Now, I'll set the a attribute:

[donnie@localhost ~]$ sudo chattr +a perm_demo.txt
[sudo] password for donnie:
[donnie@localhost ~]$

You use + to add an attribute and - to delete it. Also, it doesn't matter that the file belongs to me and is in my own home directory. I still need sudo privileges to add or delete this attribute.

Now, let's see what happens when I try to overwrite this file:

[donnie@localhost ~]$ echo "I want to overwrite this file." > perm_demo.txt
-bash: perm_demo.txt: Operation not permitted

[donnie@localhost ~]$ sudo echo "I want to overwrite this file." > perm_demo.txt
-bash: perm_demo.txt: Operation not permitted
[donnie@localhost ~]$

With or without sudo privileges, I can't overwrite it. So, how about if I try to append something to it?

[donnie@localhost ~]$ echo "I want to append this to the end of the file." >> perm_demo.txt

[donnie@localhost ~]$

There's no error message this time. Let's see what's in the file:

This is Donnie's sensitive file that he doesn't want to have overwritten.
I want to append this to the end of the file.

In addition to not being able to overwrite the file, I'm also unable to delete it:

[donnie@localhost ~]$ rm perm_demo.txt
rm: cannot remove ‘perm_demo.txt’: Operation not permitted

[donnie@localhost ~]$ sudo rm perm_demo.txt
[sudo] password for donnie:
rm: cannot remove ‘perm_demo.txt’: Operation not permitted
[donnie@localhost ~]$

So, the a works. However, I've decided that I no longer want this attribute to be set, so I'll remove it:

[donnie@localhost ~]$ sudo chattr -a perm_demo.txt
[donnie@localhost ~]$ lsattr perm_demo.txt
---------------- perm_demo.txt
[donnie@localhost ~]$