Spack Spec Command


The commands spack install and spack uninstall take a package name with an optional version of the package’s name. You can also specify the compiler, compiler version, architecture, compile options, and dependency options for a build. In Spack, that descriptor is called a spec. Spack uses specs to refer to a particular build configuration (or configurations) of a package.

For example, the following configuration is a spec of the tcl package.

tcl@8.6.10%gcc@9.2.1 ^zlib@1.2.11%gcc@9.2.1~optimize

If the above spec is provided to spack install command, it will install tcl package at version 8.6.10 using gcc compiler at version 9.2.1. Additionally, it says to link tcl with the zlib library (which it depends on) if it’s listed as a dependency of tcl. It also builds zlib with gcc@9.2.1 with the optimize option enabled.

A package can be installed using different specs.

Each spec must start with the package name (tcl). It also may consist of some optional pieces as shown in the following table:

Option Description Example


version specifier



compiler specifier


+ or ~

variant specifiers for boolean variants



variant specifiers that aren’t restricted to boolean variants


compiler flag specifiers



architecture specifier



Dependency specs


For more details, visit Spack Documentation.

Spack Spec Command

As mentioned above, a spec of a package should contain the package name and other optional pieces such as version, and compiler specifications. For example, you can install the zlib package as follows:

spack install zlib

zlib is a spec that’s used to install zlib package. You can see that the version, compiler and its version, dependencies, variants, and flags are missing. In this case, the default spec will be used to install zlib.

The general syntax of spack spec command is:

spack spec [options] <package_spec_or_hash>

The following table shows the options that can be used with spack spec command.

Flag Description


show dependency hashes as well as versions


show full dependency hashes as well as versions

show install status of packages: [+] installed, [-] not installed


print concrete spec as YAML


print concrete spec as JSON


show fully qualified package names


show dependency types

To find the spec that will be used to install the default version of the zlib package you can run spack spec as follows:

spack spec zlib


Input spec

zlib@1.2.11%gcc@9.2.1+optimize+pic+shared arch=linux-ubuntu19.10-skylake

The output shows Input spec which is the spec that you type in the command line. Also, Concretized which is the concretized version of the Input spec. You can see in the concretized spec that the package version, compiler, and variants were specified.

To show the concretized spec of zlib@1.2.8 spec, you simply type:

spack spec zlib@1.2.8


Input spec

zlib@1.2.8%gcc@9.2.1+optimize+pic+shared arch=linux-ubuntu19.10-skylake

Consider tcl as another example, you can find the concretized version of tcl using its hash. You first need to find its hash by typing:

spack find -l


-- linux-centos7-x86_64 / gcc@4.8.5 -----------------------------
w2rk6fp autoconf@2.69        fsuch7d isl@0.20         6uccmsd ncurses@6.2
h5atxgm automake@1.16.2      66scwnb libiconv@1.16    emv5ut4 perl@5.32.0
bwuaqje berkeley-db@18.1.40  lctnabj libsigsegv@2.12  vztwdsa pkgconf@1.7.3
7g5yr5g diffutils@3.7        bcbiddk libtool@2.4.6    ojil344 readline@8.0
vxq35mm gcc@9.3.0            kafywd7 m4@1.4.18        3vbkxar tcl@8.6.10

From the above output, you can see that 3vbkxar is the hash of the installed package tcl. To find the spec of that package and show dependency hashes as well as versions using its hash, you type:

spack spec -l /3vbkxar


3vbkxar  tcl@8.6.10%gcc@4.8.5 arch=linux-centos7-x86_64
64vg6e4      ^zlib@1.2.11%gcc@4.8.5+optimize+pic+shared arch=linux-centos7-x86_64

The above output shows the dependencies of the tcl package along with their hashes. You can also look at the spec of a dependency using its hash. To find the spec of zlib and check if it’s installed or not, you type:

spack spec -I /64vg6e4

The following output shows that the package has been installed [+].

Input spec
[+]  zlib@1.2.11%gcc@4.8.5+optimize+pic+shared arch=linux-centos7-x86_64

[+]  zlib@1.2.11%gcc@4.8.5+optimize+pic+shared arch=linux-centos7-x86_64