bash script: repeat command if it returns an error

Question

I would like to create a loop that repeats a ncftp transfer if it returns an error.

I'm a little unsure how the exit code variable can be used in a loop. Would something like this work?

until [$? == 0]; do
    ncftpput -DD -z -u user -p password remoteserver /remote/dir /local/file
done

Answer 1

I found the basis for this elegant loop elsewhere on serverfault. Turns out there is no need save the exit code, as you can test directly on the command itself;

until ncftpput -DD -z -u user -p password remoteserver /remote/dir /local/file; do
  echo Tansfer disrupted, retrying in 10 seconds...
  sleep 10
done

Answer 2

Almost. You are probably better saving the return value as a variable so that you can pre-set it before the loop. Otherwise it will be affected by the last-run command.

You might also want to sling a sleep in there to stop it respawning too quickly.

RET=1
until [ ${RET} -eq 0 ]; do
    ncftpput -DD -z -u user -p password remoteserver /remote/dir /local/file
    RET=$?
    sleep 10
done

Answer 3

Bit hacky but my solution was to just create a bash function which runs itself if it exits with a failure

function retry { the_command_you_want && echo "success" || (echo "fail" && retry) }; retry

Answer 4

Some of the solutions shown on this page will loop an infinite number of times if the command continues to fail.

If you're not a fan of infinite loops try this:

function retry {
    command="$*"
    retval=1
    attempt=1
    until [[ $retval -eq 0 ]] || [[ $attempt -gt 5 ]]; do
        # Execute inside of a subshell in case parent
        # script is running with "set -e"
        (
            set +e
            $command
        )
        retval=$?
        attempt=$(( $attempt + 1 ))
        if [[ $retval -ne 0 ]]; then
            # If there was an error wait 10 seconds
            sleep 10
        fi
    done
    if [[ $retval -ne 0 ]] && [[ $attempt -gt 5 ]]; then
        # Something is fubar, go ahead and exit
        exit $retval
    fi
}

Now, whenever you need a retry loop, just run:

retry mycommand and args

e.g.:

retry ncftpput -DD -z -u user -p password remoteserver /remote/dir /local/file

Answer 5

You can do a loop while your command returns error:

    while [ -n $(ncftpput -DD -z -u user -p password remoteserver /remote/dir /local/file) ]; do
            sleep 1;
    done;

Answer 6

I've had a similar problem with needing to repeat curl requests, and the scripting was getting out of hand. As a result I created the retry tool:

retry ncftpput -DD -z -u user -p password remoteserver /remote/dir /local/file

In this case, the ncftpput command will be retried indefinitely until it returns a success status code, backing off by default for ten seconds in between.

Retry is available here: https://github.com/minfrin/retry