Shebang

Bash · Reference cheat sheet

📋 Overview

The shebang (#!) is the first line of a script. It tells the kernel which interpreter to run. Prefer #!/usr/bin/env bash so the script finds bash on PATH across systems.

🔧 Core concepts

Requirements: shebang must be line 1, no BOM, Unix line endings (LF). The file needs execute permission (chmod +x). Options after the interpreter (e.g. #!/usr/bin/env bash -e) are not portable with env on all systems—set options inside the script instead.

💡 Examples

Portable bash script:

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

echo "Running under: $BASH_VERSION"

Make executable and run:

chmod +x ./deploy.sh
./deploy.sh          # uses shebang
bash ./deploy.sh     # ignores shebang; forces bash

Detect interpreter:

head -n1 "$0"        # show shebang of current script
readlink -f "$(command -v bash)"

Avoid:

#!/usr/bin/env bash -e   # often broken: env treats "-e" as a program name

Use set -e in the body instead.

⚠️ Pitfalls

• Windows editors may save CRLF; the kernel then looks for /usr/bin/env bash\r and fails.
• #!/bin/bash breaks on systems where bash lives elsewhere (e.g. some BSDs, Nix).
• Running with sh script.sh forces POSIX sh and ignores bashisms even if the shebang says bash.
• env may not pass flags; put set -o / set -euo pipefail after the shebang.
• Scripts without a shebang inherit the caller’s shell when run as ./script only if the kernel allows—always include one.

🔗 Related

• scripts_best_practices.md
• debugging.md
• exit_codes.md
• variables.md