wrap

#!/bin/sh

set -eu

die() {
    echo "$(basename "$0"): $*"
    exit 1
}

show_usage() {
    cat <<EOF
Usage: $(basename "$0") INPUT [OUTPUT]

Creates self-contained spawn launchers.

Attaches the contents of the spawn script to the end of the provided input file
and defines the "spawn" shell function which, unless the 'spawn' is found in
PATH, extracts the attachment and runs it instead.

EOF
}

main() {
    local in_file="${1-}" out_file="${2-}"
    if [ -z "$in_file" ]; then
        show_usage; exit 0
    fi
    case $in_file in
        -h|--help)
            show_usage; exit 0 ;;
    esac
    local own_dir extractor spawn shebang
    [ -f "$in_file" ] || \
        die "could not find input file: $in_file"
    if [ -z "$out_file" ]; then
        out_file="${in_file}.wrapped"
    fi
    own_dir=$(dirname "$0")
    extractor="$own_dir/extractor"
    spawn="$own_dir/spawn"
    [ -f "$extractor" ] || \
        die "could not find extractor: $extractor"
    [ -f "$spawn" ] || \
        die "could not find spawn: $spawn"
    shebang=$(head -1 "$in_file")
    if [ "${shebang#\#!}" != "$shebang" ]; then
        echo "$shebang" | cat - "$extractor" > "$out_file"
    else
        cat "$extractor" > "$out_file"
    fi
    cat "$in_file" >> "$out_file"
    echo 'exit $?' >> "$out_file"
    echo "__SPAWN__" | cat - "$spawn" >> "$out_file"
    chmod "$(stat -c%a "$in_file")" "$out_file"
}

main "$@"