Bash Error Handling - set -e, trap, Exit Codes, Retry Patterns

Error Handling in Bash

Bash scripts fail silently by default. Without proper error handling, a failed command can cause subsequent commands to run with incorrect data or in an unexpected state. Robust error handling is essential for production scripts.

set -euo pipefail

The most important line in any bash script:

set -euo pipefail

• -e (errexit): Exit immediately if any command returns a non-zero exit code
• -u (nounset): Treat references to unset variables as errors
• -o pipefail: A pipeline fails if any command in it fails (not just the last)

Trap for Cleanup

The trap command executes code when the script receives a signal or exits:

cleanup() {
  echo "Cleaning up..."
  rm -rf "$TEMP_DIR"
  # Restore original state
}

trap cleanup EXIT        # always run on exit
trap cleanup ERR         # run on error
trap 'echo "Interrupted"; exit 1' INT TERM  # handle Ctrl+C

Checking Exit Codes

# Method 1: if statement
if ! command_that_might_fail; then
  echo "Command failed with exit code $?"
  exit 1
fi

# Method 2: || operator
command_that_might_fail || {
  echo "Command failed"
  exit 1
}

# Method 3: explicit check
command_that_might_fail
status=$?
if [ $status -ne 0 ]; then
  echo "Failed with status $status"
fi

Retry Pattern

retry() {
  local max_attempts="$1"
  local delay="$2"
  shift 2
  local cmd="$@"

  local attempt=1
  while [ $attempt -le $max_attempts ]; do
    if eval "$cmd"; then
      return 0
    fi
    echo "Attempt $attempt/$max_attempts failed. Retrying in ${delay}s..."
    sleep "$delay"
    attempt=$((attempt + 1))
  done

  echo "All $max_attempts attempts failed"
  return 1
}

# Usage
retry 3 5 curl -sf https://api.example.com/health

Error Reporting

# Custom error handler
error() {
  local line="$1"
  local msg="$2"
  echo "ERROR at line $line: $msg" >&2
}

trap 'error $LINENO "Script failed"' ERR

# Detailed error logging
log_error() {
  echo "[$(date '+%Y-%m-%d %H:%M:%S')] ERROR: $*" | tee -a error.log >&2
}

Safe Temporary Files

TEMP_DIR=$(mktemp -d) || { echo "Failed to create temp dir"; exit 1; }
TEMP_FILE=$(mktemp) || { echo "Failed to create temp file"; exit 1; }

trap 'rm -rf "$TEMP_DIR" "$TEMP_FILE"' EXIT