to give it an insufficient MPI implementation (e.g., one that provides only mpi@:1), then Spack will raise an error. Likewise, if you try to plug in some package that doesn’t provide MPI, Spack will raise an error.

Specifying Specs by Hash

Complicated specs can become cumbersome to enter on the command line, especially when many of the qualifications are necessary to distinguish between similar installs. To avoid this, when referencing an existing spec, Spack allows you to reference specs by their hash. We previously discussed the spec hash that Spack computes. In place of a spec in any command, substitute /<hash> where <hash> is any amount from the beginning of a spec hash.

For example, lets say that you accidentally installed two different mvapich2 installations. If you want to uninstall one of them but don’t know what the difference is, you can run:

$ spack find --long mvapich2
==> 2 installed packages.
-- linux-centos7-x86_64 / gcc@6.3.0 ----------
qmt35td mvapich2@2.2%gcc
er3die3 mvapich2@2.2%gcc

You can then uninstall the latter installation using:

$ spack uninstall /er3die3

Or, if you want to build with a specific installation as a dependency, you can use:

$ spack install trilinos ^/er3die3

If the given spec hash is sufficiently long as to be unique, Spack will replace the reference with the spec to which it refers. Otherwise, it will prompt for a more qualified hash.

Note that this will not work to reinstall a dependency uninstalled by spack uninstall --force.

spack providers

You can see what packages provide a particular virtual package using spack providers. If you wanted to see what packages provide mpi, you would just run:

$ spack providers mpi
cray-mpich     intel-mpi              mpich@:1.0  mpich@:3.2     mpt     mvapich        mvapich2-gdr  openmpi@1.6.5
cray-mvapich2  intel-oneapi-mpi       mpich@:1.1  mpich          mpt@1:  mvapich2       mvapich2x     openmpi@1.7.5:
fujitsu-mpi    intel-parallel-studio  mpich@:1.2  mpilander      mpt@3:  mvapich2@2.1:  nvhpc         openmpi@2.0.0:
hpcx-mpi       mpi-serial             mpich@:3.1  mpitrampoline  msmpi   mvapich2@2.3:  openmpi       spectrum-mpi

And if you only wanted to see packages that provide MPI-2, you would add a version specifier to the spec:

$ spack providers mpi@2
hpcx-mpi               mpi-serial  mpich@:3.1  mpt      mvapich2       mvapich2x      openmpi@1.7.5:
intel-mpi              mpich@:1.0  mpich@:3.2  mpt@3:   mvapich2@2.1:  nvhpc          openmpi@2.0.0:
intel-oneapi-mpi       mpich@:1.1  mpich       msmpi    mvapich2@2.3:  openmpi        spectrum-mpi
intel-parallel-studio  mpich@:1.2  mpilander   mvapich  mvapich2-gdr   openmpi@1.6.5

Notice that the package versions that provide insufficient MPI versions are now filtered out.

Deprecating insecure packages

spack deprecate allows for the removal of insecure packages with minimal impact to their dependents.


The spack deprecate command is designed for use only in extraordinary circumstances. This is a VERY big hammer to be used with care.

The spack deprecate command will remove one package and replace it with another by replacing the deprecated package’s prefix with a link to the deprecator package’s prefix.


The spack deprecate command makes no promises about binary compatibility. It is up to the user to ensure the deprecator is suitable for the deprecated package.

Spack tracks concrete deprecated specs and ensures that no future packages concretize to a deprecated spec.

The first spec given to the spack deprecate command is the package to deprecate. It is an abstract spec that must describe a single installed package. The second spec argument is the deprecator spec. By default it must be an abstract spec that describes a single installed package, but with the -i/--install-deprecator it can be any abstract spec that Spack will install and then use as the deprecator. The -I/--no-install-deprecator option will ensure the default behavior.

By default, spack deprecate will deprecate all dependencies of the deprecated spec, replacing each by the dependency of the same name in the deprecator spec. The -d/--dependencies option will ensure the default, while the -D/--no-dependencies option will deprecate only the root of the deprecate spec in favor of the root of the deprecator spec.

spack deprecate can use symbolic links or hard links. The default behavior is symbolic links, but the -l/--link-type flag can take options hard or soft.

Verifying installations

The spack verify command can be used to verify the validity of Spack-installed packages any time after installation.

At installation time, Spack creates a manifest of every file in the installation prefix. For links, Spack tracks the mode, ownership, and destination. For directories, Spack tracks the mode, and ownership. For files, Spack tracks the mode, ownership, modification time, hash, and size. The Spack verify command will check, for every file in each package, whether any of those attributes have changed. It will also check for newly added files or deleted files from the installation prefix. Spack can either check all installed packages using the -a,–all or accept specs listed on the command line to verify.

