Bash Set Options to the rescue

A small bash script worked locally in GitHub Actions. The script counted successes and failures from a string, then branched on the result. Local run was fine; the CI run failed with exit code 1.

#!/bin/bash

set -x
values="success success success success"

count_success=$(echo ${values[@]} | grep -o 'success' | grep -c '^')
count_failure=$(echo ${values[@]} | grep -o 'failure' | grep -c '^')

if [[ $count_success -eq 4 ]]; then
  echo "success"
elif [[ $count_failure -ge 1 ]]; then
  echo "failure"
else
  echo "undefined"
fi

Why it fails in CI

The value of count_failure is 0, which is expected. The problem is not the final branch; it is the pipeline that produces count_failure.

When grep -o 'failure' finds no matches it exits with status 1. GitHub Actions runs shell steps with set -e and often set -o pipefail enabled by default, so a failing command in a pipeline aborts the whole script even though the output you care about (count_failure = 0) is correct.

set -x only prints each command; it does not change how the shell reacts to a failing command.

The fix: set +e

set +e tells the shell to keep running even when a command returns a non-zero exit status. Use it around the pipeline that is expected to produce zero matches:

#!/bin/bash

set +e
values="success success success success"

count_success=$(echo ${values[@]} | grep -o 'success' | grep -c '^')
count_failure=$(echo ${values[@]} | grep -o 'failure' | grep -c '^')
set -e

if [[ $count_success -eq 4 ]]; then
  echo "success"
elif [[ $count_failure -ge 1 ]]; then
  echo "failure"
else
  echo "undefined"
fi

A safer alternative is to make the pipeline itself robust so it never fails:

count_failure=$(echo ${values[@]} | grep -o 'failure' | grep -c '^' || true)

|| true forces the substitution to return 0 regardless of whether grep found a match.

The four set options you reach for first

Option	Shorthand	What it does	When you need it
set -e	exit on error	Aborts the script when any command returns non-zero	CI pipelines; default in GitHub Actions
set +e	disable exit on error	Keeps the script running after a non-zero command	Around commands where failure is expected data
set -u	unset variable guard	Exits if an undefined variable is expanded	Catching typos in variable names
set -x	xtrace	Prints each command before executing it	Debugging: seeing exactly what runs and with what values
set -o pipefail	pipefail	A pipeline fails if any command in it fails, not just the last	CI pipelines with multiple-stage pipes

Completion criterion

After reading this, you should be able to:

1. Explain why a script that works locally can fail in GitHub Actions.
2. Choose between set +e, || true, and set -o pipefail to handle a pipeline that legitimately produces no matches.
3. Use set -x for debugging and set -u to catch unset variables.

Comments