Caja-Actions script to compare files or directories by using Meld in Caja, should be placed to `~/.config/caja/scripts`, was found sometime ago on the Deutsch forum

meld

#!/bin/bash
#
#   meld script for caja, place it to ~/.config/caja/scripts
#
#   Copyright 2014 Norbert ..... <norbert@Entwicklung>
#
#   This program is free software; you can redistribute it and/or modify
#   it under the terms of the GNU General Public License as published by
#   the Free Software Foundation; either version 2 of the License, or
#   (at your option) any later version.
#
#   This program is distributed in the hope that it will be useful,
#   but WITHOUT ANY WARRANTY; without even the implied warranty of
#   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
#   GNU General Public License for more details.
#
#   You should have received a copy of the GNU General Public License
#   along with this program; if not, write to the Free Software
#   Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston,
#   MA 02110-1301, USA.
#

function ScanSelectedEntries() {

    # Scan selected entries in primary pane

    let idx=0
    PrepArray=$(
        echo -e "${CAJA_SCRIPT_SELECTED_FILE_PATHS}" | while read line; do
            if [ ${#line} -gt 0 ]; then
                echo Files1[$idx]="'$line'"
                let idx=$idx+1
            fi
        done
    )
    eval "$PrepArray"
    let numFiles1=${#Files1[@]}

    for((i = 0; i < $numFiles1; i++)); do
        Type1[$i]="-"
        [ -f "${Files1[$i]}" ] && Type1[$i]="f"
        [ -d "${Files1[$i]}" ] && Type1[$i]="d"
        [ -h "${Files1[$i]}" ] && Type1[$i]="h"
    done

    # Scan selected entries in 'next' pane  (opened with F3)

    let idx=0
    PrepArray=$(
        echo -e "${CAJA_SCRIPT_NEXT_PANE_SELECTED_FILE_PATHS}" | while read line; do
            if [ ${#line} -gt 0 ]; then
                echo Files2[$idx]="'$line'"\;
                let idx=$idx+1
            fi
        done
    )
    eval "$PrepArray"
    let numFiles2=${#Files2[@]}

    for((i = 0; i < $numFiles2; i++)); do
        Type2[$i]=""
        [ -f "${Files2[$i]}" ] && Type2[$i]="f"
        [ -d "${Files2[$i]}" ] && Type2[$i]="d"
        [ -h "${Files2[$i]}" ] && Type2[$i]="h"
    done
}

function DebugOutput() {
    T=/tmp/tmpout

    (
    echo -e "CAJA_SCRIPT_CURRENT_URI                   " $CAJA_SCRIPT_CURRENT_URI
    echo -e "CAJA_SCRIPT_SELECTED_FILE_PATHS           " $CAJA_SCRIPT_SELECTED_FILE_PATHS
    echo -e "CAJA_SCRIPT_SELECTED_URIS                 " $CAJA_SCRIPT_SELECTED_URIS
    echo -e "CAJA_SCRIPT_NEXT_PANE_CURRENT_URI         " $CAJA_SCRIPT_NEXT_PANE_CURRENT_URI
    echo -e "CAJA_SCRIPT_NEXT_PANE_SELECTED_FILE_PATHS " $CAJA_SCRIPT_NEXT_PANE_SELECTED_FILE_PATHS
    echo -e "CAJA_SCRIPT_NEXT_PANE_SELECTED_URIS       " $CAJA_SCRIPT_NEXT_PANE_SELECTED_URIS

    for((i = 0; i < $numFiles1; i++)); do
        echo "--- ${Type1[$i]} ${Files1[$i]}"
    done

    for((i = 0; i < $numFiles2; i++)); do
        echo "--- ${Type2[$i]} ${Files2[$i]}"
    done
    ) >>$T

    geany $T &
}

ScanSelectedEntries
# DebugOutput

[ $numFiles1 -eq 2 ] && \
    meld "${Files1[0]}" "${Files1[1]}"

[ $numFiles1 -eq 3 ] && \
    meld "${Files1[0]}" "${Files1[1]}" "${Files1[2]}"

[ $numFiles1 -eq 1 ] && [ $numFiles2 -eq 1 ] && \
    meld "${Files1[0]}" "${Files2[0]}"

exit