MySQL DB backup script

db_backup.sh

#! /usr/bin/env bash
PRESERVE_FILES_COUNT=3
ARCHIVE_EXTENSION='.sql.bz2'

if [ ! -z $1 ] && [ ! -z $2 ] &&  [ ! -z $3 ] && [ ! -z $4 ]; then
  backup_dir=$1
  db_user=$2
  db_pass=$3
  db_name=$4
  archive_name="$(date +\%m\%d\%Y-\%H\%M\%S)_$db_name.sql.bz2"

  function count_files_by_type() {
   full_dir_path="$( dirname "$1" )/$( basename $1 )"
   file_extension=$2

   if [ -d $full_dir_path ] && [ ! -z $file_extension ]; then
     ls $full_dir_path | grep "$file_extension" | wc -l | tr -s ' '
   fi
  }

  set -o pipefail

  cd $backup_dir && mysqldump --single-transaction -u$db_user -p$db_pass $db_name | bzip2 -9 -c > ./$archive_name
  #cd $backup_dir && echo 'some SQL data' | bzip2 -9 -c > ./$archive_name # test backup (without MySQL)

  result=$?

  bz2_files_count=$( count_files_by_type $backup_dir $ARCHIVE_EXTENSION )

  if [ "$bz2_files_count" -gt $PRESERVE_FILES_COUNT ] && [ $result -eq 0 ]; then
    echo "$bz2_files_count is grater then $PRESERVE_FILES_COUNT... I'm going to delete excess files!"
    rm -f $backup_dir/$(ls -tr1 $backup_dir|grep $ARCHIVE_EXTENSION|head -1) > /dev/null 2>&1
  fi
else
  echo "Usage: ~/db_backup.sh ~/backup/path db_user db_pass db_name"
fi