ubuntu-core image missing awk

[Michael Nelson]

I'm trying to snap a tool which uses awk. Now I can try including awk in my snap directly, but I checked the default apparmor profile for any snap, and it seems to indicate that awk is installed and enabled by the profile, but when checking (via a test snap), /usr/bin/awk symlinks to /etc/alternatives/awk symlinks to /usr/bin/gawk which doesn't exist (in ubuntu-core 16.04.0-24):

michael@dev-xenial2:~/dev/todo.txt⟫ cat /snap/todo/100001/testawk.sh
#! /bin/bash

ls -l /usr/bin/{,g,m}awk
ls -l /etc/alternatives/awk
/usr/bin/awk -h
/etc/alternatives/awk -h

michael@dev-xenial2:~/dev/todo.txt⟫ todo.testawk
ls: cannot access '/usr/bin/gawk': No such file or directory
lrwxrwxrwx 1 root root 21 Mar 8 11:29 /usr/bin/awk -> /etc/alternatives/awk
-rwxr-xr-x 1 root root 117768 Mar 24 2014 /usr/bin/mawk
lrwxrwxrwx 1 root root 13 Apr 12 14:18 /etc/alternatives/awk -> /usr/bin/gawk
/snap/todo/100001/testawk.sh: line 5: /usr/bin/awk: No such file or directory
/snap/todo/100001/testawk.sh: line 6: /etc/alternatives/awk: No such file or directory

[Jamie Strandboge (jdstrand)]

In 16.04+20160420.05-01:
$ ls -l /usr/bin/awk
lrwxrwxrwx 1 root root 21 Apr 20 04:58 /usr/bin/awk -> /etc/alternatives/awk

$ ls -l /etc/alternatives/awk
lrwxrwxrwx 1 root root 13 Apr 20 04:58 /etc/alternatives/awk -> /usr/bin/mawk

After running 'snap refresh ubuntu-core' and getting 16.04+20160419.20-55:
$ ls -l /usr/bin/awk
lrwxrwxrwx 1 root root 21 Apr 19 20:52 /usr/bin/awk -> /etc/alternatives/awk
$ ls -l /etc/alternatives/awk
lrwxrwxrwx 1 root root 13 Apr 19 20:52 /etc/alternatives/awk -> /usr/bin/mawk

It seems your image is out of date. What is the output of 'snap list'? What architecture/device is this for?

[Michael Nelson (michael.nelson)]

Thanks Jamie. My ubuntu-core was out of date, but updating doesn't change the symlinks which my snap'd app sees:

michael@dev-xenial2:~/dev/todo.txt⟫ snap list
Name Version Developer
hello-world 6.0 canonical
todo 2.1
ubuntu-core 16.04.0-24 canonical

michael@dev-xenial2:~/dev/todo.txt⟫ sudo snap refresh ubuntu-core
64.64 MB / 64.64 MB [============================================================================================] 100.00 % 26.19 MB/s

michael@dev-xenial2:~/dev/todo.txt⟫ snap list
Name Version Developer
hello-world 6.0 canonical
todo 2.1
ubuntu-core 16.04+20160419.20-55 canonical

michael@dev-xenial2:~/dev/todo.txt⟫ rm -rf parts/testawk && sudo snap remove todo && snapcraft && sudo snap install todo_2.1_amd64.
snap
[-] Remove snap "todo" from the system
...
Snapped todo_2.1_amd64.snap
[/] Setup snap "todo" security profiles

michael@dev-xenial2:~/dev/todo.txt⟫ todo.testawk
ls: cannot access '/usr/bin/gawk': No such file or directory
lrwxrwxrwx 1 root root 21 Apr 19 20:52 /usr/bin/awk -> /etc/alternatives/awk
-rwxr-xr-x 1 root root 117768 Mar 24 2014 /usr/bin/mawk
lrwxrwxrwx 1 root root 13 Apr 12 14:18 /etc/alternatives/awk -> /usr/bin/gawk
/snap/todo/100001/testawk.sh: line 5: /usr/bin/awk: No such file or directory
/snap/todo/100001/testawk.sh: line 6: /etc/alternatives/awk: No such file or directory

michael@dev-xenial2:~/dev/todo.txt⟫ cat /snap/todo/100001/testawk.sh
#! /bin/bash

ls -l /usr/bin/{,g,m}awk
ls -l /etc/alternatives/awk
/usr/bin/awk -h
/etc/alternatives/awk -h

michael@dev-xenial2:~/dev/todo.txt⟫ uname -a
Linux dev-xenial2 4.4.0-18-generic #34-Ubuntu SMP Wed Apr 6 14:01:02 UTC 2016 x86_64 x86_64 x86_64 GNU/Linux

This is running on a (canonistack) xenial instance .

[Jamie Strandboge (jdstrand)]

This is odd-- the os snap I use specifies mawk as the alternative, but yours has gawk.

$ snap list
Name Version Developer
...
ubuntu-core 16.04+20160419.20-55 canonical

$ ls -l /etc/alternatives/awk
lrwxrwxrwx 1 root root 13 Apr 20 04:58 /etc/alternatives/awk -> /usr/bin/mawk

I can easily see that things work with the hello-world snap:

$ sudo snap install hello-world
...

$ hello-world.sh # run a confined shell

bash-4.3$ gawk -W version # try gawk
bash: gawk: command not found # expected, gawk is not on the system

bash-4.3$ mawk -W version # try mawk
mawk 1.3.3 Nov 1996, Copyright (C) Michael D. Brennan
...
bash-4.3$ awk -W version # try awk
mawk 1.3.3 Nov 1996, Copyright (C) Michael D. Brennan

How did you generate the image?

[John Lenton (chipaca)]

It's because /etc isn't hidden; the snap sees the host's /etc.

[Zygmunt Krynicki (zyga)]

I'm discussing the changes to ubuntu-core-launcher that would hide /etc/alternatives from the classic system when snaps are running.

[Kyle Fazzari (kyrofa)]

This just bit the nextcloud snap as well.

[Zygmunt Krynicki (zyga)]

Hmm, so shall we bind-mount the core-snap /etc/alternatives over the already bind-mounted /etc from classic?

[Oliver Grawert (ogra)]

well, only the individual ones i guess, if you mount the whole dir over the host one you might hide bits you dont want to hide ...

[John Lenton (chipaca)]

Can't we nuke alternatives in core? In what situation is it useful to still have that in the core image?

[John Lenton (chipaca)]

By "nuke the alternatives" I mean, as part of the image build process, iterate over the alternatives and just point the symlink to the alternative, instead of to /etc/alternatives/whatever.

[Oliver Grawert (ogra)]

sure, but thats an extra hack we need to maintain and it might not work with all packages (there are some that change the binary name via the alternative or point to some non-bin path for an executable, not to forget that many packages also use it for config files in random places).

so this requires some extra code (perhaps only remove the ones with a proper executable path etc) and probably a black or whitelist ... pretty much like you would have it at runtime with the bind mounts ...

[Michael Vogt (mvo)]

Fixed with zyga updated snap-confine.