CLI to prepend a string to a file (compatible with POSIX shell, Bash or Dash)

prepend

#!/bin/sh

set -eu

if [ $# -eq 0 ] || [ "$1" = -h ] || [ "$1" = --help ]
then
  cat <<EOF
Usage: $0 file prefix

Add a prefix to a file.

No new lines are added, either after the prefix or at the end of the file.
EOF
  exit
fi

# Read the file without introducing or removing new lines at the end
content=$(cat "$1"; echo _)
content=${content%_}

printf '%s' "$2" "$content" > "$1"