Bash scripting – loop until return value is 0

bashshell-scriptunmounting

I need to umount something in my script, but sometimes it unmount's before all of the data has finished being copied and causes the umount to fail. I looked for a way to do a "blocking" umount, but I didn't find anything. So, I tried to write a script to loop until it could be unmounted, but it doesn't work. 

while [ `sudo umount mount` ]
do
    sleep 0.1
done
rmdir mount

When run outputs:

umount: /home/evantandersen/mount: device is busy.
        (In some cases useful info about processes that use
         the device is found by lsof(8) or fuser(1))
rmdir: failed to remove `mount': Device or resource busy

Shouldn't it loop until the return value of sudo umount mount is 0, meaning it was successfully umounted?

Best Answer

The [ command is to evaluate conditional expressions. It's of no use here.

Because umount doesn't output anything on its standard output (the errors go to stderr), `sudo umount mount` expands to nothing.

So it's like:

while [ ]
do
  sleep 0.1
done

The [ command, when not passed any argument beside [ and ] returns false (a non-zero exit status), so you will not enter the loop.

Even if umount had output its errors on stdout, using the [ command would not have made sense, because the words resulting of that output would never have made up a valid conditional expression.

Here you want:

until sudo umount mount
do
  sleep 0.1
done

That is, you want to check the exit status of sudo/umount, not of a [ command.

If you wanted to check if umount output any error or warning on its stderr, that's where the [ could have been useful. The -n "some-string" is a conditional expression recognised by the [ command to test whether "some-string" is empty or not, so something like:

while [ -n "$(sudo umount mount 2>&1 > /dev/null)" ]; do
  sleep 0.1
done

But looking for the presence of error or warning messages is generally a bad idea. The umount command tells us whether or not it succeeds with its exit code, that's much more reliable. It could succeed and still output some warning message. It could fail and not output an error (like when it's killed).

In this particular case, note that umount might fail because the directory is not mounted, and you would loop forever in that case, so you could try another approach like:

while mountpoint -q mount && ! sudo umount mount; do
  sleep 0.1
done

Or if "mount" may be mounted several times and you want to unmount them all:

while mountpoint -q mount; do
  sudo umount mount || sleep 0.1
done
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