Shell Scripting

Shell Scripting

Core Principles

1. Fail Explicitly

A script that encounters an error should stop, not continue with corrupted state. Use defensive coding: validate assumptions, check exit codes, and never assume success.

2. Clarity Over Cleverness

Shell scripting is already cryptic enough. Write scripts that are easy to read, not impressive one-liners. Your future self will thank you.

3. Portability By Default

Unless you have a specific reason to require Bash 4+, write for POSIX sh. Your script may need to run in a container, an embedded system, or a legacy environment.

4. Defensive Safety

Every variable could be empty. Every command could fail. Every file could be missing. Write scripts that survive these realities.

5. Principle of Least Surprise

Scripts should behave predictably. Use consistent exit codes, clear error messages, and help text. No silent failures, no hidden side effects.

Scripting Maturity Model

Target at least Level 3 for any script that runs unattended.

Script Structure

Shebang and Set Flags

Every script begins with a shebang and safety flags:

#!/usr/bin/env bash
# ─────────────────────────────────────────────────────────────
# Script:     deploy.sh
# Author:     Your Name
# Description: Builds and deploys the application to staging
# Usage:      ./deploy.sh [--env staging|production] [--skip-tests]
# ─────────────────────────────────────────────────────────────

set -euo pipefail

# -e:  Exit immediately if any command exits with non-zero status
# -u:  Treat unset variables as an error (exit)
# -o pipefail: Pipeline's exit status is last non-zero command, or zero
# -x:  (debug) Print commands and their arguments as they execute

Note: set -e has edge cases — it won't catch failures in conditionals or in the left side of &&/||. Don't rely on it exclusively; also check exit codes explicitly where it matters.

Functions

Organize logic into reusable, named functions:

# ── Logging ──────────────────────────────────────────────────
log_info()  { echo "[INFO]  $*" >&2; }
log_warn()  { echo "[WARN]  $*" >&2; }
log_error() { echo "[ERROR] $*" >&2; }

# ── Core Functions ───────────────────────────────────────────
validate_environment() {
    local env="${1:-}"
    case "$env" in
        staging|production) return 0 ;;
        *) log_error "Invalid environment: $env"; return 1 ;;
    esac
}

build_application() {
    log_info "Starting build..."
    npm run build || return 1
    log_info "Build completed successfully"
}

deploy_application() {
    local env="$1"
    log_info "Deploying to $env..."
    # ... deployment logic ...
}

Function Rules:

• Use local for all variables inside functions to avoid global scope pollution
• Return meaningful exit codes (0 = success, 1 = general error, 2 = usage error)
• Name functions with verbs (validate_, build_, deploy_, cleanup_)
• Keep functions focused — one function, one responsibility

main() Guard

Always use a main function with a guard to control execution:

main() {
    local env="staging"
    local skip_tests=false

    # Parse arguments
    while [[ $# -gt 0 ]]; do
        case "$1" in
            --env) env="$2"; shift 2 ;;
            --skip-tests) skip_tests=true; shift ;;
            --help) show_help; exit 0 ;;
            *) log_error "Unknown option: $1"; show_help; exit 2 ;;
        esac
    done

    validate_environment "$env" || exit 1
    build_application || exit 1
    deploy_application "$env" || exit 1
}

# ── Entry Point ──────────────────────────────────────────────
if [[ "${BASH_SOURCE[0]}" == "${0}" ]]; then
    main "$@"
fi

The main guard allows the script to be sourced (for testing individual functions) without executing the main flow.

Exit Codes

Standard Unix exit codes:

Best practice: Use exit codes consistently. exit 1 for generic errors, more specific codes where useful.

