What does aux mean in `ps aux`?

man ps describes these options as follows:

   a      Lift the BSD-style "only yourself" restriction, which is imposed 
          upon the set of all processes when some BSD-style (without "-") 
          options are used or when the ps personality setting is BSD-like.  
          The set of processes selected in this manner is in addition to the 
          set of processes selected by other means.  An alternate 
          description is that this option causes ps to list all processes 
          with a terminal (tty), or to list all processes when used together 
          with the x option.

   u      Display user-oriented format.

   x      Lift the BSD-style "must have a tty" restriction, which is imposed 
          upon the set of all processes when some BSD-style (without "-") 
          options are used or when the ps personality setting is BSD-like.
          The set of processes selected in this manner is in addition to the 
          set of processes selected by other means.  An alternate 
          description is that this option causes ps to list all processes 
          owned by you (same EUID as ps), or to list all processes when used 
          together with the a option.

Example

$ ps aux | head -10
USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
root         1  0.0  0.0  51120  2796 ?        Ss   Dec22   0:09 /usr/lib/systemd/systemd --system --deserialize 22
root         2  0.0  0.0      0     0 ?        S    Dec22   0:00 [kthreadd]
root         3  0.0  0.0      0     0 ?        S    Dec22   0:04 [ksoftirqd/0]
root         5  0.0  0.0      0     0 ?        S<   Dec22   0:00 [kworker/0:0H]
root         7  0.0  0.0      0     0 ?        S    Dec22   0:15 [migration/0]
root         8  0.0  0.0      0     0 ?        S    Dec22   0:00 [rcu_bh]
root         9  0.0  0.0      0     0 ?        S    Dec22   2:47 [rcu_sched]
...
saml      3015  0.0  0.0 117756   596 pts/2    Ss   Dec22   0:00 bash
saml      3093  0.9  4.1 1539436 330796 ?      Sl   Dec22  70:16 /usr/lib64/thunderbird/thunderbird
saml      3873  0.0  0.1 1482432 8628 ?        Sl   Dec22   0:02 gvim -f
root      5675  0.0  0.0 124096   412 ?        Ss   Dec22   0:02 /usr/sbin/crond -n
root      5777  0.0  0.0  51132  1068 ?        Ss   Dec22   0:08 /usr/sbin/wpa_supplicant -u -f /var/log/wpa_supplica
saml      5987  0.7  1.5 1237740 119876 ?      Sl   Dec26  14:05 /opt/google/chrome/chrome --type=renderer --lang=en-
root      6115  0.0  0.0      0     0 ?        S    Dec27   0:06 [kworker/0:2]
...

With the above switches you’ll get output regarding your processes like above.

The switches aux will show you:

• all the users’ processes
• show you the process listed in a user oriented fashion (by user names)
• show you all processes, not just ones attached to a terminal. This will include processes such as services like crond, upowerd, etc.

The key to understanding the manpage is not to search for “aux” (which I tried first), but to focus on the section that describes the kinds of parameter ps takes:

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.

From this, we know that aux is a set of (grouped) BSD options, a, u and x, which makes them slightly easier to look up.

• a and x control which processes are selected, and used together are explicitly described to select all processes.

• u outputs using the “user-oriented” format, which gives more columns, including the user id and CPU/memory usage.

Because u alone controls the output format, you can get “ps aux” style output just for specific processes with ps u $pid1 $pid2 ....

ps: process status

for me, both /bin/ps --version and /usr/bin/ps --version gets: ps from procps-ng 3.3.16

ps is a command provided by procps:
Command line and full screen utilities for browsing procfs, a "pseudo" file system dynamically generated by the kernel to provide information about the status of entries in its process table.

2 kinds of syntax

To see every process on the system using:

1. standard syntax

• ps -e
• ps -ef

BTW, example of ps -u:

• ps -u root –format=user,pid,command # which is same as this line:
• ps -uroot –format=user,pid,command #(What an ugly syntax!)

output is:

    USER         PID COMMAND
    root           1 /sbin/init splash
    root           2 [kthreadd]
    root           3 [rcu_gp]
    root           4 [rcu_par_gp]
    root           6 [kworker/0:0H-kblockd]
    root           9 [kworker/u80:0-ixgbe]

By this POSIX and UNIX standards, ps -aux means ps -a -ux , printing:
1. processes with tty (except session leaders) [by -a]
2. all processes owned by a user named "x" [by -ux] (-u: select processes by effective user)

Process selection options are additive (A process will be shown if it meets any of the given selection criteria)

If the user named "x" (the current user?) does not exist, ps -aux may fall to ps aux , and print a warning

there is no -x option in manpage, but ps -x can find the same processes as ps -ux, (in a different output format)

• ps -ux --format=user gets:
  username does not exist
• ps -x --format=user
  gets proper results (corespond to the current user)

no wonder manpage reminds:

This behavior is intended to aid in transitioning old
scripts and habits. It is fragile, and thus should not be relied upon

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2. BSD syntax:
   ps ax

• a: all users

• x: list all processes owned by you (same EUID as ps). Or more concretely, include processes with no controlling terminal (include processes not attached to a terminal).

  so, ax will list all processes

• u: output in resource usage format
  exampale: ps axu

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This scene shows the convinience of zsh’ s autocomplete (press tab and get hint)

This manpage is reader friendly: https://www.mankier.com/1/ps