Question

I want to do this:

1. run a command
2. capture the output
3. select a line
4. select a column of that line

Just as an example, let’s say I want to get the command name from a $PID (please note this is just an example, I’m not suggesting this is the easiest way to get a command name from a process id – my real problem is with another command whose output format I can’t control).

If I run ps I get:

  PID TTY          TIME CMD
11383 pts/1    00:00:00 bash
11771 pts/1    00:00:00 ps

Now I do ps | egrep 11383 and get

11383 pts/1    00:00:00 bash

Next step: ps | egrep 11383 | cut -d" " -f 4. Output is:

<absolutely nothing/>

The problem is that cut cuts the output by single spaces, and as ps adds some spaces between the 2nd and 3rd columns to keep some resemblance of a table, cut picks an empty string. Of course, I could use cut to select the 7th and not the 4th field, but how can I know, specially when the output is variable and unknown on beforehand.

Answer 1

One easy way is to add a pass of tr to squeeze any repeated field separators out:

$ ps | egrep 11383 | tr -s ' ' | cut -d ' ' -f 4