Debug snaps with snap try
snap try command installs a snap from its unpackaged components within a directory. After installation, most changes to the components in that directory will immediately by visible in the installed snap.
But it’s sometimes useful to be able to experiment with a snap locally, to help debug an issue, or to make changes to a snap when you don’t have upstream access to the packaging process. This is when
snap try is useful.
- If you only have access to the packaged snap itself, you first need to unsquash the snap and then use snap try to test it.
- If, instead, you are a developer with access to the source code of the snap, you first need to use snapcraft try to build an unpacked snap, and then use snap try to test it.
Unsquashing a snap
In order to use
snap try, you first need to get the unpacked contents of the snap. The first step is to get the snap itself. The most practical source of snaps is the snap store, where a snap can be downloaded with the
snap download command:
$ snap download hello-world Fetching snap "hello-world" Fetching assertions for "hello-world" Install the snap with: snap ack hello-world_29.assert snap install hello-world_29.snap
The download includes the snap itself and a signed set of assertions from the store to validate the snap’s default state.
Alternatively, .snap files for any installed snaps can be found at
/var/lib/snapd/snaps/, from where they can be copied across to your current working directory.
To uncompress the SquashFS .snap file, use
unsquashfs <snap filename>:
$ unsquashfs hello-world_29.snap Parallel unsquashfs: Using 8 processors 6 inodes (6 blocks) to write [===========================|] 6/6 100% created 6 files created 4 directories created 0 symlinks created 0 device
The files associated with the snap can now be found in the `squashfs-root’ directory. You can use snap try to install this unpacked snap.
Using snap try
snap try <directory> installs an unpacked snap using a bind mount.
$ snap try squashfs-root hello-world 6.4 mounted from /home/user/squashfs-root $ which hello-world /snap/bin/hello-world
Most changes now made to files in the
squashfs-root folder will be immediately reflected in the installed snap. This can be helpful when debugging an application within a snap, or the snap itself.
Certain changes, such as adjusting a snap’s interfaces or confinement will not be reflected in the installed snap until after a reinstall. These changes include:
- changes to snap interfaces, such as adding or removing a plug
- changes to layouts
- changes to a snap’s confinement
Using the above hello-world snap, for example, we could edit the
bin/echo script to change its output without rebuilding or remounting the snap:
$ hello-world Hello world! $ sed -i 's/World/Everyone/g' /home/user/squashfs-root/bin/echo $ hello-world Hello Everyone!
Using snapcraft try
When developing a snap with snapcraft, the
snapcraft try command can be used in combination with
snap try to quickly test a snap and fix issues.
snapcraft try command runs through the build process to the completion of the prime stage (see Parts lifecycle for further details). It then exposes the resultant prime directory to the snapcraft directory, even from within a virtual machine or container.
This prime directory includes all the staged components of a snap, which can then be installed and tested with the
snap try <prime directory> command.
The following, example, will build a hello-world snap within LXD and offer its prime directory locally:
$ snapcraft try --use-lxd Pulling hello-world + snapcraftctl pull Building hello-world + snapcraftctl build + cp --archive --link --no-dereference . /root/parts/hello-world/install Staging hello-world + snapcraftctl stage Priming hello-world + snapcraftctl prime You can now run `snap try /home/user/hello-world/prime`.
The above snap can then be installed and tested with
snap try and the prime directory as its target:
$ snap try /home/user/hello-world/prime hello-world 0.1 mounted from /home/user/hello-world/prime
Last updated 6 months ago.