Error Handling in Scripts

By default, Bash keeps going after a command fails. In a script, that can turn one error into a disaster — cd fails, and the next rm runs in the wrong folder. Robust scripts check for failure.

Check important commands

die() { echo "Error: $*" >&2; exit 1; }

mkdir -p output
cd output || die "cannot enter output"
echo "data" > result.txt || die "cannot write result.txt"
echo "Saved $(pwd | sed 's#.*/##')/result.txt"
cd /no/such/folder || die "cannot enter /no/such/folder"
echo "never printed"

Saved output/result.txt
script.sh: line 7: cd: /no/such/folder: No such file or directory
Error: cannot enter /no/such/folder

command || die "..." is short and readable. Note that the failing cd printed its own error message too.

Checking in if statements

if ! grep -q "ERROR" <<< "all good"; then
  echo "No errors found"
fi

if output=$(ls /no/such/folder 2>&1); then
  echo "Listing: $output"
else
  echo "ls failed with: $output"
fi

No errors found
ls failed with: ls: cannot access '/no/such/folder': No such file or directory

if output=$(command) captures the output and checks the status in one step.

set -e: stop on the first error

set -e makes the script exit as soon as a command fails:

set -e
echo "Step 1"
false
echo "Step 2 — never reached"

Step 1

It's a useful safety net — Level 3's strict mode combines it with other options — but it has surprising exceptions (for example, failures inside if conditions and && chains don't trigger it), so explicit checks on important commands are still good practice.

Meaningful exit codes

Exit 0 on success and a non-zero code on failure. Some scripts use different codes for different problems (for example, 1 general error, 2 bad usage), and document them in the usage message.

Common mistakes

• Assuming a pipeline fails when an early command fails. By default, a pipeline's status is that of its last command: false | true succeeds. set -o pipefail (Level 3) changes that.
• Printing errors to stdout. Send them to stderr with >&2, so they aren't mixed into output that another program is reading.