Bash Error Handling: set -euo pipefail and trap

Exit codes

Every command returns an exit status: 0 success, anything else failure. $? holds the last one.

Strict mode

Put this at the top of scripts:

set -euo pipefail

• -e exits when a command fails (with exceptions such as conditions in if).
• -u treats unset variables as errors (catches typos).
• -o pipefail makes a pipeline fail if any command in it fails, not only the last.

cat > strict.sh <<'EOF'
set -euo pipefail
echo "step 1"
grep -q needle missing.txt | sort
echo "never printed"
EOF
bash strict.sh 2>&1 || echo "strict.sh failed with status $?"
bash -c 'set -u; echo "value: $UNDEFINED"' 2>&1 | sed 's/^.*: line [0-9]*: //' || true

step 1
grep: missing.txt: No such file or directory
strict.sh failed with status 2
UNDEFINED: unbound variable

Cleaning up with trap

trap 'commands' EXIT runs on exit, whether the script succeeds or fails: ideal for temporary files.

cat > job.sh <<'EOF'
set -euo pipefail
workdir=$(mktemp -d)
trap 'rm -rf "$workdir"; echo "cleaned up"' EXIT
echo "working in a temp dir"
echo data > "$workdir/file"
false
EOF
bash job.sh || echo "job.sh exited with $?"

working in a temp dir
cleaned up
job.sh exited with 1