prepend

#!/usr/bin/env bash
# author: andreasl

show_help() {
    script_name="${0##*/}"
    msg="${script_name}\n"
    msg+="Prepend a given file with a given string from stdin.\n"
    msg+="\n"
    msg+="Usage:\n"
    msg+="  ${script_name} [OPTIONS] <file> -o <output-file> <<< <input-string>\n"
    msg+="  echo <input-string> | ${script_name} <file> -o <output-file>\n"
    msg+="\n"
    msg+="Options:\n"
    msg+="  -h, --help:                     Print the help message.\n"
    msg+="  -i, --inline:                   Write output to same file as input file.\n"
    msg+="  -o <file>, --output <file>:     Write output to specified output file.\n"
    msg+="\n"
    msg+="Examples:\n"
    msg+="  ${script_name} -i script.py <<< '#!/usr/env/bin python3\\\n# -*- coding: utf-8 -*-'\n"
    msg+="  printf 'Step 0: ' | ${script_name} myfile.txt -o otherfile.txt\n"
    printf "$msg"
}

inline=false
while [ "$#" -gt 0 ]; do
    case "$1" in
    -i | --inline)
        inline=true
        ;;
    -o | --output)
        output_file="$2"
        shift # past argument
        ;;
    -h | --help)
        show_help
        exit 0
        ;;
    *) # unknown option
        input_file="$1"
        ;;
    esac
    shift # past argument or value
done
if [ -z "$input_file" ]; then
    printf "Error: No input file specified.\n"
    exit 1
fi
if [ "$inline" == true ]; then
    output_file="$input_file"
fi
if [ -z "$output_file" ]; then
    printf "Error: No output file specified.\n"
    exit 1
fi

# read all of stdin at once; the sentinel `x` preserves trailing newlines.
# `read -N` would be shorter but needs Bash 4.1+, which macOS does not ship.
input_to_prepend="$(
    cat
    printf x
)"
input_to_prepend="${input_to_prepend%x}"

file_content="$(
    cat "$input_file"
    printf x
)"
file_content="${file_content%x}"

# `%b` expands escapes like `\n` in the prepended string, so we can have e.g. newlines in the
# prepended input.
# The file's own content goes through `%s`, so that nothing in it is interpreted at all.
printf '%b' "$input_to_prepend" >"$output_file"
printf '%s' "$file_content" >>"$output_file"