Shell Scripting

Shell Scripting Patterns

This skill auto-applies when writing shell scripts. Follow these conventions.

Script Header

All bash scripts must start with:

#!/bin/bash
set -euo pipefail

Zsh scripts (.zshrc, .zsh_prompt, etc.) don't use set -euo pipefail — it breaks interactive shell behavior.

Platform Detection

if [[ "$(uname)" == "Darwin" ]]; then
    # macOS
else
    # Linux
fi

For commands that differ between platforms (e.g., sed -i on macOS needs ''):

if [[ "$(uname)" == "Darwin" ]]; then
    sed -i '' "s/old/new/" "$file"
else
    sed -i "s/old/new/" "$file"
fi

Command Availability

Always check before using optional commands:

command -v foo >/dev/null 2>&1 && foo_available=true || foo_available=false

# Or for guard clauses:
if ! command -v foo &>/dev/null; then
    echo "Skipping (foo not installed)"
    return 0  # or exit 0 in scripts
fi

Idempotency

Scripts should be safe to run repeatedly:

# Symlinks: -sf overwrites existing
ln -sf "$source" "$dest"

# Directories: -p is no-op if exists
mkdir -p "$dir"

# Only install if missing
if ! command -v foo &>/dev/null; then
    install_foo
fi

Paths

Resolve symlinks when capturing the script's own location — use pwd -P, not pwd. Plain pwd returns the logical path, so a script run via a symlink ends up with a directory string that doesn't match its physical location. If any subsequent code does a same-path guard ([[ "$src" == "$dest" ]]), it fails to fire and the script can overwrite its own source.

script_dir="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd -P)"
dest_dir="$(cd "$(dirname "$dest_file")" && pwd -P)"
[[ "$script_dir" == "$dest_dir" ]] && return  # same file, skip

Variables

• Quote all variable expansions: "$var", "${var}", "$@"
• Use ${var:-default} for optional variables with defaults
• Use ${var:?error message} for required variables
• Use local in functions: local result="$(command)"

Output

• Use echo for user-facing messages
• Use >&2 for errors: echo "Error: failed" >&2
• Silent success is fine for automation scripts
• Use || true to suppress expected failures

ShellCheck

All .sh files must pass shellcheck. Common directives:

# shellcheck disable=SC2034  # Variable appears unused (but is exported/sourced)
# shellcheck source=/dev/null  # Can't follow dynamic source