What is the effect of "chattr +a" on a directory

Question

The a Linux file attribute is often documented as applicable to files.

chattr(1) manpage:

A file with the 'a' attribute set can only be open in append mode for writing.

ext4 wiki:

0x20 File can only be appended (EXT4_APPEND_FL).

My questions are:

• Is chattr +a restricted to files only?
• Is it recursive on new subdirectories and files?

Solution

The append only flag (chattr +a) prevent from removing the directory, a well as files and directories created directly inside that directory:

Create test directory and files:

# mkdir     /tmp/foo
# chattr +a /tmp/foo

That directory can't be deleted:

# rmdir     /tmp/foo
rmdir: failed to remove ‘/tmp/foo’: Operation not permitted

Now create files and directory inside it:

# touch     /tmp/foo/bar
# mkdir     /tmp/foo/baz

Let's inspect that:

# lsattr -d /tmp/foo /tmp/foo/ba*
-----a-------e-- /tmp/foo
-------------e-- /tmp/foo/bar
-------------e-- /tmp/foo/baz

Try to erase stuffs:

# rm     /tmp/foo/bar
rm: cannot remove ‘/tmp/foo/bar’: Operation not permitted
# rmdir  /tmp/foo/baz
rmdir: failed to remove ‘/tmp/foo/baz’: Operation not permitted
rm -Rf /tmp/foo
rm: cannot remove ‘/tmp/foo/bar’: Operation not permitted
rm: cannot remove ‘/tmp/foo/baz’: Operation not permitted

Finally, sub-sub-directory and files in sub-directories are not protected:

# mkdir            /tmp/foo/baz/bat
# touch            /tmp/foo/baz/baff
# rm --verbose -Rf /tmp/foo/baz
removed directory: ‘/tmp/foo/baz/bat’
removed ‘/tmp/foo/baz/baff’
rm: cannot remove ‘/tmp/foo/baz’: Operation not permitted

Again, note that only /tmp/foo had the append flag:

# lsattr -d  /tmp/foo /tmp/foo/baz
-----a-------e-- /tmp/foo
-------------e-- /tmp/foo/baz