How to check if any errors occurred during ssh?

Question

I'm writing a deployment script, and I need to rollback if any errors occurred.

Eg:

#!/bin/bash
ssh myapp '
    mkdir /some/dir
    # check if above command failed, and execute rollback script if needed
'
# or maybe do it from here?

Right now when I execute this script, if mkdir fails, it prints it on my screen and continues execution. I need to catch that error and do something about it.

Answer 1

The exit status of ssh will be the exit status of the remote command. For instance

ssh myapp 'exit 42'
echo $?

should print 42 ($? is the exit status of the last command executed).

One option is to exit right away if the mkdir fails:

ssh myapp 'mkdir /some/dir || exit 42; do-more-stuff'
if [[ $? = 1 ]]; then
   echo "Remote mkdir failed"
fi

It is probably better to try to handle any remote failures in your script, if possible.

Answer 2

If you really need to catch the error message you can try this:

#!/bin/bash
result=`ssh myapp 'mkdir /some/dir' 2>&1`
if [[ -n $result ]]; then
    echo "the following error occurred: $result"
fi

By this you redirect the standard error output to the standard output and save the output of the ssh command to $result. If you just need the error code / exit status, see this answer.