Not able to modify the Exec property in the desktop file

snapcraft
1

I am trying to set a default flag that would be passed when my snap app would be launched.
For this, I tried to update my desktop file from

[Desktop Entry]
...
Exec=/path/to/myApp %U

to

[Desktop Entry]
...
Exec=/path/to/myApp --my-default-flag %U

The app is built correctly. I checked the contents of the desktop file inside the snap package that was generated using

sudo mount -t squashfs /path/to/myApp.snap /some/mount-point

But once I install the package locally, the contents are overwritten to

[Desktop Entry]
...
Exec=env BAMF_DESKTOP_FILE_HINT=/var/lib/snapd/desktop/applications/myApp.desktop /snap/bin/my-app %U

Specific Details:
the custom flag I want to pass is actually --no-sandbox

2

Hi @harryi3t - this is done this way for security reasons.

What you can do is have a simple wrapper script:

#!/usr/bin/bash

exec ${SNAP}/path/to/myApp --my-default-flag "$@"
3

how and when exactly in the build process do you do this ?
while the Exec line is indeed re-written during build to add the necessary BAMF environment, it should still use the existing Exec line appended to the added environment bits …

here i’m modifying the Exec line of an upstream .desktop file from an override-build: command:

(and later use that file from a desktop: entry in the respective apps: section (line 34))

4

What happens if you use the following?

Exec=my-app --my-default-flag %U

You should be using a command that is provided by your snap in the Exec line, rather than a path to an executable distributed within the snap. The Exec line will still be rewritten, but it should include the additional parameters you’ve specified.

5

That should not affect the additional flags being passed

As @Saviq suggested, looks like snap would overwrite the Exec line removing all flags.

6

how and when exactly in the build process do you do this ?

It’s hardcoded in the desktop file. During build time, I just fix the path, the flags are hardcoded. As I have mentioned in the post, I have verified that the file is correctly getting packaged. After the snap package was generated, I was able to open it up and see the flag in that file. It is only after I install the app, that it is overwritten for security reasons

Also, in your case, you are not passing any additional flags.

7

This is the code that rewrites the exec line:

As you can see, it checks to see if the Exec line starts with one of the snap’s exported commands, and preserves the remainder of the line if it finds a match.

If your .desktop file in $SNAP/meta/gui reads something like Exec=/path/to/myApp, you won’t get a match because of that leading path. It needs to be one of the commands your snap exports to /snap/bin.

8

Thanks @Saviq for the workaround.

I have one doubt, how is VSCode able to do this?
I see this in their desktop file

Exec=env BAMF_DESKTOP_FILE_HINT=/var/lib/snapd/desktop/applications/code_code.desktop /snap/bin/code --force-user-env --no-sandbox --unity-launch %F

They are able to pass these additional flags: --force-user-env --no-sandbox --unity-launch
The first one is replaced during build time and rest two are hardcoded.

9

Thanks @jamesh!

That looks like the core issue. Will try it out. Thanks again.