linux-bash-scripting

name	linux-bash-scripting
description	Defensive Bash scripting for Linux: safe foundations, argument parsing, production patterns, ShellCheck compliance. Use when writing "bash script", "shell script", "linux automation", "system script", "cron job", "deployment script", "CLI tool in bash", or "automate with bash".

Linux Bash Scripting

Target: GNU Bash 4.4+ on Linux. No macOS/BSD workarounds, no Windows paths, no POSIX-only restrictions.

Script Foundation

#!/usr/bin/env bash
set -Eeuo pipefail
shopt -s inherit_errexit

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

trap 'printf "Error at %s:%d\n" "${BASH_SOURCE[0]}" "$LINENO" >&2' ERR
trap 'rm -rf -- "${_tmpdir:-}"' EXIT

• -E propagates ERR traps into functions
• inherit_errexit propagates errexit into $() command substitutions
• Always create temp dirs under the EXIT trap: _tmpdir=$(mktemp -d)

Core Rules

• Quote every expansion: "$var", "$(cmd)", "${array[@]}"
• local for function variables, local -r for function constants, readonly for script constants
• printf '%s\n' over echo — predictable behavior, no flag interpretation
• [[ ]] for conditionals; (( )) for arithmetic; $() over backticks
• End options with --: rm -rf -- "$path", grep -- "$pattern" "$file"
• Require env vars: : "${VAR:?must be set}"
• Never eval user input; build commands as arrays: cmd=("grep" "--" "$pat" "$f"); "${cmd[@]}"
• Separate local from assignment to preserve exit codes: local val; val=$(cmd)

Safe Iteration

# NUL-delimited file processing
while IFS= read -r -d '' f; do
    process "$f"
done < <(find /path -type f -name '*.log' -print0)

# Array from command output
readarray -t lines < <(command)
readarray -d '' files < <(find . -print0)

# Glob with no-match guard
for f in *.txt; do [[ -e "$f" ]] || continue; process "$f"; done

Argument Parsing

verbose=false; output=""
while [[ $# -gt 0 ]]; do
    case "$1" in
        -v|--verbose) verbose=true; shift ;;
        -o|--output)  output="$2"; shift 2 ;;
        -h|--help)    usage; exit 0 ;;
        --)           shift; break ;;
        -*)           printf 'Unknown: %s\n' "$1" >&2; exit 1 ;;
        *)            break ;;
    esac
done

Production Patterns

Dependency check:

require() { command -v "$1" &>/dev/null || { printf 'Missing: %s\n' "$1" >&2; exit 1; }; }
require jq; require curl

Dry-run wrapper:

run() { if [[ "${DRY_RUN:-}" == "1" ]]; then printf '[dry] %s\n' "$*" >&2; else "$@"; fi; }
run cp "$src" "$dst"

Atomic file write — write to temp, rename into place:

atomic_write() { local tmp; tmp=$(mktemp); cat >"$tmp"; mv -- "$tmp" "$1"; }
generate_config | atomic_write /etc/app/config.yml

Retry with backoff:

retry() { local n=0 max=5 delay=1; until "$@"; do ((++n>=max)) && return 1; sleep $delay; ((delay*=2)); done; }
retry curl -fsSL "$url"

Script locking — prevent concurrent runs:

exec 9>/var/lock/"${0##*/}".lock
flock -n 9 || { printf 'Already running\n' >&2; exit 1; }

Idempotent operations — safe to rerun:

ensure_dir()  { [[ -d "$1" ]] || mkdir -p -- "$1"; }
ensure_link() { [[ -L "$2" ]] || ln -s -- "$1" "$2"; }

Logging

log() { printf '[%s] [%s] %s\n' "$(date -Iseconds)" "$1" "${*:2}" >&2; }
info()  { log INFO "$@"; }
warn()  { log WARN "$@"; }
error() { log ERROR "$@"; }
die()   { error "$@"; exit 1; }

Anti-Patterns

Bad	Fix
for f in $(ls)	for f in *; do or find -print0 | while read
local x=$(cmd)	local x; x=$(cmd) — preserves exit code
echo "$data"	printf '%s\n' "$data"
cat file | grep	grep pat file
kill -9 $pid first	kill "$pid" first, -9 as last resort
cd dir; cmd	`cd dir

Performance

• Parameter expansion over externals: ${path%/*} not dirname, ${path##*/} not basename, ${var//old/new} not sed
• (( )) over expr; [[ =~ ]] over echo | grep
• Cache results: val=$(cmd) once, reuse $val
• xargs -0 -P "$(nproc)" for parallel work
• declare -A map for lookups instead of repeated grep

Bash 4.4+ / 5.x

• ${var@Q} shell-quoted, ${var@U} uppercase, ${var@L} lowercase
• declare -n ref=varname nameref for indirect access
• wait -n wait for any background job
• $EPOCHSECONDS, $EPOCHREALTIME — timestamps without forking date

Linux-Specific

• /proc/self/status, /proc/cpuinfo, /proc/meminfo for system info
• systemctl for services; journalctl -u svc for logs
• GNU coreutils: sed -i (no ''), grep -P (PCRE), readlink -f
• timeout 30s cmd to prevent hangs
• flock for script locking (see above)
• Package install: apt-get install -y / dnf install -y / pacman -S --noconfirm

ShellCheck

Run shellcheck --enable=all script.sh. Key rules:

• SC2155: Separate declaration from assignment
• SC2086: Double-quote variables
• SC2046: Quote command substitutions
• SC2164: cd dir || exit
• SC2327/SC2328: Use ${BASH_REMATCH[n]} not $n for regex captures

Pre-commit: shellcheck *.sh && shfmt -i 2 -ci -d *.sh