Error Handling

set -euo pipefail (The Holy Trinity)

set -euo pipefail

# What each flag protects against:
# -e:  Prevents: "command fails but script continues merrily"
# -u:  Prevents: "typo in variable name leads to silent empty string"
# -o pipefail: Prevents: "grep fails but '| wc -l' returns 0, hiding the error"

Caveat: set -e is disabled inside conditionals (if, while, until) and on the left side of || or &&. This is intentional — it allows you to test command success. But be aware of it:

# This does NOT exit on error (because `command` is in a conditional)
if command_might_fail; then
    log_info "Command succeeded"
fi

# This does NOT exit on error (because `||` suppresses it)
command_might_fail || log_warn "Command failed, but continuing..."

trap for Cleanup

Use trap to guarantee cleanup when a script exits — whether normally, by error, or by signal:

# ── Cleanup actions ──────────────────────────────────────────
CLEANUP_FILES=()

cleanup() {
    local exit_code=$?
    log_info "Cleaning up..."
    for file in "${CLEANUP_FILES[@]}"; do
        [[ -f "$file" ]] && rm -f "$file"
    done
    exit "$exit_code"
}

trap cleanup EXIT  # Runs on any exit (success, error, signal)
trap 'exit' INT TERM  # Ensures cleanup() is called on Ctrl+C or SIGTERM

# Usage: register temp files for cleanup
create_temp_file() {
    local tmp
    tmp=$(mktemp) || exit 1
    CLEANUP_FILES+=("$tmp")
    echo "$tmp"
}

Multiple trap patterns:

# ── Trap reference ───────────────────────────────────────────
trap 'cleanup' EXIT           # Normal exit
trap 'exit 1' INT             # Ctrl+C — exit, which triggers EXIT trap
trap 'exit 1' TERM             # kill signal — exit, which triggers EXIT trap
trap 'emergency_cleanup; exit 1' ERR  # Runs on any error (with set -e)

# ── Emergency cleanup for partial writes ─────────────────────
emergency_cleanup() {
    log_error "Emergency cleanup triggered"
    # Remove partial output files, release locks, etc.
}

Error Functions

Dedicated error handling improves clarity:

# ── Fatal error: exits immediately ──────────────────────────
fatal() {
    log_error "$*"
    exit 1
}

# ── Non-fatal error: continues but warns ─────────────────────
warn() {
    log_warn "$*"
    return 1
}

# ── Assertion: checks a condition, fails if false ───────────
assert() {
    local condition="$1"
    local message="${2:-Assertion failed}"
    if ! eval "$condition"; then
        fatal "$message"
    fi
}

# Usage
assert '[[ -f "$CONFIG_FILE" ]]' "Config file not found: $CONFIG_FILE"
assert '[[ -n "${AWS_REGION:-}" ]]' "AWS_REGION is not set"

Portability

Bash vs POSIX sh

Rule of thumb: Start with #!/bin/sh unless you genuinely need Bash-specific features. If you need arrays or associative maps, use Bash but document the requirement.

OS Differences

# ── macOS vs Linux detection ─────────────────────────────────
detect_os() {
    case "$(uname -s)" in
        Darwin) echo "macos" ;;
        Linux)  echo "linux" ;;
        CYGWIN*|MINGW*|MSYS*) echo "windows" ;;
        *)      echo "unknown" ;;
    esac
}

OS=$(detect_os)

# ── sed differences ───────────────────────────────────────────
# macOS sed requires an argument for -i
sed_in_place() {
    local file="$1"
    local pattern="$2"
    if [[ "$OS" == "macos" ]]; then
        sed -i '' "$pattern" "$file"
    else
        sed -i "$pattern" "$file"
    fi
}

# ── date differences ──────────────────────────────────────────
# macOS: date -r <timestamp> -u, Linux: date -d @<timestamp> -u
format_timestamp() {
    local ts="$1"
    if [[ "$OS" == "macos" ]]; then
        date -r "$ts" -u '+%Y-%m-%dT%H:%M:%SZ'
    else
        date -d "@$ts" -u '+%Y-%m-%dT%H:%M:%SZ'
    fi
}

Checking for Command Availability

# ── Check if a command exists ────────────────────────────────
command_exists() {
    command -v "$1" >/dev/null 2>&1
}

# ── Validate required dependencies ───────────────────────────
check_dependencies() {
    local missing=()
    for cmd in "$@"; do
        if ! command_exists "$cmd"; then
            missing+=("$cmd")
        fi
    done

    if [[ ${#missing[@]} -gt 0 ]]; then
        fatal "Missing required commands: ${missing[*]}"
    fi
}

