This script can be run before ElkarBackup does the backup of a nextcloud-data direcory. It stores the SQL Dump within the data directory.

elckarbackupNextcloudBackupScript.sh

#!/bin/bash

# inspired by https://github.com/elkarbackup/elkarbackup-scripts/tree/master/backup-mysql
# access to the nexecloud-DB from the user doing the backup via mysqldump must be granted somehow

URL=`echo $ELKARBACKUP_URL | cut -d ":" -f1`    # user@serverip
USER="${URL%@*}"                                # user
HOST="${URL#*@}"                                # host
DIR=`echo $ELKARBACKUP_URL | cut -d ":" -f2`    # path
DIR=$DIR"/sqlbkp"                               # path inside the main backup folder
TMP=/tmp/ebmysqldump

# which DB should be backed up
DB="nextcloud"

SSHPARAMS="-i /var/lib/elkarbackup/.ssh/id_rsa -o StrictHostKeyChecking=no $ELKARBACKUP_SSH_ARGS"

if [ "$ELKARBACKUP_LEVEL" != "JOB" ]
then
    echo "Only allowed at job level" >&2
    exit 1
fi

if [ "$ELKARBACKUP_EVENT" == "PRE" ]
then
    # If backup directory doesn't exist, create it
    TEST=`ssh $SSHPARAMS $USER@$HOST "test -d $DIR && echo $?"`
    if [ ! ${TEST} ]; then
        echo "[INFO] Backup directory $DIR doesn't exist. Creating..."
        ssh $SSHPARAMS $USER@$HOST "mkdir -p $DIR"
    fi

    # If tmp directory doesn't exist, create it
    TEST=`ssh $SSHPARAMS $USER@$HOST "test -d $TMP && echo $?"`
    if [ ! ${TEST} ]; then
        echo "[INFO] TMP directory $TMP doesn't exist. Creating..."
        ssh $SSHPARAMS $USER@$HOST "mkdir -p $TMP"
    fi

    # If .htaccess doesn't exist, create it
    TEST=`ssh $SSHPARAMS $USER@$HOST "test -f $DIR/.htaccess && echo $?"`
    if [ ! ${TEST} ]; then
        ssh $SSHPARAMS $USER@$HOST "echo \"# line below if for Apache 2.4
<ifModule mod_authz_core.c>
Require all denied
</ifModule>

# line below if for Apache 2.2
<ifModule !mod_authz_core.c>
deny from all
Satisfy All
</ifModule>

# section for Apache 2.2 and 2.4
<ifModule mod_autoindex.c>
IndexIgnore *
</ifModule>\" > $DIR/.htaccess"
    fi
    
    # enable Nextcloud Maintenance Mode
    ssh $SSHPARAMS $USER@$HOST "sudo -u www-data php /var/www/nextcloud/occ maintenance:mode --on"

    # Dump it!
    ssh $SSHPARAMS $USER@$HOST "mysqldump --force --opt --databases $DB --single-transaction --quick --lock-tables=FALSE > $TMP/$DB.sql"
    # If we already have an old version...
    TEST=`ssh $SSHPARAMS $USER@$HOST "test -f $DIR/$DB.sql && echo $?"`
    if [ ${TEST} ]; then
        # Diff
        #echo "making a diff"
        TEST=`ssh $SSHPARAMS $USER@$HOST "diff -q <(cat $TMP/$DB.sql|head -n -1) <(cat $DIR/$DB.sql|head -n -1) > /dev/null && echo $?"`
        #echo "diff result: [$TEST]"
        # If Diff = false, copy tmp dump file
        if [ ! ${TEST} ]; then
            ssh $SSHPARAMS $USER@$HOST "cp $TMP/$DB.sql $DIR/$DB.sql"
            echo "[$DB.sql] Changes detected. New dump saved."
        else
            echo "[$DB.sql] No changes detected. Nothing to save."
        fi
    else
        echo "[$DB.sql] First dump created!"
        ssh $SSHPARAMS $USER@$HOST "cp $TMP/$DB.sql $DIR/$DB.sql"
    fi
else
    # disable Nextcloud Maintenance Mode
    ssh $SSHPARAMS $USER@$HOST "sudo -u www-data php /var/www/nextcloud/occ maintenance:mode --off"
    echo "";
fi