How can you run a command in bash over and over until success?

Question

until passwd
do
  echo "Try again"
done

or

while ! passwd
do
  echo "Try again"
done

You need to test $? instead, which is the exit status of the previous command. passwd exits with 0 if everything worked ok, and non-zero if the passwd change failed (wrong password, password mismatch, etc...)

passwd
while [ $? -ne 0 ]; do
    passwd
done

With your backtick version, you're comparing passwd's output, which would be stuff like Enter password and confirm password and the like.

To elaborate on @Marc B's answer,

$ passwd
$ while [ $? -ne 0 ]; do !!; done

Is nice way of doing the same thing that's not command specific.

If anyone looking to have retry limit:

max_retry=5
counter=0
until $command
do
   sleep 1
   [[ counter -eq $max_retry ]] && echo "Failed!" && exit 1
   echo "Trying again. Try #$counter"
   ((counter++))
done