# Usage
check_dependencies "jq" "curl" "aws" "docker"

# ── Optional dependency with fallback ────────────────────────
if command_exists "jq"; then
    JSON_FMT="jq"
else
    log_warn "jq not found, falling back to grep-based parsing (fragile)"
    JSON_FMT="grep"
fi

Best Practices

Quoting Variables

# ❌ BAD — unquoted variables
file_path=$HOME/dir/$filename    # Breaks on spaces/special chars
if [ $status = "ok" ]; then      # Syntax error if $status is empty

# ✅ GOOD — always quote
file_path="$HOME/dir/$filename"
if [ "$status" = "ok" ]; then

# ✅ Quote even in [[ ]]
if [[ "$name" == "$pattern" ]]; then

Golden Rule: If a variable contains user input, a filename, or any path, quote it. When in doubt, quote it. Unquoted variables are a leading cause of shell script bugs.

Using [[ ]] Over [ ]

# ❌ BAD: [ ] — older, more error-prone
if [ "$var" = "value" ] && [ -f "$file" ]; then

# ✅ GOOD: [[ ]] — safer, more features
if [[ "$var" == "value" && -f "$file" ]]; then

# [[ ]] advantages:
# - No word splitting or glob expansion inside
# - Supports pattern matching (== with glob, =~ with regex)
# - Supports && and || inside (no need for -a and -o)
# - Can safely handle empty variables without extra quoting

Avoiding ls Parsing

# ❌ BAD: Don't parse ls output
for file in $(ls *.txt); do       # Breaks on spaces, newlines, special chars
    process "$file"
done

# ✅ GOOD: Use globs
for file in *.txt; do
    [[ -f "$file" ]] && process "$file"
done

# ✅ GOOD: Use find for recursive operations
find /path -name "*.txt" -type f -print0 | while IFS= read -r -d '' file; do
    process "$file"
done

Temporary File Handling

# ❌ BAD: Unsafe temp file
tempfile="/tmp/myscript.tmp"      # Predictable name — security risk
echo "$data" > "$tempfile"

# ✅ GOOD: Use mktemp
create_temp_dir() {
    local tmpdir
    tmpdir=$(mktemp -d) || fatal "Failed to create temp directory"
    CLEANUP_FILES+=("$tmpdir")
    echo "$tmpdir"
}

TMPDIR=$(create_temp_dir)
TMPFILE="$TMPDIR/data.txt"

# Always clean up via trap (see Error Handling section)

Argument Parsing

Using getopts

getopts is the POSIX-compliant way to parse short options:

#!/usr/bin/env bash

usage() {
    cat <<EOF
Usage: $(basename "$0") [OPTIONS] <input-file>

Options:
  -e <env>    Target environment (staging|production)
  -v          Verbose output
  -n          Dry run (no actual changes)
  -h          Show this help message

Examples:
  $(basename "$0") -e staging data.csv
  $(basename "$0") -v -n data.csv
EOF
    exit 0
}

# ── Parse arguments ──────────────────────────────────────────
env="staging"
verbose=false
dry_run=false

while getopts "e:vn h" opt; do
    case "$opt" in
        e) env="$OPTARG" ;;
        v) verbose=true ;;
        n) dry_run=true ;;
        h) usage ;;
        ?) echo "Invalid option: -$OPTARG" >&2; usage; exit 2 ;;
    esac
done
shift $((OPTIND - 1))

# Positional arguments
if [[ $# -lt 1 ]]; then
    echo "Error: Missing input file" >&2
    usage
    exit 2
fi
INPUT_FILE="$1"

Manual Parsing with shift (for long options)

# ── Parse long and short options manually ────────────────────
parse_args() {
    while [[ $# -gt 0 ]]; do
        case "$1" in
            --env|-e)
                env="${2:?Error: --env requires an argument}"
                shift 2
                ;;
            --verbose|-v)
                verbose=true
                shift
                ;;
            --dry-run|-n)
                dry_run=true
                shift
                ;;
            --config|-c)
                config_file="${2:?Error: --config requires an argument}"
                shift 2
                ;;
            --help|-h)
                show_help
                exit 0
                ;;
            --)
                shift
                positional+=("$@")
                break
                ;;
            -*)
                log_error "Unknown option: $1"
                show_help
                exit 2
                ;;
            *)
                positional+=("$1")
                shift
                ;;
        esac
    done
}

Help Text Best Practices

show_help() {
    cat <<EOF
${SCRIPT_NAME:-$(basename "$0")} — ${SCRIPT_DESCRIPTION:-"No description"}

USAGE:
  $(basename "$0") [OPTIONS] <input> [<input>...]

