swap

#!/usr/bin/env bash

######################################################################
# Swaps two files by renaming them with `mv`.                        #
######################################################################
# Usage:                                                             #
#   swap <file1> <file2>                                             #
######################################################################
# Exit Status Codes:                                                 #
#   0  Success                                                       #
#   1  Not enough arguments                                          #
#   2  First file does not exist                                     #
#   3  First file is a directory                                     #
#   4  Second file does not exist                                    #
#   5  Second file is a directory                                    #
#   6  Failed to move the first file to a temporary location         #
#   7  Failed to move the second file to the first file location     #
#   8  Failed to move the temporary file to the second file location #
######################################################################
# TODO: -v support

_swap() {
    if [ $# -lt 2 ]; then
        echo "ERROR: Must provide two files to swap. You specified $#." >&2
        return 1
    fi
    local file1="$1"
    local file2="$2"

    # Validate first file
    if [ ! -e "$file1" ]; then
        echo "ERROR: File '$file1' does not exist." >&2
        return 2
    elif [ -d "$file1" ]; then
        echo "ERROR: '$file1' is a directory." >&2
        return 3
    fi

    # Validate second file
    if [ ! -e "$file2" ]; then
        echo "ERROR: File '$file2' does not exist." >&2
        return 4
    elif [ -d "$file2" ]; then
        echo "ERROR: '$file2' is a directory." >&2
        return 5
    fi

    # Create a unique temporary filename for swapping
    local basename; basename="$(basename "$file1")"
    local temp_file="/tmp/temp-swap-$basename"
    while [ -e "$temp_file" ]; do
        temp_file="/tmp/temp-swap-$basename-$$-$RANDOM"
    done

    # Perform the swap using `mv`. Return distinct codes for each failure point.
    if ! mv "$file1" "$temp_file"; then # file1 -> temp
        echo "ERROR: Failed to move '$file1' to temporary file '$temp_file'." >&2
        return 6
    fi
    if ! mv "$file2" "$file1"; then # file2 -> file1
        echo "ERROR: Failed to move '$file2' to '$file1'." >&2
        # Move failed - Attempt to restore the original file
        if ! mv "$temp_file" "$file1"; then
            echo "ERROR: Failed to restore '$temp_file' to '$file1'." >&2
        fi
        return 7
    fi
    if ! mv "$temp_file" "$file2"; then # temp -> file2
        echo "ERROR: Failed to move temporary file to '$file2'." >&2
        return 8
    fi

    # 0 status code is implied, but we will be explicit
    return 0
}

_swap "$@"