This is the third and last "command that should be simple, but isn't" that is discussed here. The ps command lists processes and gives you an extraordinary amount of control over its operation.
The first thing to know is that ps is typically used with what are known as BSD-style parameters. Back in the section "Finding Files by Searching with find," we discussed UNIX-STYLE, GNU-style, and X-style parameters (-c, --dosomething, and -dosomething, respectively), but BSD-style parameters are different because they use single letters without a dash.
So, the default use of ps lists all processes that you are running that are attached to the terminal. However, you can ask it to list all your processes attached to any terminal (or indeed no terminal) by adding the x parameter: ps x. You can ask it to list all processes for all users with the a parameter or combine that with x to list all processes for all users, attached to a terminal or otherwise: ps ax.
However, both of these are timid compared with the almighty u option, which enables user-oriented output. In practice, that makes a huge difference because you get important fields like the username of the owner, how much CPU time and RAM are being used, when the process was started, and more. This outputs a lot of information, so you might want to try adding the f parameter, which creates a process forest by using ASCII art to connect parent commands with their children. You can combine all the options so far with this command: ps faux (yes, with a little imagination you spell words with the parameters!).
You can control the order in which the data is returned by using the --sort parameter. This takes either a + or a - (although the + is default) followed by the field you want to sort by: command, %cpu, pid, and user are all popular options. If you use the minus sign, the results are reversed. This next command lists all processes, ordered by CPU usage descending:
There are many other parameters for ps, including a huge amount of options for compatibility with other UNIXes. If you have the time to read the man page, you should give it a try!
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