How does Bash path completion work with sudo?

Question

Tab path completion does not work (at least on Ubuntu and AFAIK Arch) with

sudo mount <whatever>

The iso file I am trying to mount is not in /etc/fstab. If I just type

mount <whatever>

the completion works (but of course the command fails as I am not root). Apparently it is sudo that breaks it.

How can make completion work with sudo?

It is surprising that with

sudo umount <whatever>

completion works. How is it achieved? Does it look into /etc/fstab?

Solution: I just put a shell script into /usr/local/bin that calls sudo mount ... with the arguments passed to it. Completion works when calling this script since there is no sudo in the way.

Answer 1

This has not at all to do with bash, but it depends on the completions programmed in the package bash-completion.

From some comments in the file /etc/bash_completion.d/mount:

# mount(8) completion. This will pull a list of possible mounts out of
# /etc/{,v}fstab, unless the word being completed contains a ':', which
# would indicate the specification of an NFS server. In that case, we
# query the server for a list of all available exports and complete on
# that instead.
#

# umount(8) completion. This relies on the mount point being the third
# space-delimited field in the output of mount(8)
#

Also, you find in the main file /etc/bash_completion the following comment, that explicitly talk about mount and umount commands:

# A meta-command completion function for commands like sudo(8), which need to
# first complete on a command, then complete according to that command's own
# completion definition - currently not quite foolproof (e.g. mount and umount
# don't work properly), but still quite useful.
#

Update:
The comment about mount and umount commands was removed from bash_completion in the commit:

_command_offset: Restore compopts used by called command.

This fixes completions that rely on their compopts, most notably
mount(8).
Fixes bash-completion bug #313183.

Released in bash-completion 1.90

Answer 2

This can easily be solved by looking into the Arch Wiki:

It is useful to have the auto-complete feature (pressing the Tab key twice on the keyboard) after you type some command like sudo.

To do this, add line(s) in this format to your ~/.bashrc file:

complete -cf your_command

For example, to enable auto-complete after sudo and man:

complete -cf sudo
complete -cf man

Answer 3

No it doesn't look in fstab. Bash completion looks for commands in path, aliases in bashrc and/or bash_profile and paths. So if you type something like sudo mount /mnt/some_folder it should auto-complete. Maybe when you did the umount you were in a relative path for your destination and it did an auto-complete.

Answer 4

I guess bash doesn't want to be too complex and stay portable, so they won't look in /dev to propose completion for a mount operation.

However when calling umount, it can easily look the output of df/mount/etc to know what devices are mounted and where.

Maybe zsh will provide completion for mount, heck zsh even autocomplete when doing an scp command...