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 (Require 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.
