Shell Script Preferences

Project conventions take precedence unless user says otherwise.

Shebang

POSIX sh (default):

#!/usr/bin/env sh

Only use bash/zsh when their specific features are needed.

Error Handling

Always set error flags at script start:

set -eu

• -e: exit on first error
• -u: error on undefined variables
• Add -x for debugging when needed

Trap for cleanup:

trap 'cleanup' EXIT
trap 'cleanup; exit 1' INT TERM

Error messages to stderr:

echo 'error: cannot find config' >&2
exit 1

Variable Naming

Variable Expansion

Always use braces: ${VAR} not $VAR

Exception: Special parameters use bare form: $#, $@, $?, $0, $1, $2, etc.

# Correct
echo "${HOME}"
printf '%s\n' "${message}"
[ $# -gt 0 ]
main "$@"

# Wrong
echo $HOME
printf '%s\n' "$message"
[ ${#} -gt 0 ]
main "${@}"

Quoting

• Double quotes for variable expansion: "${VAR}"
• Single quotes for literals: 'literal string'
• Always quote "$@" and path variables

Functions

function_name() {
    # body
}

• No function keyword (POSIX compatibility)
• Opening brace on same line
• 4-space indent

Indentation

4 spaces. No tabs.

Output

Colors:

YELLOW='\033[1;33m'
RESET='\033[0m'

log() {
    printf '%b%s%b\n' "${YELLOW}" "$1" "${RESET}"
}

Argument Parsing

while [ $# -gt 0 ]; do
    case "$1" in
        -h|--help)
            show_help
            exit 0
            ;;
        --force)
            FORCE=1
            shift
            ;;
        *)
            echo "error: unknown option '$1'" >&2
            exit 1
            ;;
    esac
done

Help Text

show_help() {
    cat << EOF
Usage: $(basename "$0") [options]

Options:
    -h, --help    Show this help
    --force       Force operation
EOF
}

Command Checking

if ! command -v program > /dev/null; then
    echo 'error: program not found' >&2
    exit 1
fi

Script Structure

1. Shebang
2. set directives
3. Constants (UPPERCASE)
4. Color definitions (if used)
5. Helper functions
6. Main logic / argument parsing
7. Execution

Example:

#!/usr/bin/env sh
set -eu

YELLOW='\033[1;33m'
RESET='\033[0m'

log() {
    printf '%b%s%b\n' "${YELLOW}" "$1" "${RESET}"
}

show_help() {
    cat << EOF
Usage: $(basename "$0") [options]
EOF
}

main() {
    while [ $# -gt 0 ]; do
        case "$1" in
            -h|--help)
                show_help
                exit 0
                ;;
            *)
                shift
                ;;
        esac
    done

    log "Starting..."
}

main "$@"

Parameter Expansion

${VAR:-default}     # default if unset/empty
${VAR##*/}          # basename-like
${VAR%.*}           # remove extension

Temporary Files

temp_file="$(mktemp)"
trap 'rm -f "${temp_file}"' EXIT

Common Mistakes

• Using $VAR instead of ${VAR} (except special params: $#, $@, $1, etc.)
• Unquoted variables: ${VAR} instead of "${VAR}"
• Using bashisms in POSIX scripts (arrays, [[, source)
• Missing set -eu at start
• Error messages not going to stderr
• Using function keyword
• Tab indentation instead of 4 spaces