#!/bin/bash
# bak - move file to file.bak, rotating any existing backup chain
#
# Usage:
#   bak [options] [--] [file ...]
#
# Options:
#   -v, --verbose  Print filenames as they are backed up
#   -n, --dry-run  Simulate actions without making changes
#   -h, --help     Show help message
#
# Backs up files by moving (renaming) them from "file.ext"->"file.ext.bak",
# "file.ext.bak"->"file.ext.bak.bak", etc. which ensures the file with a single
# ".bak" is always the most recent version

_bak() (
    local SCRIPT_NAME; SCRIPT_NAME="$(basename "${BASH_SOURCE[0]}")"
    case "${BASH_SOURCE[0]}" in /dev/*|/proc/*) SCRIPT_NAME="" ;; esac
    case "$SCRIPT_NAME" in ""|bash|sh|zsh|dash) SCRIPT_NAME="bak" ;; esac
    local args=() # Positional args
    local verbose=false
    local dry_run=false

    # Print usage (help) info
    _show_help() {
        # Underline only if output is a terminal (not a pipe or file)
        local s; [ -t 1 ] && s=$'\033[4m'
        local r; [ -t 1 ] && r=$'\033[24m'
        echo "NAME"
        echo "  $SCRIPT_NAME - move ${s}file${r} to ${s}file.bak${r}"
        echo "SYNOPSIS"
        echo "  $SCRIPT_NAME [-v | -n] [--] [${s}file${r} ${s}...${r}]"
        echo "  $SCRIPT_NAME -h"
        echo "OPTIONS"
        echo "  -v, --verbose      Print filenames as they are backed up"
        echo "  -n, --dry-run      Simulate actions without making changes"
        echo "  -h, --help         Show this help message"
        echo "EXIT STATUS"
        echo "  0  Success"
        echo "  1  Runtime failure (source file missing, mv failed)"
        echo "  2  Usage error (unknown option)"
    }
    _error() { echo "[ERR][$SCRIPT_NAME] $*" >&2; }

    # True if verbose flag is set
    _verbose() { [ "$verbose" = true ]; }

    # True if dry-run flag is set
    _dry_run() { [ "$dry_run" = true ]; }

    # Recursive backup function which ultimately calls mv
    _backup() {
        local file="$1"

        # Helper function to handle dry-run and verbose output
        _move_file() {
            local src="$1"
            local dest="$2"
            if [ ! -e "$src" ]; then
                _error "$src: No such file"
                return 1
            fi
            if _dry_run; then
                echo "$SCRIPT_NAME: Would move: $src -> $dest"
            else
                if _verbose; then
                    echo "$SCRIPT_NAME: Moving $src -> $dest"
                fi
                mv "$src" "$dest"
            fi
        }

        # If neither the file nor its backup exists, error out
        if [ ! -e "$file" ] && [ ! -e "$file.bak" ]; then
            _error "$file: No such file"
            return 1
        fi

        # If a backup already exists, recursively rotate it
        if [ -e "$file.bak" ]; then
            _backup "$file.bak" || return 1
        fi

        # Move the original file to its backup
        _move_file "$file" "$file.bak" || return 1
    }

    _expand_short_opts() {
        # $1 = string of short-opt letters that take a value (e.g. "nXHd"); "" for flag-only scripts
        # $2..$N = "$@"
        # Populates _EXPANDED; caller does: set -- "${_EXPANDED[@]}"; unset _EXPANDED
        local value_opts="$1"; shift
        _EXPANDED=()
        local passthru=""
        local arg
        local rest
        local c
        for arg in "$@"; do
            if [ -n "$passthru" ]; then _EXPANDED+=("$arg"); continue; fi
            case "$arg" in
                --)          passthru=1; _EXPANDED+=("$arg") ;;
                --*|-|"")    _EXPANDED+=("$arg") ;;
                -[a-zA-Z]?*)
                    rest="${arg#-}"
                    while [ -n "$rest" ]; do
                        c="${rest%"${rest#?}"}"; rest="${rest#?}"
                        _EXPANDED+=("-$c")
                        case "$value_opts" in *"$c"*)
                            [ -n "$rest" ] && _EXPANDED+=("$rest")
                            rest="" ;;
                        esac
                    done ;;
                *)        _EXPANDED+=("$arg") ;;
            esac
        done
    }

    # Clean up inner functions when returning
    _expand_short_opts "" "$@"
    set -- "${_EXPANDED[@]}"; unset _EXPANDED

    # Parse parameters
    while [ $# -gt 0 ]; do
        case "$1" in
            -v|--verbose)
                verbose=true
                shift
                ;;
            -n|--dry-run)
                dry_run=true
                shift
                ;;
            -h|--help)
                _show_help
                return 0
                ;;
            # End of options, parse remainder as positional
            --)
                shift # discard --
                while [ $# -gt 0 ]; do
                    args+=("$1")
                    shift
                done
                ;;
            # Unknown option
            -*)
                _error "Unknown argument '$1'. Run \`$SCRIPT_NAME -h\` for usage"
                return 2
                ;;
            # Positional argument
            *)
                args+=("$1")
                shift
                ;;
        esac
    done

    # Show help if no filenames provided
    if [ ${#args[@]} -eq 0 ]; then
        _show_help
        return 0
    fi

    # Process each argument
    for f in "${args[@]}"; do
        _backup "$f"
    done
)

_bak "$@"
__bak_rc=$?
unset -f _bak
if [ -n "${BASH_SOURCE[0]}" ] && [ "${BASH_SOURCE[0]}" != "$0" ]; then
    eval "unset __bak_rc; return $__bak_rc"
fi
eval "unset __bak_rc; exit $__bak_rc"