How to protect a file on Linux against accidental delete/modify

… even from superuser

Use chattr

touch foobar
lsattr foobar     # show attributes
chattr +i foobar  # set immutable attribute
lsattr foobar
# rm, edit foobar all you want, you will fail
chattr -i foobar  # clear immutable attribute

For once, Wikipedia more readable than the venerable manpages.

Not to be confused with ACLs which only works when a filesystem is mounted with acl option.