Bash

#!/usr/bin/env bash
set -Eeuo pipefail
IFS=$'\n\t'

main() {
  echo "hello ${1:-world}"
}

main "$@"

files=(a.txt "b c.txt" c.txt)
files+=(d.txt)
echo "${files[0]}"        # first element
echo "${files[@]}"        # all elements (quote it)
echo "${#files[@]}"       # length
echo "${files[@]:1:2}"    # slice: offset 1, length 2
for f in "${files[@]}"; do echo "$f"; done

mapfile -t lines < <(grep -v '^#' list.txt)   # file/command to array

declare -A ports=([web]=80 [ssh]=22)          # associative (Bash 4+)
ports[db]=5432
echo "${ports[web]}"
echo "${!ports[@]}"       # keys
echo "${ports[@]}"        # values
[[ -v ports[ssh] ]] && echo "key exists"
unset 'ports[web]'
for k in "${!ports[@]}"; do echo "$k=${ports[$k]}"; done

if [[ -f "$file" && -r "$file" ]]; then
  echo "readable file"
elif [[ -d "$file" ]]; then
  echo "directory"
else
  echo "missing"
fi

case "$1" in
  start|up)  echo "starting" ;;
  stop)      echo "stopping" ;;
  *)         echo "usage: $0 {start|stop}" >&2; exit 2 ;;
esac

[[ "$a" == prefix* ]]        # glob match (unquoted right side)
[[ "$a" =~ ^[0-9]+$ ]]       # regex match, groups in BASH_REMATCH
(( count > 3 )) && echo big  # arithmetic test

for i in 1 2 3; do echo "$i"; done
for i in {1..10..2}; do echo "$i"; done          # 1 3 5 7 9
for ((i = 0; i < 5; i++)); do echo "$i"; done
for f in *.log; do gzip "$f"; done

while [[ $n -lt 5 ]]; do ((n++)); done
until ping -c1 -W1 host >/dev/null; do sleep 2; done

while IFS= read -r line; do echo "$line"; done < file.txt
break; continue          # leave the loop / skip to the next iteration

greet() {
  local name="${1:?name required}"
  echo "Hello, $name"
  return 0                 # return codes only; output goes to stdout
}

greeting="$(greet Bob)"    # capture stdout
greet Bob || echo "failed"

cleanup() { rm -rf -- "$tmpdir"; }
trap cleanup EXIT                         # runs on any exit
trap 'echo "line $LINENO failed" >&2' ERR # with set -E
trap 'echo interrupted; exit 130' INT TERM
trap - EXIT                               # remove a trap
trap -l                                   # list signals

usage() { echo "Usage: $0 [-v] [-n NAME] FILE..." >&2; exit 2; }

verbose=0 name=""
while getopts ":vn:h" opt; do
  case "$opt" in
    v) verbose=1 ;;
    n) name="$OPTARG" ;;
    h) usage ;;
    :) echo "-$OPTARG needs an argument" >&2; usage ;;
    \?) echo "unknown option -$OPTARG" >&2; usage ;;
  esac
done
shift $((OPTIND - 1))     # remaining args are the positional ones

cat <<EOF                 # variables expand
Hello $USER
EOF

cat <<'EOF'               # quoted delimiter: no expansion
Literal $USER
EOF

cat <<-EOF                # <<- strips leading TABs (not spaces)
	indented
	EOF

grep -c foo <<< "$text"   # here-string
ssh host bash -s <<'EOF'  # run a script remotely
uptime
EOF

if ! grep -q foo file; then echo "no match"; fi
cmd; rc=$?                       # capture the code
echo "${PIPESTATUS[@]}"          # exit code of each pipeline stage
exit 3                           # your own code