Bash Strict Mode & Error Handling

Advancedv1.0.0

Write robust Bash scripts with strict mode (set -euo pipefail), trap handlers for cleanup, proper error messages, and exit code conventions for production automation.

Content

Overview

Bash scripts silently continue after failures by default. Strict mode changes this behavior, making scripts fail fast on errors, undefined variables, and pipeline failures.

Why This Matters

-Fail fast — catch errors immediately instead of cascading silently
-No silent failures — unset variables cause errors, not empty strings
-Pipeline safety — catch failures in any command of a pipeline
-Production reliability — scripts behave predictably in CI/CD

Step 1: Enable Strict Mode

bash

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

# What each flag does:
# -e  Exit immediately when a command fails
# -u  Treat unset variables as errors
# -o pipefail  Pipeline returns failure if ANY command fails

Step 2: Trap for Cleanup

bash

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

TEMP_DIR=""

cleanup() {
  local exit_code=$?
  if [[ -n "$TEMP_DIR" && -d "$TEMP_DIR" ]]; then
    rm -rf "$TEMP_DIR"
    echo "Cleaned up temp directory"
  fi
  exit "$exit_code"
}
trap cleanup EXIT  # Runs on ANY exit (success, error, signal)

TEMP_DIR=$(mktemp -d)
echo "Working in $TEMP_DIR"
# ... do work ...
# cleanup runs automatically when script ends

Step 3: Error Reporting

bash

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

# Log with timestamp and severity
log() {
  local level="$1"
  shift
  echo "[$(date '+%Y-%m-%d %H:%M:%S')] [$level] $*" >&2
}

info()  { log "INFO" "$@"; }
warn()  { log "WARN" "$@"; }
error() { log "ERROR" "$@"; }

die() {
  error "$@"
  exit 1
}

# Usage
info "Starting deployment"
command -v docker >/dev/null 2>&1 || die "Docker is not installed"
info "Docker found"

Step 4: Exit Code Conventions

bash

# Standard exit codes
# 0  Success
# 1  General error
# 2  Usage error (wrong arguments)
# 126  Command not executable
# 127  Command not found
# 130  Interrupted (Ctrl+C)

main() {
  if [[ $# -lt 1 ]]; then
    echo "Usage: $0 <environment>" >&2
    exit 2
  fi

  local env="$1"
  case "$env" in
    prod|staging|dev) deploy "$env" ;;
    *) die "Unknown environment: $env" ;;
  esac
}

main "$@"

Step 5: Safe Variable Defaults

bash

# With -u enabled, unset variables cause errors
# Use defaults for optional variables
PORT="${PORT:-8080}"
LOG_LEVEL="${LOG_LEVEL:-info}"
VERBOSE="${VERBOSE:-false}"

# Required variables — check explicitly
: "${DATABASE_URL:?ERROR: DATABASE_URL is required}"
: "${API_KEY:?ERROR: API_KEY is required}"

Best Practices

-Always start with set -euo pipefail
-Use trap cleanup EXIT for guaranteed cleanup
-Use ${VAR:-default} for optional variables
-Use ${VAR:?error message} for required variables
-Use die() function for fatal errors with messages
-Log to stderr (>&2), output data to stdout

Common Mistakes

-Not using strict mode (scripts continue after failures)
-Not quoting variables ($var instead of "$var")
-Using rm -rf $DIR with unset DIR (deletes /)
-Not cleaning up temp files on error
-Ignoring exit codes from critical commands

FAQ

Discussion

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#!/usr/bin/env bash set -euo pipefail # What each flag does: # -e Exit immediately when a command fails # -u Treat unset variables as errors # -o pipefail Pipeline returns failure if ANY command fails

Step 2: Trap for Cleanup

bash

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

TEMP_DIR=""

cleanup() {
  local exit_code=$?
  if [[ -n "$TEMP_DIR" && -d "$TEMP_DIR" ]]; then
    rm -rf "$TEMP_DIR"
    echo "Cleaned up temp directory"
  fi
  exit "$exit_code"
}
trap cleanup EXIT  # Runs on ANY exit (success, error, signal)

TEMP_DIR=$(mktemp -d)
echo "Working in $TEMP_DIR"
# ... do work ...
# cleanup runs automatically when script ends

Step 3: Error Reporting

bash

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

# Log with timestamp and severity
log() {
  local level="$1"
  shift
  echo "[$(date '+%Y-%m-%d %H:%M:%S')] [$level] $*" >&2
}

info()  { log "INFO" "$@"; }
warn()  { log "WARN" "$@"; }
error() { log "ERROR" "$@"; }

die() {
  error "$@"
  exit 1
}

# Usage
info "Starting deployment"
command -v docker >/dev/null 2>&1 || die "Docker is not installed"
info "Docker found"

Step 4: Exit Code Conventions

bash

# Standard exit codes
# 0  Success
# 1  General error
# 2  Usage error (wrong arguments)
# 126  Command not executable
# 127  Command not found
# 130  Interrupted (Ctrl+C)

main() {
  if [[ $# -lt 1 ]]; then
    echo "Usage: $0 <environment>" >&2
    exit 2
  fi

  local env="$1"
  case "$env" in
    prod|staging|dev) deploy "$env" ;;
    *) die "Unknown environment: $env" ;;
  esac
}

main "$@"

Step 5: Safe Variable Defaults

bash

# With -u enabled, unset variables cause errors
# Use defaults for optional variables
PORT="${PORT:-8080}"
LOG_LEVEL="${LOG_LEVEL:-info}"
VERBOSE="${VERBOSE:-false}"

# Required variables — check explicitly
: "${DATABASE_URL:?ERROR: DATABASE_URL is required}"
: "${API_KEY:?ERROR: API_KEY is required}"