How to sort ps output

The ps command is key to understanding what’s running on your Linux system and the resources that each process is using. It’s useful to know how to display the information that ps provides in whatever way helps you focus on the problem you’re trying to resolve. One aspect of this is being able to sort the output of the ps aux command by any column to highlight particular information, such as how much memory processes are using or how long they’ve been running.

The trick involves using the ps command’s –sort option and knowing how to specify the column that you want to use for the sort. By default, ps sorts by process IDs (PIDs), showing the smallest first. PID 1 will appear at the top of the list, right under the column headings. The rest will follow in numeric order.

Here’s an example of the standard ps aux output. It probably looks familiar.

$ ps aux | head -5
USER         PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
root           1  0.0  0.2 169820 14028 ?        Ss   Nov01   0:52 /lib/systemd/systemd --system --deserialize 22
root           2  0.0  0.0      0     0 ?        S    Nov01   0:00 [kthreadd]
root           3  0.0  0.0      0     0 ?        I
Using the --sort option, you will get the same output if you sort by PID. Notice that you need to enter "pid" in lower case. However, you can change "pid" to any other column and sort by that one instead.
$ ps aux --sort pid | head -5
USER         PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
root           1  0.0  0.2 169820 14028 ?        Ss   Nov01   0:52 /lib/systemd/systemd --system --deserialize 22
root           2  0.0  0.0      0     0 ?        S    Nov01   0:00 [kthreadd]
root           3  0.0  0.0      0     0 ?        I
To see the largest values for the selected column listed first, you would have to change the sort option (e.g., -pid in place of pid). Notice how the display in this case will then start with the highest process ID. Think of - as meaning descending values (largest first) and just the name (e.g., pid) or the name with a plus sign prepended (e.g., +pid) as meaning ascending. The command shown below sorts by process ID showing the largest first.
$ ps aux --sort -pid | head -5
USER         PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
shs       316468  0.0  0.0   8092   588 pts/0    S+   14:15   0:00 head -5
shs       316467  0.0  0.0  12052  4080 pts/0    R+   14:15   0:00 ps aux --sort -pid
root      316463  0.0  0.0      0     0 ?        I    14:14   0:00 [kworker/u8:1-events_power_efficient]
root      316453  0.0  0.0      0     0 ?        I    14:09   0:00 [kworker/0:0-events]

To sort by some other column, you need to use its column name. In this example "pmem" means the percentage of memory usage and using "-pmem" means we'll see the processes using the most memory first. You could also use "%mem" and "-%mem".
$ ps aux --sort -pmem | head -5
USER         PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
gdm         1842  0.0  2.5 3765092 156612 tty1   Sl+  Nov01   3:44 /usr/bin/gnoe-shell
root      171907  0.0  0.9 130096 60232 ?        S0.8 146272 53900 ?        Sl   Nov01  17:58 nessusd -q
gdm         2310  0.0  0.8 798716 50032 tty1     Sl+  Nov01   0:00 /usr/bin/Xwaland :1024 -rootless -noreset -accessx -core -auth /run/user/121/.mutter-Xwayladauth.BPIVS0 -listen 4 -listen 5 -displayfd 6 -listen 7

If you only want to see select columns of the ps output, you can specify them as in this example where only commands and their CPU usages are shown and the output is sorted by the largest CPU usage.
$ ps -eo comm,pcpu --sort -pcpu | head -5
COMMAND         %CPU
snmp             0.5
nessusd          0.1
systemd          0.0
kthreadd         0.0

Here's a list of the column names and strings that you can use to sort by them:
Column    Ascending Descending
Heading   Sort      Sort       Alternatives
===============================================
USER      user      -user
PID       pid       -pid
%CPU      pcpu      -pcpu      %cpu and -%cpu
%MEM      pmem      -pmem      %pmem and -%pmem
VSZ       vsz       -vsz
RSS       rss       -rss
TTY       tty       -tty
STAT      stat      -stat
START     start     -start
TIME      time      -time
COMMAND   comm      -comm

Using the sort command
You can also pipe the output of the ps aux command to the sort command and select the column to use for sorting by using the column number (1 through 11). There are a couple of downsides to this approach: 1) that your column headings will be sorted along with the rest of the ps output, likely ending up somewhere you didn't expect to see them; and 2) that you need to add an "n" to your sort command when the sort needs to be numerical. The ps command understand this distinction; sort does not and needs to be nudged.
A command like this one would sort on memory usage. The sort command uses "-nk 4" to (-n) sort numerically on the (-k 4) 4th data column in the sort output.
$ ps aux | head -1; ps aux | sort -r -nk 4 | head -6
USER         PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
gdm         1842  0.0  2.5 3765092 156612 tty1   Sl+  Nov01   3:46 /usr/bin/gnome-shell
root      171907  0.0  0.9 130096 60380 ?        S
Using aliases
Even if the ps command's flexibility meets your needs, you can always create aliases so that you don't have count columns and/or think about whether column values are numeric. Here are some examples:
alias ByCPU='ps aux --sort -%cpu'
alias ByMem='ps aux --sort -%mem'
alias TopCPU='ps aux --sort -%cpu | head -11'
alias TopMem='ps aux --sort -%mem | head -11'