Bash – Preventing Grep from Causing Premature Termination in bash -e Script

basherror handlingexitgrep

This script does not echo "after":

#!/bin/bash -e

echo "before"

echo "anything" | grep e # it would if I searched for 'y' instead

echo "after"
exit

It also would if I removed the -e option on the shebang line, but I wish to keep it so my script stops if there is an error. I do not consider grep finding no match as an error. How may I prevent it from exiting so abruptely?

Best Answer

echo "anything" | { grep e || true; }

Explanation:

$ echo "anything" | grep e
### error
$ echo $?
1
$ echo "anything" | { grep e || true; }
### no error
$ echo $?
0
### DopeGhoti's "no-op" version
### (Potentially avoids spawning a process, if `true` is not a builtin):
$ echo "anything" | { grep e || :; }
### no error
$ echo $?
0

The "||" means "or". If the first part of the command "fails" (meaning "grep e" returns a non-zero exit code) then the part after the "||" is executed, succeeds and returns zero as the exit code (true always returns zero).

Subshells

There is no way for a subshell to kill the whole script. Under the effect of set -u, a subshell has no way to tell its parent “hey, I want you to treat this as a fatal error”. This wouldn't even make sense if set -u was in effect in the subshell but not in its parent. All the subshell can do is return a nonzero exit code. It's up to you to handle that exit code in the parent, either manually or through set -e.

(set -u; …; echo $undefined_variable)
# here $? is nonzero
ret=$?; if [ $ret -ne 0 ]; then exit $ret; fi

Subprocesses

What you've done here is not a subshell: you're launching another program. That other program happens to be bash, which is also the interpreter that's executing the present script, but it's a coincidence. There is absolutely no reason why a setting in a program invoked by bash would cause bash itself to exit.

set +e
bash -c 'do something'
echo "A - should and does reach here"

This is no different than if you'd written perl -e 'use strict; print $barf' — Perl would die and return a nonzero status but that doesn't cause the bash script to exit (except under set -e). Use set -e if you want to exit when a subprocess returns a nonzero return code.

Shell – Keep exit codes when trapping SIGINT and similar

All you need to do is change the EXIT handler inside your cleanup handler. Here's an example:

#!/bin/bash
cleanup() {
    echo trapped exit
    trap 'exit 0' EXIT
}
trap cleanup EXIT
read -p 'If you ctrl-c me now my return code will be the default for SIGTERM. '

Best Answer

Related Solutions

Bash ‘nounset’ on sub-shell/child doesn’t force main script to exit, how to workaround that globally

Subshells

Subprocesses

Shell – Keep exit codes when trapping SIGINT and similar

Related Question