Create a backup copy of a file, with a timestamp in its name

backup

#!/bin/sh

errecho() {
    >&2 printf '%s: %s\n' "%0" "$*"
}

if [ "$#" != 1 ] || [ -z "$1" ]
then
    errecho 'Requires a single filename as the argument'
    exit 1
fi

add_suffix_to_filename() {
    # because of course the output of dirname can't be blindly joined with slash and the basename
    DIRNAME=$(dirname -- "$1")
    if [ "$DIRNAME" = / ]
    then
        DIRNAME=
    fi

    FILENAME=$(basename -- "$1")
    # because of course extracting the filename's extension is not supported out of the box
    case "$FILENAME" in
        .*.* )
            EXTENSION=.${FILENAME#.*.}
            BASENAME=$(printf '%s' "$FILENAME" | cut -c-$((${#FILENAME} - ${#EXTENSION})) )
            ;;
        .* )
            EXTENSION=
            BASENAME=$FILENAME
            ;;
        *.[^.]* )
            BASENAME=${FILENAME%%.*}
            EXTENSION=${FILENAME#*.}
            ;;
        * )
            BASENAME=${FILENAME%%.*}
            EXTENSION=$(printf '%s' "$FILENAME" | cut -c$((${#BASENAME} + 1))- )
            ;;
    esac

    if [ "$FILENAME" != "$BASENAME$EXTENSION" ] || [ -z "$BASENAME" ]
    then
        errecho 'Failed to properly split the extension from the filename:' "$FILENAME"
        return 1
    fi

    printf '%s/%s%s%s' "$DIRNAME" "$BASENAME" "$2" "$EXTENSION"
}

ORIGINAL=$1
if [ "${ORIGINAL#./}" = "$ORIGINAL" ] && [ "${ORIGINAL#/}" = "$ORIGINAL" ]
then
    ORIGINAL=./"$ORIGINAL"
fi

TIMESTAMP=$(date --utc --date=@"$(stat --format %Y "$ORIGINAL")" +'_%Y-%m-%d_%H%M%S')
if [ -z "$TIMESTAMP" ]
then
    errecho 'Failed to get the timestamp of the file: ' "$ORIGINAL"
    exit 1
fi

NEW_FILENAME=$(add_suffix_to_filename "$ORIGINAL" "$TIMESTAMP")
if [ "$?" != 0 ]
then
    exit 1
fi

if [ "$NEW_FILENAME" = "$ORIGINAL" ]
then
    errecho 'Somehow the name for the backup is the same as the original file:' "$NEW_FILENAME"
    exit 1
fi

# this preserves the timestamps somewhat better
if ! mv --no-clobber --no-copy -- "$ORIGINAL" "$NEW_FILENAME"
then
    errecho 'Failed to backup the file: ' "$ORIGINAL" "$NEW_FILENAME"
    exit 1
fi

if ! cp -- "$NEW_FILENAME" "$ORIGINAL"
then
    errecho 'Failed to backup the file: ' "$ORIGINAL" "$NEW_FILENAME"
    mv --no-clobber --no-copy -- "$NEW_FILENAME" "$ORIGINAL"
fi