Build Environment


You may wish to customize your build environment by doing things such as specifying a custom cache directory for images, or sending your Docker Credentials to the registry endpoint. In this section, we will discuss these and other topics related to the build environment.

Cache Folders

Apptainer will cache SIF container images generated from remote sources, and any OCI/docker layers used to create them. The cache is created at $HOME/.apptainer/cache by default. The location of the cache can be changed by setting the APPTAINER_CACHEDIR environment variable.

If you run builds as root, using sudo, images will be cached in root’s home directory at /root, rather than your user’s home directory. If you have set the APPTAINER_CACHEDIR environment variable, you may use sudo’s -E option to pass the value of APPTAINER_CACHEDIR through to the root user’s environment. This allows you to control where images will be cached even when running builds under sudo.

$ export APPTAINER_CACHEDIR=/tmp/user/temporary-cache

# Running a build under your user account
$ apptainer build --fakeroot myimage.sif mydef.def

# Running a build with sudo, must use -E to pass env var
$ sudo -E apptainer build myimage.sif mydef.def

If you change the value of APPTAINER_CACHEDIR be sure to choose a location that is:

  • Unique to you. Permissions are set on the cache so that private images cached for one user are not exposed to another. This means that Apptainer cache directories cannot be shared across users.

  • Located on a filesystem with sufficient space for the number and size of container images you anticipate using.

  • Located on a filesystem that supports atomic rename, if possible.


If you are not certain that your $HOME or APPTAINER_CACHEDIR filesystems support atomic rename, do not run Apptainer in parallel using remote container URLs. Instead, use apptainer pull to create a local SIF image, and then run this SIF image in a parallel step. Alternatively, you may use the --disable-cache option, but this will result in each Apptainer instance independently fetching the container from the remote source, into a temporary location.

Inside the cache location you will find separate directories for the different kinds of data that are cached:


You can safely delete these directories, or content within them. Apptainer will re-create any directories and data that are needed in future runs.

You should not add any additional files, or modify files in the cache, as this may cause checksum / integrity errors when you run or build containers. If you experience problems use, apptainer cache clean to reset the cache to a clean, empty state.

BoltDB Corruption Errors

The library that Apptainer uses to retrieve and cache Docker/OCI layers keeps track of them using a single-file database. If your home directory is on a network filesystem which experiences interruptions, or you run out of storage, it is possible for this database to become inconsistent.

If you observe error messages that mention when trying to run Apptainer, then you should remove the database file:

rm ~/.local/share/containers/cache/blob-info-cache-v1.boltdb

Cache commands

The cache command for Apptainer allows you to view and clean up your cache, without needing to manually inspect the cache directories.


If you have built images as root, directly or via sudo, the default cache location for those builds is /root/.apptainer. You will need to use sudo when running cache clean or cache list to manage these cache entries.

Listing the Cache

To view a summary of cache usage, use apptainer cache list:

$ apptainer cache list
There are 4 container file(s) using 59.45 MB and 23 oci blob file(s) using 379.10 MB of space
Total space used: 438.55 MB

To view more detailed information, use apptainer cache list -v:

$ apptainer cache list -v
NAME                     DATE CREATED           SIZE             TYPE
0ed5a98249068fe0592edb   2020-05-27 12:57:22    192.21 MB        blob
1d9cd1b99a7eca56d8f2be   2020-05-28 15:19:07    0.35 kB          blob
219c332183ec3800bdfda4   2020-05-28 12:22:13    0.35 kB          blob
2adae3950d4d0f11875568   2020-05-27 12:57:16    51.83 MB         blob
376057ac6fa17f65688c56   2020-05-27 12:57:12    50.39 MB         blob
496548a8c952b37bdf149a   2020-05-27 12:57:14    10.00 MB         blob
5a63a0a859d859478f3046   2020-05-27 12:57:13    7.81 MB          blob
5efaeecfa72afde779c946   2020-05-27 12:57:25    0.23 kB          blob
6154df8ff9882934dc5bf2   2020-05-27 08:37:22    0.85 kB          blob
70d0b3967cd8abe96c9719   2020-05-27 12:57:24    26.61 MB         blob
8f5af4048c33630473b396   2020-05-28 15:19:07    0.57 kB          blob
95c3f3755f37380edb2f8f   2020-05-28 14:07:20    2.48 kB          blob
96878229af8adf91bcbf11   2020-05-28 14:07:20    0.81 kB          blob
af88fdb253aac46693de78   2020-05-28 12:22:13    0.58 kB          blob
bb94ffe723890b4d62d742   2020-05-27 12:57:23    6.15 MB          blob
c080bf936f6a1fdd2045e3   2020-05-27 12:57:25    1.61 kB          blob
cbdbe7a5bc2a134ca8ec91   2020-05-28 12:22:13    2.81 MB          blob
d51af753c3d3a984351448   2020-05-27 08:37:21    28.56 MB         blob
d9cbbca60e5f0fc028b13c   2020-05-28 15:19:06    760.85 kB        blob
db8816f445487e48e1d614   2020-05-27 12:57:25    1.93 MB          blob
fc878cd0a91c7bece56f66   2020-05-27 08:37:22    32.30 kB         blob
fee5db0ff82f7aa5ace634   2020-05-27 08:37:22    0.16 kB          blob
ff110406d51ca9ea722112   2020-05-27 12:57:25    7.78 kB          blob
sha256.02ee8bf9dc335c2   2020-05-29 13:45:14    28.11 MB         library
sha256.5111f59250ac94f   2020-05-28 13:14:39    782.34 kB        library
747d2dbbaaee995098c979   2020-05-28 14:07:22    27.77 MB         oci-tmp
9a839e63dad54c3a6d1834   2020-05-28 12:22:13    2.78 MB          oci-tmp