The spack verify command can also verify for individual files that they haven’t been altered since installation time. If the given file is not in a Spack installation prefix, Spack will report that it is not owned by any package. To check individual files instead of specs, use the -f,--files option.

Spack installation manifests are part of the tarball signed by Spack for binary package distribution. When installed from a binary package, Spack uses the packaged installation manifest instead of creating one at install time.

The spack verify command also accepts the -l,--local option to check only local packages (as opposed to those used transparently from upstream spack instances) and the -j,--json option to output machine-readable json data for any errors.

Extensions & Python support

Spack’s installation model assumes that each package will live in its own install prefix. However, certain packages are typically installed within the directory hierarchy of other packages. For example, Python packages are typically installed in the $prefix/lib/python-2.7/site-packages directory.

In Spack, installation prefixes are immutable, so this type of installation is not directly supported. However, it is possible to create views that allow you to merge install prefixes of multiple packages into a single new prefix. Views are a convenient way to get a more traditional filesystem structure. Using extensions, you can ensure that Python packages always share the same prefix in the view as Python itself. Suppose you have Python installed like so:

$ spack find python
==> 1 installed packages.
-- linux-debian7-x86_64 / gcc@4.4.7 --------------------------------

spack extensions

You can find extensions for your Python installation like this:

$ spack extensions python
==> python@2.7.8%gcc@4.4.7 arch=linux-debian7-x86_64-703c7a96
==> 36 extensions:
geos          py-ipython     py-pexpect    py-pyside            py-sip
py-basemap    py-libxml2     py-pil        py-pytz              py-six
py-biopython  py-mako        py-pmw        py-rpy2              py-sympy
py-cython     py-matplotlib  py-pychecker  py-scientificpython  py-virtualenv
py-dateutil   py-mpi4py      py-pygments   py-scikit-learn
py-epydoc     py-mx          py-pylint     py-scipy
py-gnuplot    py-nose        py-pyparsing  py-setuptools
py-h5py       py-numpy       py-pyqt       py-shiboken

==> 12 installed:
-- linux-debian7-x86_64 / gcc@4.4.7 --------------------------------
py-dateutil@2.4.0    py-nose@1.3.4       py-pyside@1.2.2
py-dateutil@2.4.0    py-numpy@1.9.1      py-pytz@2014.10
py-ipython@2.3.1     py-pygments@2.0.1   py-setuptools@11.3.1
py-matplotlib@1.4.2  py-pyparsing@2.0.3  py-six@1.9.0

The extensions are a subset of what’s returned by spack list, and they are packages like any other. They are installed into their own prefixes, and you can see this with spack find --paths:

$ spack find --paths py-numpy
==> 1 installed packages.
-- linux-debian7-x86_64 / gcc@4.4.7 --------------------------------
    py-numpy@1.9.1  ~/spack/opt/linux-debian7-x86_64/gcc@4.4.7/py-numpy@1.9.1-66733244

However, even though this package is installed, you cannot use it directly when you run python:

$ spack load python
$ python
Python 2.7.8 (default, Feb 17 2015, 01:35:25)
[GCC 4.4.7 20120313 (Red Hat 4.4.7-11)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import numpy
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ImportError: No module named numpy

Using Extensions in Environments

The recommended way of working with extensions such as py-numpy above is through Environments. For example, the following creates an environment in the current working directory with a filesystem view in the ./view directory:

$ spack env create --with-view view --dir .
$ spack -e . add py-numpy
$ spack -e . concretize
$ spack -e . install

We recommend environments for two reasons. Firstly, environments can be activated (requires Shell support):

$ spack env activate .

which sets all the right environment variables such as PATH and PYTHONPATH. This ensures that

$ python
>>> import numpy

works. Secondly, even without shell support, the view ensures that Python can locate its extensions:

$ ./view/bin/python
>>> import numpy

See Environments (spack.yaml) for a more in-depth description of Spack environments and customizations to views.

Using spack load

A more traditional way of using Spack and extensions is spack load (requires Shell support). This will add the extension to PYTHONPATH in your current shell, and Python itself will be available in the PATH:

$ spack load py-numpy
$ python
>>> import numpy

The loaded packages can be checked using spack find --loaded

Loading Extensions via Modules

Apart from spack env activate and spack load, you can load numpy through your environment modules (using environment-modules or lmod). This will also add the extension to the PYTHONPATH in your current shell.

$ module load <name of numpy module>

If you do not know the name of the specific numpy module you wish to load, you can use the spack module tcl|lmod loads command to get the name of the module from the Spack spec.

