Snap data and file storage

Environment variables are widely used across Linux to provide convenient access to system and application properties. Both snapcraft and snapd consume, set, and pass-through specific environment variables to support building and running snaps.

Snap makes available certain environment variables to identify data and file storage location at run-time.

Snaps runs in a custom environment specifically made for them. Additionally, our snap is strictly confined and immutable. As a result, they have dedicated locations where they can write data. Data environment variables provide different locations for snaps to write data depending on the purpose and lifespan of those data.

Environment variable	Data backed up over updates	Data kept over updates	Accessible from host	Accessible from other snaps
SNAP_USER_DATA	Yes	No	Yes	No
SNAP_USER_COMMON	No	Yes	Yes	No
SNAP_DATA	Yes	No	Read - Yes/Write - Root only	No
SNAP_COMMON	No	Yes	Read - Yes/Write - Root only	No
SNAP_REAL_HOME (Requires additional HOME interface)	No	Yes	Yes	Yes

All the following examples are assuming the snap hello-world revision 27.

Note that SNAP_USER_DATA, SNAP_USER_COMMON depend on the user. When the user is ROOT (for daemons and command launched with sudo), $HOME correspond to /root.

The following diagram shows the relationships and location of a filesystem.

There is a dedicated location for each installed snap for storing data and files.

SNAP_USER_DATA

The SNAP_USER_DATA variables store the path to the directory for the user data of a snap. This directory is owned and writable by the current user.

All the content of this directory is backed-up and restored across updates and reverts. This also means that the data stored here won’t be available for the next update.

This variable is typically pointing to the host location:

$HOME/snap/hello-world/27

Also corresponding to:

$HOME/snap/hello-world/current

This directory can be accessed with read/write permissions by the user even outside the snap.

The typical use case of this variable would be:

• Log files

• Revision specific configuration files

• Temporary runtime created files

SNAP_USER_COMMON

The SNAP_USER_COMMON variable stores the path to the directory for the user data that are common across revisions of a snap. This directory is owned and writable by the user.

Unlike SNAP_USER_DATA this directory is not backed up and restored across snap refresh and revert operations.

This variable is typically pointing to the host location:

$HOME/snap/hello-world/common

The typical use case of this variable would be:

• Revision specific configuration files

• Any file that we want to keep over the updates

Similarly, there is SNAP_DATA and SNAP_COMMON for system specific data and not user specific data. They are respectively pointing to /var/snap/hello-world/27 and /var/snap/hello-world/common.

The snap documentation for environment variables describe many more features that could be useful in other projects.