Bash

Bourne Again SHell, the standard shell for Linux and macOS.

When to Use

• Linux System Administration & Automation: Scripting OS maintenance, cron jobs, file backups, and package management.
• CI/CD Pipeline Glue Code: Orchestrating build, test, and container push steps in GitHub Actions and GitLab CI.
• Docker Entrypoint Scripts: Configuring environment variables, secrets, and database wait-for loops during container startup.
• Command-Line Wrappers: Wrapping multi-step CLI commands into simple, repeatable internal developer utilities.

Quick Start

#!/bin/bash

NAME="World"
echo "Hello, $NAME!"

if [ -f "file.txt" ]; then
    echo "File exists"
else
    echo "File not found"
fi

Core Concepts

Strict Error Handling (set -euo pipefail)

Guarantees script execution aborts on unhandled errors, unset variables, and piped command failures:

#!/usr/bin/env bash
set -euo pipefail
IFS=$'
	'

# -e: Exit immediately if a command exits with a non-zero status
# -u: Treat unset variables as an error and exit immediately
# -o pipefail: Pipeline exit status matches the last failing command

Robust Parameter Expansion & Default Values

Safely handles optional and required script variables:

# Provide fallback default if variable unset
TARGET_ENV="${1:-staging}"

# Throw descriptive error if variable is unset or empty
DATABASE_URL="${DATABASE_URL:?Error: DATABASE_URL environment variable is required}"

# Extract filename without extension
filename="archive_2026.tar.gz"
base="${filename%%.*}" # "archive_2026"

Trap Signals for Reliable Cleanup

Ensures temporary files and child processes are cleaned up upon exit or interruption:

TEMP_DIR="$(mktemp -d)"
cleanup() {
  echo "Cleaning up temporary files..."
  rm -rf "${TEMP_DIR}"
}
trap cleanup EXIT INT TERM

Common Patterns

Safe Script Header with Error and Signal Traps

Problem: Shell scripts fail silently halfway through, leaving orphaned temp files and corrupted state.

Solution: Use unofficial bash strict mode with automatic cleanup trap:

#!/usr/bin/env bash
set -euo pipefail
IFS=$'\n\t'

TMP_DIR=$(mktemp -d)
cleanup() {
    rm -rf "$TMP_DIR"
    echo "Cleaned up temporary workspace."
}
trap cleanup EXIT ERR INT TERM

# Script business logic
echo "Working in: $TMP_DIR"

Best Practices

Do:

• Always Quote Variables: Wrap variables in double quotes ("${MY_VAR}") to prevent word splitting and glob expansion bugs.
• Lint with ShellCheck: Run shellcheck in CI to catch syntax pitfalls, portability bugs, and quoting errors automatically.
• Use [[ ... ]] for Conditions: Prefer modern Bash conditional evaluation [[ $var == "val" ]] over legacy [ ... ].
• Use Local Variables in Functions: Always declare variables inside functions with local var="value".

Don't:

• Parse ls output: Use globbing loops (for file in *.txt; do ... done) instead of parsing ls.
• Use unquoted eval: eval executes arbitrary string inputs and introduces catastrophic shell injection vulnerabilities.
• Write multi-thousand line monolithic bash scripts: Transition complex scripts to Python or Go when logic grows large.

Troubleshooting

References

• ShellCheck
• Google Shell Style Guide