Is "ps -u" Really a Bad Syntax?

The correct syntax, which returns the same output, would be:

ps u

There is a good reason why modern syntax for ps is a mess. Historically, there were two incompatible version of ps. Options with a leading dash were inherited from the AT&T Unix version of ps. Options without a leading dash were inherited from BSD. The version of ps that Linux distributions generally use is GNU which has merged both sets of options together, as well as added its own set of options that start with a leading double-dash.

Thus, ps u is BSD-style and ps -u $USER is AT&T-style. The fact that GNU ps allows you to run ps -u and, other than the warning, get the same output as ps u shows that GNU is attempting to make the best of a bad situation.

The ps command historically had wildly different syntax in BSD and System V Unix.

• In BSD ps, the u option (no dash) takes no parameter and shows the "user-oriented output" with the additional columns.

• In SunOS ps, the -u option (with dash) takes a username as parameter and only includes processes owned by that user, but without changing the display format.

(As another very common example, BSD e means "show environment", while SunOS -e means "show everyone's processes".)

Linux procps ps tries to support both styles. So if you use the 'dash' option -u, it will expect it to be the SunOS "filter this user" option, not the extended columns option. The two are confused frequently enough, however, that procps tries to Do What You Meant – if the username is missing, it'll assume you gave it a BSD option but used SunOS syntax.

(There were in fact so many different variants of ps that the procps has an actual table of "personalities" to force ambiguous behavior to be interpreted as one style or another or yet another – in addition to knobs like "UNIX95", "CMD_ENV", "_XPG", "I_WANT_A_BROKEN_PS"...)