Filesystem requirements

By default, Spack needs to be run from a filesystem that supports flock locking semantics. Nearly all local filesystems and recent versions of NFS support this, but parallel filesystems or NFS volumes may be configured without flock support enabled. You can determine how your filesystems are mounted with mount. The output for a Lustre filesystem might look like this:

$ mount | grep lscratch
mds1-lnet0@o2ib100:/lsd on /p/lscratchd type lustre (rw,nosuid,lazystatfs,flock)
mds2-lnet0@o2ib100:/lse on /p/lscratche type lustre (rw,nosuid,lazystatfs,flock)

Note the flock option on both Lustre mounts.

If you do not see this or a similar option for your filesystem, you have a few options. First, you can move your Spack installation to a filesystem that supports locking. Second, you could ask your system administrator to enable flock for your filesystem.

If none of those work, you can disable locking in one of two ways:

  1. Run Spack with the -L or --disable-locks option to disable locks on a call-by-call basis.

  2. Edit config.yaml and set the locks option to false to always disable locking.


If you disable locking, concurrent instances of Spack will have no way to avoid stepping on each other. You must ensure that there is only one instance of Spack running at a time. Otherwise, Spack may end up with a corrupted database file, or you may not be able to see all installed packages in commands like spack find.

If you are unfortunate enough to run into this situation, you may be able to fix it by running spack reindex.

This issue typically manifests with the error below:

$ ./spack find
Traceback (most recent call last):
File "./spack", line 176, in <module>
File "./spack", line 154,' in main
  return_val = command(parser, args)
File "./spack/lib/spack/spack/cmd/", line 170, in find
  specs = set(spack.installed_db.query(\**q_args))
File "./spack/lib/spack/spack/", line 551, in query
  with self.read_transaction():
File "./spack/lib/spack/spack/", line 598, in __enter__
  if self._enter() and self._acquire_fn:
File "./spack/lib/spack/spack/", line 608, in _enter
  return self._db.lock.acquire_read(self._timeout)
File "./spack/lib/spack/llnl/util/", line 103, in acquire_read
  self._lock(fcntl.LOCK_SH, timeout)   # can raise LockError.
File "./spack/lib/spack/llnl/util/", line 64, in _lock
  fcntl.lockf(self._fd, op | fcntl.LOCK_NB)
IOError: [Errno 38] Function not implemented

A nicer error message is TBD in future versions of Spack.


The spack audit command:

$ spack audit -h
usage: spack audit [-h] SUBCOMMAND ...

audit configuration files, packages, etc.

positional arguments:
    configs       audit configuration files
    externals     check external detection in packages
                  check https in packages
    packages      audit package recipes
    list          list available checks and exits

  -h, --help      show this help message and exit

can be used to detect a number of configuration issues. This command detects configuration settings which might not be strictly wrong but are not likely to be useful outside of special cases.

It can also be used to detect dependency issues with packages - for example cases where a package constrains a dependency with a variant that doesn’t exist (in this case Spack could report the problem ahead of time but automatically performing the check would slow down most runs of Spack).

A detailed list of the checks currently implemented for each subcommand can be printed with:

$ spack -v audit list
  Generic checks relying on global variables

  Sanity checks on compilers.yaml
    1. Report compilers with the same spec and two different definitions

  Sanity checks on packages.yaml
    1. Search for duplicate specs declared as externals

  Sanity checks on specs used in directives
    1. Ensure stand-alone test method is not included in build-time callbacks
    2. Ensure that patches fetched from GitHub have stable sha256 hashes.
    3. Report unknown or wrong variants in directives for this package
    4. Report unknown dependencies and wrong variants for dependencies
    5. Ensures that variant defaults are present and parsable from cli
    6. Ensures that all variants have a description.
    7. Report if version constraints used in directives are not satisfiable

  Sanity checks on reserved attributes of packages
    1. Ensure that packages don't override reserved names

  Sanity checks on properties a package should maintain
    1. Ensure package names are lowercase and consistent
    2. Ensure that package objects are pickleable
    3. Ensure that all packages can unparse and that unparsed code is valid Python
    4. Ensure all versions in a package can produce a fetcher
    5. Ensure the package has a docstring and no fixmes
    6. Ensure no packages use md5 checksums
    7. Ensure that methods modifying the build environment are ported to builder classes.

  Sanity checks on https checks of package urls, etc.
    1. Check for correctness of links

  Sanity checks for external software detection
    1. Test drive external detection for packages

Depending on the use case, users might run the appropriate subcommands to obtain diagnostics. Issues, if found, are reported to stdout:

% spack audit packages lammps
PKG-DIRECTIVES: 1 issue found
1. lammps: wrong variant in "conflicts" directive
    the variant 'adios' does not exist
    in /home/spack/spack/var/spack/repos/builtin/packages/lammps/

Getting Help

spack help

If you don’t find what you need here, the help subcommand will print out out a list of all of spack’s options and subcommands:

$ spack help
usage: spack [-hkV] [--color {always,never,auto}] COMMAND ...

