====== Bash Recipes ======

Reusable patterns for writing shell scripts: boilerplate, flags, config,
logging, error handling, and testing.

===== Skeleton =====

<code bash>
#!/usr/bin/env bash
set -euo pipefail          # die on error, unset vars, pipe failures
IFS=$'\n\t'                # optional: safer word splitting
</code>

===== Arguments and flags =====

<code bash>
for arg in "$@"; do
  case $arg in
    -v|--verbose) VERBOSE=1 ;;
    -o|--output)  shift; OUT=$1 ;;
    *) echo "unknown: $arg" >&2; exit 1 ;;
  esac
  shift
done
</code>

''$0'' script name · ''$1..$9'' args · ''$@'' all args · ''$#'' count · ''$$'' PID ·
''$?'' last exit code · ''$!'' last background PID

Env-var tunables with defaults:

<code bash>
TURBO="${TURBO:-off}"        # ${VAR:-default}
die "needs TURBO"            # if unset: ${VAR:?message}
</code>

===== Conditionals =====

<code bash>
[[ -f file ]]   [[ -d dir ]]   [[ -z $str ]]   [[ -n $str ]]
[[ $a == $b ]]  [[ $a == foo* ]]  [[ $n -gt 3 ]]   # < -eq -ne -lt -le -ge
[[ $x =~ ^[0-9]+$ ]]             # regex match
command -v foo >/dev/null        # command exists
</code>

Files: ''-e'' exists, ''-r/-w/-x'' readable/writable/executable, ''-s'' non-empty,
''-nt''/''-ot'' newer/older.

===== Loops =====

<code bash>
for f in *.txt; do echo "$f"; done
for i in {1..10}; do ...; done
while IFS= read -r line; do ...; done < file
find . -type f -print0 | while IFS= read -r -d '' f; do ...; done   # safe
</code>

===== Functions / output / errors =====

<code bash>
die()  { printf 'Error: %s\n' "$*" >&2; exit 1; }
say()  { printf '==> %s\n' "$*"; }

trap 'echo "interrupted"; exit 130' INT
trap cleanup EXIT                # run on any exit
</code>

Return values: ''return 1'' sets ''$?''. Get output: ''result="$(myfunc)"''.

===== Strings =====

<code bash>
${s#prefix}  ${s%suffix}    # strip shortest
${s##*/}                    # basename
${s%/*}                     # dirname
${s/old/new}  ${s//o/n}     # replace first / all
${#s}                       # length
${s:2:5}                    # substring
${s^^} ${s,,}               # upper / lower
</code>

===== Arrays =====

<code bash>
a=(one two three)
echo "${a[1]}" "${a[@]}" "${#a[@]}"
a+=(four)
mapfile -t lines < file     # file -> array
</code>

===== Heredocs =====

<code bash>
cat <<EOF        # expands $vars
template for $USER
EOF
cat <<'EOF'      # literal, no expansion
no $expansion here
EOF
</code>

===== Debugging =====

<code bash>
bash -x script.sh           # trace every command
shellcheck script.sh        # linter: catches real bugs
set -x; ... ; set +x        # trace a section
</code>

===== Gotchas =====

  * <nowiki>[[</nowiki> not ''['': ''['' is a command with worse semantics.
  * Quote everything: ''"$var"'': unquoted vars split on spaces.
  * ''cd'' in a script doesn't affect your shell; use ''( cd dir && ... )'' subshells.
  * ''local'' only exists inside functions.
  * ''exit'' inside ''$(...)'' exits the subshell, not the script.
  * ''pipefail'' + ''set -o'' at top or ''foo | grep x'' returns 0 even when foo fails.

===== Reading input =====

<code bash>
read -rp "Name? " name
read -rsp "Password: " pw; echo   # -s silent
read -t 5 -n1 -p "continue? " || exit
</code>

===== Running as root =====

<code bash>
[[ $EUID -ne 0 ]] && exec sudo -E bash "$0" "$@"   # re-exec via sudo
</code>


===== See Also =====

  * [[terminal:scripting:start|Scripting Index]]
  * [[terminal:tools:start|CLI Tools]]
  * [[terminal:start|Return to terminal]]
  * [[start|Return to wiki home]]