Someone
public text v1 · immutable$ mkdir blah
$ touch blah/test.1
$ sudo chattr +i blah
$ touch blah/test.2
touch: cannot touch `blah/test.2': Permission denied
$
$ mkdir blah
$ touch blah/test.1
$ sudo chattr +i blah
$ touch blah/test.2
touch: cannot touch `blah/test.2': Permission denied
$