OPTIONS:
  -e, --env ENV       Target environment (default: staging)
  -v, --verbose       Enable verbose output
  -n, --dry-run       Show what would be done without doing it
  -c, --config FILE   Path to config file
  -h, --help          Show this help message

ARGUMENTS:
  <input>             One or more input files or directories

EXAMPLES:
  $(basename "$0") -e production data.csv
  $(basename "$0") --verbose --dry-run ./config.json

EXIT CODES:
  0  Success
  1  General error
  2  Usage error (invalid options or arguments)

EOF
}

Debugging

set -x (Execution Trace)

# ── Enable debug mode for a specific section ─────────────────
debug_section() {
    set -x  # Print commands and their arguments as they execute
    # ... debug this section ...
    set +x  # Disable debug mode
}

# ── Conditional debug mode ───────────────────────────────────
if [[ "$verbose" == "true" ]]; then
    set -x
fi

shellcheck

ShellCheck is the single most important tool for writing safe shell scripts:

# Install shellcheck
# macOS: brew install shellcheck
# Linux: apt install shellcheck / yum install shellcheck

# Run shellcheck on your scripts
shellcheck script.sh

# Example output:
# In script.sh line 42:
#     if [ $status = "ok" ]
#          ^—————^ SC2086: Double quote to prevent globbing and word splitting.

# Fix warnings before running scripts in production

Common ShellCheck warnings and fixes:

bash -n Syntax Checking

# ── Check syntax without executing ───────────────────────────
bash -n script.sh
# Returns silently if syntax is valid
# Outputs errors and exits non-zero if syntax is broken

# ── Integrate into CI pipeline ───────────────────────────────
ci_check() {
    local errors=0
    for script in scripts/*.sh; do
        if ! bash -n "$script"; then
            log_error "Syntax error in: $script"
            ((errors++))
        fi
    done
    return "$errors"
}

Runtime Debugging Techniques

# ── Print variable values for debugging ──────────────────────
debug() {
    [[ "$verbose" == "true" ]] && echo "[DEBUG] $*" >&2
}

debug "VARIABLES: env=$env, file=$INPUT_FILE, mode=$mode"

# ── Trace execution with timestamps ──────────────────────────
trace() {
    echo "[$(date '+%H:%M:%S.%3N')] $*" >&2
}

trace "Starting deployment to $env"

# ── Trap for unexpected errors ───────────────────────────────
error_trap() {
    local line=$1
    local command=$2
    local code=$3
    log_error "Error on line $line: '$command' exited with code $code"
}
trap 'error_trap $LINENO "$BASH_COMMAND" $?' ERR

Performance

Avoiding Subshells

# ❌ BAD: Subshells are expensive
result=$(cat file.txt | grep "pattern" | head -1)

# ✅ GOOD: Use built-ins and redirects
result=$(grep "pattern" file.txt | head -1)
# Or even better (avoiding the pipe entirely):
while IFS= read -r line; do
    [[ "$line" == *"pattern"* ]] && { result="$line"; break; }
done < file.txt

