Last post we introduced ps, this post we’ll talk about ps and flags. So, brief recap:

Alpha:~ computerlamp$ ps
  PID TTY          TIME CMD
17245 pts/2    00:00:00 bash
17248 pts/2    00:00:00 ps

Tada! ps with no flags.

Let’s start with a simple flag, x. In the first example, we saw every command that has a tty, or a direct connection to a terminal. That’s the default action of ps. What if we want to see every thing even if it doesn’t have a terminal?

Alpha:~ computerlamp$ ps x
12779 pts/2    R+     0:00 ps x
17245 pts/2    Ss     0:00 -bash

Well, that isn’t any different from the first time around, except now I see that I ran ps with a flag.

But wait, something is weird here. Since we started talking about flags, they’ve always started with a – aka a dash or a hyphen. That time, I didn’t use one. ps is an interesting command, it has three kinds of flags you can use. From the man page:

      This version of ps accepts several kinds of options:
       1   UNIX options, which may be grouped and must be preceded by a dash.
       2   BSD options, which may be grouped and must not be used with a dash.
       3   GNU long options, which are preceded by two dashes.

This means that there are flags that begin with a dash, flag that don’t begin with a dash and flags that have two dashes. If you use the kind without a dash then you can’t use the kind with a dash. It also means that ps -a and ps a are two different commands. See?

Alpha:~ computerlamp$ ps -a
  PID TTY          TIME CMD
12825 pts/2    00:00:00 ps
Alpha:~ computerlamp$ ps a
  PID TTY      STAT   TIME COMMAND
12832 pts/2    R+     0:00 ps a
13451 pts/2    Ss     0:00 -bash

And isn’t that confusing? It feels like Dr. Strange wandered in and the world went weird.

Next post, we’ll talk about processes again.  Understanding processes is key to understanding ps.