Proper Error Handling with trap and set -euo pipefail

Beginner8 min

Implement robust error handling in bash scripts using set flags, trap for cleanup, error reporting with line numbers, and safe variable expansion.

Prerequisites

-Bash 4.0+

Steps

Enable strict mode

Add set flags at the top of every script to catch errors immediately.

$ echo '#!/usr/bin/env bash' > safe_script.sh && echo 'set -euo pipefail' >> safe_script.sh

-e exits on error, -u exits on unset variables, -o pipefail catches failures in pipes.

Add an error trap with line number reporting

$ cat >> safe_script.sh << 'EOF' on_error() { echo "Error on line $1: command '$2' exited with code $?" >&2 } trap 'on_error ${LINENO} "${BASH_COMMAND}"' ERR EOF

Add a cleanup trap for temp files

Ensure temporary files and resources are cleaned up on exit, error, or interrupt.

$ cat >> safe_script.sh << 'EOF' TMPDIR=$(mktemp -d) cleanup() { rm -rf "$TMPDIR" echo "Cleaned up temp directory" } trap cleanup EXIT EOF

EXIT traps run on both success and failure. ERR traps only run on errors. Use EXIT for cleanup and ERR for error reporting.

Use safe variable defaults

Prevent unset variable errors with default values and explicit checks.

$ echo 'NAME="${1:-default_name}"' && echo 'DIR="${OUTPUT_DIR:=/tmp/output}"' && echo '[[ -z "${API_KEY:-}" ]] && { echo "API_KEY not set" >&2; exit 1; }'

${var:-default} provides a default without setting the variable. ${var:=default} also assigns the default.

Full Script

full-script.sh

#!/usr/bin/env bash
set -euo pipefail

# Error Handling Template

# Error reporting with line numbers
on_error() {
  local line=$1
  local cmd=$2
  local code=$?
  echo "" >&2
  echo "=== ERROR ===" >&2
  echo "Script:  $(basename "$0")" >&2
  echo "Line:    $line" >&2
  echo "Command: $cmd" >&2
  echo "Code:    $code" >&2
  echo "=============" >&2
}
trap 'on_error ${LINENO} "${BASH_COMMAND}"' ERR

# Cleanup on exit
TMPDIR=$(mktemp -d)
cleanup() {
  local exit_code=$?
  rm -rf "$TMPDIR"
  [[ $exit_code -ne 0 ]] && echo "Script failed with code $exit_code" >&2
  exit "$exit_code"
}
trap cleanup EXIT

# Safe variable access
NAME="${1:-}"
[[ -z "$NAME" ]] && { echo "Error: name argument required" >&2; exit 1; }

# Safe command execution
if ! output=$(some_command 2>&1); then
  echo "Command failed: $output" >&2
  exit 1
fi

echo "Success: $output"

FAQ

Discussion

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#!/usr/bin/env bash set -euo pipefail # Error Handling Template # Error reporting with line numbers on_error() { local line=$1 local cmd=$2 local code=$? echo "" >&2 echo "=== ERROR ===" >&2 echo "Script: $(basename "$0")" >&2 echo "Line: $line" >&2 echo "Command: $cmd" >&2 echo "Code: $code" >&2 echo "=============" >&2 } trap 'on_error ${LINENO} "${BASH_COMMAND}"' ERR # Cleanup on exit TMPDIR=$(mktemp -d) cleanup() { local exit_code=$? rm -rf "$TMPDIR" [[ $exit_code -ne 0 ]] && echo "Script failed with code $exit_code" >&2 exit "$exit_code" } trap cleanup EXIT # Safe variable access NAME="${1:-}" [[ -z "$NAME" ]] && { echo "Error: name argument required" >&2; exit 1; } # Safe command execution if ! output=$(some_command 2>&1); then echo "Command failed: $output" >&2 exit 1 fi echo "Success: $output"