How to make a file immutable(read-only)

Assign Special attributes to a file using "chattr".

If you want to restrict all the operation on a file except readonly including root user, chattr can help to achieve this.
[root@localhost rpm]# touch /tmp/my.txt
[root@localhost rpm]# ll /tmp/my.txt
-rw-r--r--. 1 root root 0 Jan 25 15:36 /tmp/my.txt
[root@localhost rpm]# chgrp harry /tmp/my.txt
[root@localhost rpm]# chown natasha /tmp/my.txt
[root@localhost rpm]# ll /tmp/my.txt
-rw-r--r--. 1 natasha harry 0 Jan 25 15:36 /tmp/my.txt
[root@localhost rpm]# chmod 000 /tmp/my.txt
[root@localhost rpm]# ll /tmp/my.txt
----------. 1 natasha harry 0 Jan 25 15:36 /tmp/my.txt
[root@localhost rpm]#

root user doesn't have any permission on /tmp/my.txt file, still it can do all the file operation on this file.

[root@localhost rpm]# cat >> /tmp/my.txt

root user added some data

[root@localhost rpm]# cat /tmp/my.txt

root user added some data

[root@localhost rpm]#

How to restrict root user so that any user can't perform any operation?

[root@localhost rpm]# chattr +i /tmp/my.txt

[root@localhost rpm]# lsattr /tmp/my.txt

----i----------- /tmp/my.txt

[root@localhost rpm]#

Now if any user including root try to do any file operation except read, they can't do.

[root@localhost rpm]# cat >> /tmp/my.txt

-bash: /tmp/my.txt: Permission denied

[root@localhost rpm]#

[root@localhost rpm]# rm -rvf /tmp/my.txt

rm: cannot remove ‘/tmp/my.txt’: Operation not permitted

[root@localhost rpm]#

[root@localhost rpm]# chattr -i /tmp/my.txt ---------- if want to remove

chattr has lots of option but i and a are my favorite.

a -> We can append some data but can't remove anything.

[root@localhost rpm]# chattr +a /tmp/my.txt

[root@localhost rpm]# cat > /tmp/my.txt

-bash: /tmp/my.txt: Operation not permitted

[root@localhost rpm]# cat >> /tmp/my.txt

adding few more lines

[root@localhost rpm]# cat /tmp/my.txt

root user added some data

adding few more lines

[root@localhost rpm]#