Heredocs and Script Arguments

Heredocs — embedded multi-line strings

cat <<EOF
Hello, $USER
Today is $(date +%Y-%m-%d)
EOF

Anything between the opening token (here EOF) and the closing one becomes stdin to the command. Variables and command substitutions expand by default.

Disable expansion: quote the delimiter

cat <<'EOF'
$USER stays literal
$(date) stays literal
EOF

Quoting any character of the opening delimiter (typically the whole 'EOF') disables variable expansion. Use this for embedded config, scripts, or templates that contain shell-like syntax you don't want expanded.

Indent a heredoc with <<-

cat <<-EOF
	hello
	world
	EOF

<<- strips leading tabs (not spaces) from each line — handy for keeping heredocs indented inside if blocks.

Script arguments

• $0 — script name.
• $1 $2 ... — positional args.
• $# — number of args.
• $@ — all args. Always quote: "$@".
• $* — args joined by IFS — usually wrong.
• shift — drop $1, renumber.

Parsing flags with getopts

verbose=0
while getopts ':vo:' opt; do
  case "$opt" in
    v) verbose=1 ;;
    o) output="$OPTARG" ;;
    \?) echo "bad flag: -$OPTARG" >&2; exit 2 ;;
  esac
done
shift $((OPTIND - 1))

POSIX getopts handles short flags only. For long flags, either parse them by hand or use bash's getopt variant or switch to Python's argparse. Most scripts get away with short-only.

Preserve argument and expansion boundaries

Forward caller arguments with "$@", not an unquoted aggregate. Choose quoted or unquoted heredoc delimiters deliberately: an unquoted delimiter expands substitutions while the shell constructs stdin; a quoted delimiter preserves the body literally.

The boundary: <<- never strips spaces

The dash form can indent a heredoc with tab characters; it does not remove spaces. Make invisible indentation deliberate.

A heredoc is not a safe template engine

An unquoted delimiter expands variables, commands, and arithmetic while the shell constructs stdin. Quote the delimiter for literal data and use a real template or structured serializer when substitution rules become complex.