There are 4 container file(s) using 59.45 MB and 23 oci blob file(s) using 379.10 MB of space
Total space used: 438.55 MB

All cache entries are named using a content hash, so that identical layers or images that are pulled from different URIs do not result in duplication within the cache.

Entries marked blob are OCI/docker layers and manifests, which are used to create SIF format images in the oci-tmp cache. Other caches are named for the source of the image, e.g. library or oras.

You can limit the cache list to a specific cache type with the --type / -t option.

Cleaning the Cache

To reclaim space used by the Apptainer cache, use apptainer cache clean.

By default, apptainer cache clean will remove all cache entries, after asking you to confirm:

$ apptainer cache clean
This will delete everything in your cache (containers from all sources and OCI blobs).
Hint: You can see exactly what would be deleted by canceling and using the --dry-run option.
Do you want to continue? [N/y] n

Use the --dry-run / -n option to see the files that would be deleted, or the --force / -f option to clean without asking for confirmation.

If you want to leave your most recent cached images in place, but remove images that were cached longer ago, you can use the --days / -d option. E.g. to clean cache entries older than 30 days:

$ apptainer cache clean --days 30

To remove only a specific kind of cache entry, e.g. only library images, use the --type / -T option:

$ apptainer cache clean --type library

Temporary Folders

When building a container, or pulling/running an Apptainer container from a Docker/OCI source, a temporary working space is required. The container is constructed in this temporary space before being packaged into an Apptainer SIF image.

The location for temporary directories defaults to /tmp. However, Apptainer will respect the environment variable TMPDIR, and both of these locations can be overridden by setting the environment variable APPTAINER_TMPDIR.

The temporary directory used during a build must be on a filesystem that has enough space to hold the entire container image, uncompressed, including any temporary files that are created and later removed in the course of the build. You may therefore need to set APPTAINER_TMPDIR when building a large container on a system which has a small /tmp filesystem.

Remember to use -E option to pass the value of APPTAINER_TMPDIR through to root’s environment when executing the build command with sudo.


Many modern Linux distributions use an in-memory tmpfs filesystem for /tmp when installed on a computer with a sufficient amount of RAM. This may limit the size of container you can build, as temporary directories under /tmp share RAM with runniing programs etc. A tmpfs also uses default mount options that can interfere with some container builds.

If you experience problems, set APPTAINER_TMPDIR to a disk location, or disable the tmpfs /tmp mount on your system.

Encrypted Containers

With Apptainer it is possible to build and run encrypted containers. The containers are decrypted at runtime entirely in memory, meaning that no intermediate decrypted data is ever present on disk. See encrypted containers for more details.

Environment Variables

You can set environment variables on the host to influence the behaviour of a build. Note that environment variables are not passed into the build itself, and cannot be accessed in the %post section of a definition file. To pass values into a build, use the templating / build-args support.

Environment variables that control a build are generally associated with an equivalent CLI option. The order of precendence is:

  1. If a flag is represented by both a CLI option and an environment variable, and both are set, the CLI option will take precedence. This is true for all environment variables with the exception for APPTAINER_BIND and APPTAINER_BINDPATH, which are combined with the --bind option / argument pair, if both are present.

  2. Environment variables will override default values of CLI options that have not been explicitly set in the command line.

  3. Any default values for CLI options that have not been overridden on the command line, or by corresponding environment variables, will then take effect.


The following variables have defaults that can be overridden by assigning your own values to the corresponding environment variables at runtime:


APPTAINER_DOCKER_LOGIN - Set this to login to a Docker Repository interactively.



RUNSCRIPT_COMMAND - Is not obtained from the environment, but is a hard coded default (“/bin/bash”). This is the fallback command used in the case that the docker image does not have a CMD or ENTRYPOINT.

TAG - Is the default tag, latest.

APPTAINER_NOHTTPS - This is relevant if you want to use a registry that doesn’t support https. A typical use-case for this variable is when using local registry, running on the same machine as Apptainer itself.


APPTAINER_LIBRARY - Used to specify the library to pull from. Default is the currently selected remote endpoint.


APPTAINER_ENCRYPTION_PASSPHRASE - Used to pass a plaintext passphrase to be used to encrypt a container file system (in conjunction with the --encrypt flag). The default is empty.

APPTAINER_ENCRYPTION_PEM_PATH - Used to specify the location of a public key to use for container encryption (in conjunction with the --encrypt flag). The default is empty.