A flexible package manager that supports multiple versions,
configurations, platforms, and compilers.

These are common spack commands:

query packages:
  list                  list and search available packages
  info                  get detailed information on a particular package
  find                  list and search installed packages

build packages:
  install               build and install packages
  uninstall             remove installed packages
  gc                    remove specs that are now no longer needed
  spec                  show what would be installed, given a spec

  external              manage external packages in Spack configuration

  env                   manage virtual environments
  view                  project packages to a compact naming scheme on the filesystem

create packages:
  create                create a new package file
  edit                  open package files in $EDITOR

  arch                  print architecture information about this machine
  audit                 audit configuration files, packages, etc.
  compilers             list available compilers

user environment:
  load                  add package to the user environment
  module                generate/manage module files
  unload                remove package from the user environment

  --color {always,never,auto}
                        when to colorize output (default: auto)
  -V, --version         show version number and exit
  -h, --help            show this help message and exit
  -k, --insecure        do not check ssl certificates when downloading

more help:
  spack help --all       list all commands and options
  spack help <command>   help on a specific command
  spack help --spec      help on the package specification syntax
  spack docs             open in a browser

Adding an argument, e.g. spack help <subcommand>, will print out usage information for a particular subcommand:

$ spack help install
usage: spack install [-hnvyU] [--only {package,dependencies}] [-u UNTIL] [-j JOBS] [--overwrite] [--fail-fast]
                     [--keep-prefix] [--keep-stage] [--dont-restage]
                     [--use-cache | --no-cache | --cache-only | --use-buildcache [{auto,only,never},][package:{auto,only,never},][dependencies:{auto,only,never}]]
                     [--include-build-deps] [--no-check-signature] [--show-log-on-error] [--source] [--deprecated]
                     [--fake] [--only-concrete] [--add | --no-add] [-f SPEC_YAML_FILE] [--clean | --dirty]
                     [--test {root,all}] [--log-format {junit,cdash}] [--log-file LOG_FILE] [--help-cdash] [--reuse]

build and install packages

positional arguments:
  spec                  package spec

  --add                 (with environment) add spec to the environment as a root
  --cache-only          only install package from binary mirrors
  --clean               unset harmful variables in the build environment (default)
  --deprecated          fetch deprecated versions without warning
  --dirty               preserve user environment in spack's build environment (danger!)
  --dont-restage        if a partial install is detected, don't delete prior state
  --fail-fast           stop all builds if any build fails (default is best effort)
  --fake                fake install for debug purposes
  --help-cdash          show usage instructions for CDash reporting
  --include-build-deps  include build deps when installing from cache, useful for CI pipeline troubleshooting
  --keep-prefix         don't remove the install prefix if installation fails
  --keep-stage          don't remove the build stage if installation succeeds
  --log-file LOG_FILE   filename for the log file
  --log-format {junit,cdash}
                        format to be used for log files
  --no-add              (with environment) do not add spec to the environment as a root
  --no-cache            do not check for pre-built Spack packages in mirrors
  --no-check-signature  do not check signatures of binary packages
  --only {package,dependencies}
                        select the mode of installation
                        default is to install the package along with all its dependencies. alternatively, one can decide to install only the package or only the dependencies
  --only-concrete       (with environment) only install already concretized specs
  --overwrite           reinstall an existing spec, even if it has dependents
  --show-log-on-error   print full build log to stderr if build fails
  --source              install source files in prefix
  --test {root,all}     run tests on only root packages or all packages
  --use-buildcache [{auto,only,never},][package:{auto,only,never},][dependencies:{auto,only,never}]
                        select the mode of buildcache for the 'package' and 'dependencies'
                        default: package:auto,dependencies:auto
                        - `auto` behaves like --use-cache
                        - `only` behaves like --cache-only
                        - `never` behaves like --no-cache
  --use-cache           check for pre-built Spack packages in mirrors (default)
                        read specs to install from .yaml files
  -h, --help            show this help message and exit
  -j JOBS, --jobs JOBS  explicitly set number of parallel jobs
  -n, --no-checksum     do not use checksums to verify downloaded files (unsafe)
  -u UNTIL, --until UNTIL
                        phase to stop after when installing (default None)
  -v, --verbose         display verbose build output while installing
  -y, --yes-to-all      assume "yes" is the answer to every confirmation request

concretizer arguments:
  --reuse               reuse installed packages/buildcaches when possible
  --reuse-deps          reuse installed dependencies only
  -U, --fresh           do not reuse installed deps; build newest configuration

Alternately, you can use spack --help in place of spack help, or spack <subcommand> --help to get help on a particular subcommand.