Kiraneswaran Engineering Skills Bash Shell Scripting

name: bash-shell-scripting description: Bash scripting best practices for production-grade scripts, CLI tools, and Makefiles. Covers strict mode, error handling, portability, performance patterns, and argument parsing. Use when working with .sh files, Makefiles, shell scripts, or when asking about Bash, shell scripting, CLI design, or command-line tools.

Bash & Shell Scripting

Core Principles

• DRY: Don't Repeat Yourself
• KISS: Keep It Simple
• Fail Fast: Exit on errors immediately
• Zero Warnings: Must pass shellcheck

Quick Reference

set -euo pipefail           # Strict mode (fail-fast)
set -uo pipefail            # Controlled mode (explicit error handling)
set -Euo pipefail           # Strict + ERR trap propagation
command -v cmd >/dev/null   # Check if command exists (portable)
trap 'cleanup' EXIT         # Always cleanup
flock -n 200 || exit 1      # Prevent concurrent runs
readonly VAR="value"        # Immutable constant
local var="value"           # Function-local variable

Core Standards

Error Handling Modes

Strict Mode (Fail-Fast)

set -euo pipefail  # Exit immediately on any error

Use for: Simple linear scripts, dependency installation, straightforward validation.

Controlled Mode (Explicit)

set -uo pipefail  # No -e: handle errors explicitly

Use for: Diagnostics, cleanup operations, commands where failure is expected.

Strict + ERR Trap

set -Euo pipefail

trap 'echo "ERROR in ${FUNCNAME[0]:-main} at line $LINENO"' ERR

Use for: Production scripts with comprehensive error handling.

Script Template

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

readonly SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
readonly SCRIPT_NAME="$(basename "$0")"

cleanup() {
    rm -f "${TEMP_FILE:-}" 2>/dev/null || true
}
trap cleanup EXIT

log() {
    echo "[$(date +'%Y-%m-%d %H:%M:%S')] $*" >&2
}

die() {
    log "ERROR: $*"
    exit 1
}

main() {
    local arg="${1:-}"
    [[ -z "$arg" ]] && die "Usage: $SCRIPT_NAME <argument>"
    
    log "Processing: $arg"
    # Main logic here
}

main "$@"

Best Practices

Always Quote Variables

# ✅ GOOD
echo "${var}"
[[ -n "${var:-}" ]] && echo "set"

# ❌ BAD
echo $var
[ -n $var ] && echo "set"

Use Functions

process_file() {
    local file="$1"
    [[ -f "$file" ]] || return 1
    # Process file
}

Check Command Existence

command -v docker >/dev/null 2>&1 || die "docker is required"

Temporary Files

readonly TEMP_FILE="$(mktemp)"
trap 'rm -f "$TEMP_FILE"' EXIT

echo "data" > "$TEMP_FILE"

Lock Files

exec 200>"/tmp/${SCRIPT_NAME}.lock"
flock -n 200 || { echo "Already running"; exit 1; }

Performance Patterns

Avoid Subshells in Loops

# ❌ BAD - subshell, variables don't persist
count=0
cat file.txt | while read -r line; do
    count=$((count + 1))
done
echo "$count"  # Empty! (subshell issue)

# ✅ GOOD - process substitution
count=0
while read -r line; do
    count=$((count + 1))
done < file.txt
echo "$count"  # Correct

Use Arrays

# Arrays for multiple values
files=("file1.txt" "file2.txt" "file3.txt")
for file in "${files[@]}"; do
    process "$file"
done

# Append to array
files+=("file4.txt")

Argument Parsing

show_help() {
    cat <<EOF
Usage: $SCRIPT_NAME [OPTIONS] <file>

Options:
    -h, --help      Show this help
    -v, --verbose   Enable verbose mode
    -o, --output    Output file (default: stdout)
EOF
}

parse_args() {
    VERBOSE=false
    OUTPUT=""
    
    while [[ $# -gt 0 ]]; do
        case "$1" in
            -h|--help) show_help; exit 0 ;;
            -v|--verbose) VERBOSE=true; shift ;;
            -o|--output) OUTPUT="$2"; shift 2 ;;
            -*) die "Unknown option: $1" ;;
            *) break ;;
        esac
    done
    
    [[ $# -eq 0 ]] && die "Missing required argument"
    INPUT_FILE="$1"
}

Detailed References

• Shell Utilities: See references/shell-utilities.md for curl, jq, lynx
• Makefile Patterns: See references/makefile-patterns.md
• CLI Design: See references/cli-design.md

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