Ubuntu – How to allow snap applications to access /tmp folder


Usually I'm avoiding snap-based applications, but sometime I need them.

For example – Markdown Lint Tool is shipped only as Snap named mdl.

It works great when files are in home folder –

$ echo "# header" > ~/test.md
$ /snap/bin/mdl ~/test.md ; echo $?

but it fails when file is located in /tmp directory:

$ echo "# header" > /tmp/test.md
$ cat /tmp/test.md
# header
$ /snap/bin/mdl /tmp/test.md
/snap/mdl/140/lib/ruby/gems/2.4.0/gems/mdl-0.9.0/lib/mdl/doc.rb:57:in `read': No such file or directory @ rb_sysopen - /tmp/test.md (Errno::ENOENT)
    from /snap/mdl/140/lib/ruby/gems/2.4.0/gems/mdl-0.9.0/lib/mdl/doc.rb:57:in `new_from_file'
    from /snap/mdl/140/lib/ruby/gems/2.4.0/gems/mdl-0.9.0/lib/mdl.rb:75:in `block in run'
    from /snap/mdl/140/lib/ruby/gems/2.4.0/gems/mdl-0.9.0/lib/mdl.rb:73:in `each'
    from /snap/mdl/140/lib/ruby/gems/2.4.0/gems/mdl-0.9.0/lib/mdl.rb:73:in `run'
    from /snap/mdl/140/lib/ruby/gems/2.4.0/gems/mdl-0.9.0/bin/mdl:10:in `<top (required)>'
    from /snap/mdl/140/bin/mdl:23:in `load'
    from /snap/mdl/140/bin/mdl:23:in `<main>'

For this particular application there are no options for snap connect:

$ snap connections | grep mdl
home                      mdl:home                             :home                                 -

Also I can't install it as classic:

$ snap install mdl --classic 
Warning: flag --classic ignored for strictly confined snap mdl

mdl 0.9.0 from Snapcrafters installed

What should I do to give Snap application full access to the /tmp folder?
Does it happen by Snap design or not?

Best Answer

  • It seems you may be looking for the "proper" way to solve the issue caused with snaps...

    but if you were looking for a workaround so you can use your tmp directory, you could:

    mkdir /home/you/tmp
    sudo mount --bind /tmp /home/you/tmp/

    mdl will have full access to the /tmp directory via the /home/you/tmp mountpoint:

    echo "# header" > /home/you/tmp/test.md
    mdl /home/you/tmp/test.md

    To make it permanent you can add this line to the /etc/fstab:

     # <file system> <mount point>   <type>  <options>       <dump>  <pass>
    /tmp        /home/you/tmp   auto    bind    0   3

    and rebuild the initrd with sudo update-initramfs -u -k all .