Minimizing Pipes

# ❌ BAD: Three pipes where one will do
cat data.log | grep "ERROR" | cut -d' ' -f2 | sort | uniq

# ✅ GOOD: Use awk to handle multiple operations
awk '/ERROR/ {print $2}' data.log | sort -u

Using Built-ins Over External Commands

# ❌ BAD: External commands are slow (fork + exec)
[ "$(echo "$var" | tr '[:upper:]' '[:lower:]')" = "yes" ]

# ✅ GOOD: Bash built-in parameter expansion
[[ "${var,,}" == "yes" ]]  # lowercase conversion (bash 4+)

# ❌ BAD: External grep for simple pattern match
if echo "$line" | grep -q "pattern"; then

# ✅ GOOD: Bash built-in pattern matching
if [[ "$line" == *"pattern"* ]]; then

Bulk Operations

# ❌ BAD: One call per file
for file in *.txt; do
    mv "$file" "${file%.txt}.md"
done

# ✅ GOOD: Use a tool that handles batches
# For thousands of files, use rename (Perl-based)
rename 's/\.txt$/.md/' *.txt

# Or process in bulk with find -exec
find . -name "*.txt" -exec sh -c 'mv "$1" "${1%.txt}.md"' _ {} \;

Common Mistakes

1. Forgetting to Quote Variables

# ❌ BAD
if [ $status = ok ]; then   # Breaks if $status empty or has spaces

# ✅ GOOD
if [[ "$status" == "ok" ]]; then

2. Missing Error Handling on cd

# ❌ BAD — if cd fails, script continues in wrong directory
cd /some/directory
rm -rf ./*

# ✅ GOOD — exit if cd fails
cd /some/directory || fatal "Failed to change to /some/directory"

3. Unsafe Temporary Files

# ❌ BAD — predictable name, race condition, no cleanup
echo "$data" > /tmp/output.txt

# ✅ GOOD
tmpfile=$(mktemp) || fatal "Failed to create temp file"
trap 'rm -f "$tmpfile"' EXIT
echo "$data" > "$tmpfile"

4. Parsing ls Output

# ❌ BAD — breaks on spaces, newlines, special characters
for file in $(ls *.txt); do

# ✅ GOOD
for file in *.txt; do
    [[ -f "$file" ]] || continue
    process "$file"
done

5. Not Using set -euo pipefail

# ❌ BAD — script continues after errors
#!/bin/bash
echo "Starting..."
some_command_that_fails  # Script continues as if nothing happened
echo "Done!"              # This runs even though the above failed

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

6. Useless Use of cat

# ❌ BAD — unnecessary fork
cat file.txt | grep "pattern"

# ✅ GOOD — direct input redirection
grep "pattern" file.txt

# Even better — tell the useless cat award:
< file.txt grep "pattern"

7. Forgetting to Handle Errors in Pipelines

# ❌ BAD — only exit code of last command matters
cmd_that_fails | cmd_that_succeeds
# $? is 0 (success), hiding the first command's failure

# ✅ GOOD
set -o pipefail  # Pipeline fails if ANY command fails

# Or check individually
if ! cmd_that_fails | cmd_that_succeeds; then
    log_error "Pipeline failed"
fi

8. Incorrect String Comparisons

# ❌ BAD — uses numeric comparison instead of string
if [ "$var" -eq 10 ]; then   # -eq is for integers only

# ❌ BAD — missing spaces around operator
if ["$var" = "value"]; then  # Syntax error

# ✅ GOOD
if [[ "$var" == "value" ]]; then
if [ "$var" = "value" ]; then  # POSIX compatible

9. Zeroing In on the Wrong Problem

# ❌ BAD — using eval unnecessarily (security risk)
eval "echo \$$var"  # Arbitrary code execution

# ✅ GOOD — use variable indirection properly
echo "${!var}"  # Indirect reference — safe

10. No Help or Usage Text

# ❌ BAD — user has no idea how to use the script
# No comments, no help, nothing

# ✅ GOOD — always include a usage function
# Run ./script.sh --help or ./script.sh -h for instructions

11. Modifying IFS Without Saving/Restoring

# ❌ BAD — changes persist and break everything after
IFS=',' read -ra fields <<< "$csv_line"
# Now IFS is permanently changed!

# ✅ GOOD — save and restore
old_ifs="$IFS"
IFS=','
read -ra fields <<< "$csv_line"
IFS="$old_ifs"

12. Not Using printf for Reliable Output

# ❌ BAD — echo has inconsistent behavior across shells
echo "Hello\nWorld"  # On some systems: prints literal \n

# ✅ GOOD — printf is predictable everywhere
printf 'Hello\nWorld\n'
printf '%s\n' "$var"  # Safe way to print variable contents