Nagios script that checks permissions for a file

check_permissions.sh

#!/bin/bash

################################
# Lampros Chaidas              #
# 03/12/2017                   #
################################

stat=/usr/bin/stat
help="Usage: $0 filename_to_check user group octal_permissions\nExample: $0 /usr/bin/stat root root 755"

# function to check if a file exists
check_if_file_exists () {
	ls $1 &> /dev/null
	if [ $? -ne 0 ];then
		echo "The file $1 does not exist - exiting"
		echo -e $help
		exit 1
	fi
}

check_if_file_exists $stat

# check if any arguments have been passed
if [ -z "$1" ]
  then
    echo "Please provide the filename you would like me to check"
    echo -e $help
    exit 1
fi

if [ -z "$2" ]
  then
    echo "Please provide the user that should own the file"
    echo -e $help
    exit 1
fi

if [ -z "$3" ]
  then
    echo "Please provide the group that should own the file"
    echo -e $help
    exit 1
fi

if [ -z "$4" ]
  then
    echo "Please provide the octal file permissions for the file"
    echo -e $help
    exit 1
fi

check_if_file_exists $1

filename=$1

octal_permissions=`$stat -c "%a" $filename`
user=`$stat -c "%U" $filename`
group=`$stat -c "%G" $filename`

if [ ! "$user" == "$2" ];then
        echo "CRITICAL - the users do not match - expected: $2, found: $user"
        exit 2 
fi

if [ ! "$group" == "$3" ];then
        echo "CRITICAL - the groups do not match - expected: $3, found: $group"
        exit 2
fi

if [ ! "$octal_permissions" == "$4" ];then
        echo "CRITICAL - the octal permissions do not match - expected: $4, found: $octal_permissions"
        exit 2
fi

echo "OK - all permissions match"
exit 0