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

Question

I have a script and want to ask the user for some information, but the script cannot continue until the user fills in this information. The following is my attempt at putting a command into a loop to achieve this but it doesn't work for some reason:

echo "Please change password"
while passwd
do
    echo "Try again"
done

I have tried many variations of the while loop:

while `passwd`
while [[ "`passwd`" -gt 0 ]]
while [ `passwd` -ne 0 ]]
# ... And much more

But I can't seem to get it to work.

Gagarin

Answer 1

until passwd
do
  echo "Try again"
done

or

while ! passwd
do
  echo "Try again"
done

Narcis

oneliner: until passwd; do echo "Try again"; done – Abele

Difficult to Ctrl-C out of this. – Layer

If you think you're going to need to cancel this, just open up a new instance of your shell (example, type "bash") and, when you want to cancel it, go into another terminal window and kill the newly spawned bash process. – Effectually

Easy to Ctr-C out of this: until passwd; do echo "Try again"; sleep 2; done - all you have to do is press Ctr-C right after (within the two seconds given) the echo did it's job. – Poltroonery

Ctrl-Z followed by kill %1 works here when Ctrl-C won't – Fineness

What is the most simple way to keep trying only a few times ? – Penology

@azmeuk: Try something like until passwd || (( count++ >= 5 )); do echo "foo"; done (bash only, make sure to set count to 0 if that varaible exists) If you need this for plain sh, increment the counter in the body and use [ ] – Dogear

After referring to this page many times, I decided to created a generic bash function to retry commands, here it is: gist.github.com/felipou/6fbec22c4e04d3adfae5 – Hilton

Answer 2

To elaborate on @Marc B's answer,

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

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

If you want to do this as an alias (kudos to @Cyberwiz):

alias rr='while [ $? -ne 0 ]; do eval $(history -p !!); done'

Usage:

$ passwd
$ rr

Norinenorita

Doesn't work for me unfortunately (I get '!!' command not found). How is it supposed to work? – Fireback

It is a bash trick to run the previous command. For example if you forget to write sudo in front of a command, you can simply do sudo !! to run the previous command with root privileges. – Preventive

How to sleep between runs? – Hecklau

^^ while [ $? -ne 0 ]; do !!; sleep 1; done – Appositive

adding sleep 1; won't work since it'll give out 0 as exit value and therefore end the while loop prematurely – Luralurch

If you want to do this as an alias, with sleep: alias rr='while [ $? -ne 0 ]; do sleep 1; eval $(history -p !!); done' – Iambic

Answer 3

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.

Levo

I like this because it's clear how to adapt it for the opposite situation. e.g. run a program with a non-deterministic bug until it fails – Meador

When success is indicated by a non-zero exit code, this is what you need. – Despond

I think this can be slightly improved upon by changing the first passwd with /bin/false if you have some long and complicated command that you don't want to keep two versions of. – Cyprinodont

Should be the accepted answer imho. This should work in most shells (tested in bash, mksh, and ash) and is easily adaptable to the situation. – Doting

Fails shellcheck with output: Check exit code directly with e.g. 'if mycmd;', not indirectly with $?. [SC2181] – Tutankhamen

Precisely because of the shellcheck warning I prefer the accepted until answer – Grandpapa

Answer 4

If anyone looking to have retry limit:

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

Ridglea

$command isn't a great placeholder -- see BashFAQ #50 on why using strings to store commands is innately unreliable. – Cannula

I think in this example $command should just be replaced by your arbitrary command. Not necessarily used as string. I like to use <YOUR_COMMAND> to indicate user has to replace something here – Darky

Answer 5

You can use an infinite loop to achieve this:

while true
do
  read -p "Enter password" passwd
  case "$passwd" in
    <some good condition> ) break;;
  esac
done

Kurland

This is the only answer that didn't require putting my multi-line command in a function. I did && break after my verification command though instead of the select case. – Kill

Answer 6

while [ -n $(passwd) ]; do
        echo "Try again";
done;

Incinerate

This is not the way to do it... you're negating the stdout of psswd and then doing a unary test on it. @Norinenorita is good. – Inerrable

Moreover, because there isn't quoting, the test is going to misbehave when there is output but it's more than one word, or is a glob expression that matches files in the current directory, or so forth. – Cannula

Answer 7

It becomes a little tricky if you want the strict mode (set -e) and none of above worked.

set -euo pipefail

counter=0
max_attempts=3
while ret=0; <your command> || ret=$?; [ $ret -ne 0 ]
do
  [[ $counter -eq $max_attempts ]] && echo "Command failed"; exit 1
  sleep 2
  echo "Trying again"
  counter=$((counter+1))
done

Ezarra