In this article, we are going to learn how to check the integrity of a file and how to check for tampering by writing a simple shell script. Why do we need to check integrity? The answer is simple: administrators check integrity when there are passwords and libraries present on a server, as well as when files contain highly sensitive data.
Prerequisites
Besides having a terminal open, you need to make sure the necessary files and directories are present.
Write script
We are going to write a script to check whether a file in a directory has been tampered with. Create an integrity_check.sh script and add the following code to it:
integrity_check.sh
#!/bin/bash
E_DIR_NOMATCH=50
E_BAD_DBFILE=51
dbfile=Filerec.md5
# storing records.
set_up_database ()
{
echo ""$directory"" > "$dbfile"
# Write directory name to first line of file.
md5sum "$directory"/* >> "$dbfile"
# Append md5 checksums and filenames.
}
check_database ()
{
local n=0
local filename
local checksum
if [ ! -r "$dbfile" ]
then
echo "Unable to read checksum database file!"
exit $E_BAD_DBFILE
fi
while read rec[n]
do
directory_checked="${rec[0]}"
if [ "$directory_checked" != "$directory" ]
then
echo "Directories do not match up!"
# Tried to use file for a different directory.
exit $E_DIR_NOMATCH
fi
if [ "$n" -gt 0 ]
then
filename[n]=$( echo ${rec[$n]} | awk '{ print $2 }' )
# md5sum writes recs backwards,
#+ checksum first, then filename.
checksum[n]=$( md5sum "${filename[n]}" )
if [ "${rec[n]}" = "${checksum[n]}" ]
then
echo "${filename[n]} unchanged."
else
echo "${filename[n]} : CHECKSUM ERROR!"
fi
fi
let "n+=1"
done <"$dbfile" # Read from checksum database file.
}
if [ -z "$1" ]
then
directory="$PWD" # If not specified,
else
directory="$1"
fi
clear
if [ ! -r "$dbfile" ]
then
echo "Setting up database file, \""$directory"/"$dbfile"\".";
echo
set_up_database
fi
check_database
echo
exit 0How script works
When we run this script, it will create a database file named filerec.md5, which will have data about all the files present in that directory. We’ll use those files for reference.
0 Comments