How universal is the -L (dereference symlink) switch of the 'ls' command?

Question

I have some software that, among other things, needs to:

• Assess a file's rwxrwxrwx permissions;
• Work under every possible flavor of Unix and Linux you can find in the wild.

Currently, it does that running the ls -l command. If the file is a symlink, I have to get the permissions of the target file. The -L switch works nicely for that.

The question: Are there flavors of Unix in which I run the risk of that switch not being available? If so, which ones? (If they're rare enough and old enough, I might just be able to ignore the problem.)

Answer 1

Well, it's in the GNU, FreeBSD, OpenBSD, and general BSD ls implementations, so I'd say it's pretty likely to be anywhere.

Answer 2

Don't parse ls if you can help it. Depending on what you're doing, the test utility might be a better alternative.

test -r filename will return success if the file is readable with the current user's permissions, and will follow symbolic links automatically. Likewise, -w and -x will tell you if the file is writable or executable. The test command is specified by POSIX, and so ought to work with any modern UNIX.

If you really need to know the specific permissions of the file, ls might be the most portable way, as long as you only use it on one file at a time. In that case, the -L and -l flags are also specified by POSIX, so you should be reasonably safe. (See "Don't Parse ls" for a little more commentary on the portabilty of the output fields of ls -l.)

Another option might be to write a short C program to take advantage of the stat(2) system call, which will be available on every UNIX under the sun. If this is a small part of a larger piece of software that's already being compiled from C, this might be the best option.

Answer 3

It is in SUSv3 (Posix 2001) but not in SUSv2 (alias Unix98).