Bash Shell Scripting Cheatsheet: Variables, Pipes and More

#!/usr/bin/env bash
#
# Always start scripts with safe defaults — the "unofficial strict mode"

set -euo pipefail              # -e fail on error, -u no unset vars, -o pipefail propagate pipe errors
IFS=$'\n\t'                    # safer word splitting (no spaces)

# Cleanup runs even on failure
tmpdir=$(mktemp -d)
cleanup() { rm -rf "$tmpdir"; }
trap cleanup EXIT
trap 'echo "interrupted"; exit 130' INT TERM

# Require an argument
if (( $# < 1 )); then
    echo "usage: $0 " >&2
    exit 64                    # EX_USAGE
fi

name=$1
echo "working in $tmpdir for $name"

# Capture stderr separately
if ! out=$(command_that_might_fail 2> >(tee /tmp/err.log >&2)); then
    echo "failed; see /tmp/err.log" >&2
    exit 1
fi

echo "done: $out"

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

# ── Parameter expansion — Bash's swiss army knife
path="/home/abhi/notes/draft.md"

echo "${path}"                  # /home/abhi/notes/draft.md
echo "${path##*/}"              # draft.md          (strip longest prefix matching */)
echo "${path%/*}"               # /home/abhi/notes  (strip shortest suffix matching /*)
echo "${path%.*}.bak"           # /home/abhi/notes/draft.bak
echo "${path//notes/papers}"    # /home/abhi/papers/draft.md (all replacements)
echo "${path/notes/papers}"     # first replacement only
echo "${path:6:4}"              # substring: start 6, length 4

# ── Defaults & required
echo "${NAME:-anon}"            # default if unset or empty
echo "${NAME:=anon}"            # assign default if unset
: "${REQUIRED:?must be set}"    # error + exit if unset

# ── Length
echo "${#path}"                 # 28

# ── Indirect ref
key=path
echo "${!key}"                  # value of $path

# ── Case
echo "${path^^}"                # uppercase
echo "${path,,}"                # lowercase

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

# Indexed arrays
fruits=(apple banana cherry "dragon fruit")
echo "${fruits[0]}"           # apple
echo "${fruits[@]}"           # all elements
echo "${#fruits[@]}"          # length
echo "${fruits[@]:1:2}"       # slice: banana cherry
fruits+=(elderberry)          # append

# Iterate — quote @ to preserve spaces in elements
for f in "${fruits[@]}"; do
    echo "- $f"
done

# Read lines into an array
mapfile -t lines < input.txt          # one line per element, no trailing \n
# alt: readarray -t lines < input.txt  (same thing)

# Associative arrays (Bash 4+) — declare with -A
declare -A ages=([ada]=36 [grace]=85 [linus]=54)
ages[turing]=41                       # add a key
echo "${ages[ada]}"                   # 36
for name in "${!ages[@]}"; do         # keys
    echo "$name → ${ages[$name]}"
done

# Test membership
[[ -v ages[ada] ]] && echo "found ada"

#!/usr/bin/env bash
#
# Backup all *.md files in a directory to a tar.gz with timestamped name.
# Logs progress, retries on transient failure, cleans up on exit.

set -euo pipefail
IFS=$'\n\t'

usage() {
    cat <<-EOF
		usage: $(basename "$0") [-v]  
		  -v   verbose (print each file)
	EOF
    exit 64
}

verbose=0
while getopts ":v" opt; do
    case "$opt" in
        v) verbose=1 ;;
        *) usage ;;
    esac
done
shift $((OPTIND - 1))
(( $# == 2 )) || usage

src=$1
dest=$2

[[ -d $src  ]] || { echo "no such dir: $src"  >&2; exit 66; }
mkdir -p "$dest"

timestamp=$(date +%Y%m%d-%H%M%S)
archive="$dest/notes-$timestamp.tar.gz"
tmpdir=$(mktemp -d)
trap 'rm -rf "$tmpdir"' EXIT

# Collect markdown files null-safely
mapfile -d '' -t files < <(find "$src" -type f -name '*.md' -print0)
(( ${#files[@]} > 0 )) || { echo "no markdown files in $src" >&2; exit 0; }

(( verbose )) && printf 'including: %s\n' "${files[@]}"

# Retry the tar up to 3 times
for try in 1 2 3; do
    if tar -czf "$archive" -C "$src" "${files[@]##$src/}"; then
        echo "wrote $archive ($(du -h "$archive" | cut -f1))"
        exit 0
    fi
    echo "attempt $try failed, retrying..." >&2
    sleep $((try * 2))
done

echo "failed after 3 attempts" >&2
exit 1