get the Xth argument in each line

Question

I would like to get the second argument in a line of output. For example:

ps aux

gives me an output that contains the cpu-usage %CPU in the third column.

How can I filter the output so I have only that column?

I tried

ps aux | cut -f 3

but that's not working

see also: Filtering the Xth element in a row?

Answer 1

There's a number of ways to accomplish this. Here's an example using awk:

ps aux | awk '{print $3}'

The $3 argument to the print command will print the third field, by default delimited by a sequence of whitespace, of each line.

Answer 2

Using ps format options

ps allows you to specify a custom output format.

ps ax o pcpu

The above command would return a single column of CPU usage data.

Why cut doesn't work

cut's default delmiter is the tab character. ps uses spaces in its output, so you would have to use cut -d' ' -f3 to select the third field (as cut sees it).

The problem with using cut is that it views anything adjacent to a delimiter as a field. Since ps prints multiple adjacent spaces between fields,

$ ps aux
...
para␣␣␣␣␣␣␣462␣␣0.0␣␣0.0␣␣13492␣␣1792␣tty1␣␣␣␣␣Ss+␣␣05:52␣␣␣0:00
root␣␣␣␣␣␣␣484␣␣1.8␣␣0.5␣137020␣45112␣?␣␣␣␣␣␣␣␣Ss␣␣␣05:52␣␣␣0:20
para␣␣␣␣␣␣␣490␣␣0.0␣␣0.0␣␣28072␣␣1708␣tty1␣␣␣␣␣S␣␣␣␣05:52␣␣␣0:00
...
$ ps aux | cut -d' ' -f3
...
␣
␣
␣
...

cut will print a column of spaces if you ask it for the third field. To make matters worse, the number of spaces that ps outputs between each field is not always uniform among the rows, so you can't just use cut -d' ' -f12 You would end up with results like this:

...
4.6
0.0
0.0
␣
␣
0.0
0.1
␣
␣
␣
...