[kwlug-disc] Question about `ps` and `grep`

[Ernst Rullmann]

Oh that’s fun. It’s because the grep command is altered so that it can’t match itself. With [t]mux as the argument, it appears in ps aux as something like:

root      748047  0.0  0.0   9212  2560 pts/10   S+   17:59   0:00 grep --color=auto --directories=skip [t]mux

Note the brackets. But the grep expression will only match ‘tmux’ without brackets. So it no longer finds itself. 

I think you could use any other search phrase that has control characters that it doesn’t match. Some examples might be

[a-z]mux
[:alpha:]mux
t{1}mux
t+mux

I haven’t tried these yet, but I think they’d work. 

Ernst

> On Sep 25, 2024, at 21:08, Ronald Barnes via kwlug-disc <kwlug-disc at kwlug.org> wrote:
> 
> I watched a recent video about bash on YouTube channel "You Suck At Programming" and a comment there has me completely baffled.
> 
> 
> The topic was `pgrep` and `pkill` instead of `ps aux | grep ...`.
> 
> 
> The comment had a solution to seeing the `grep` command in the output when grep'ing `ps` output:
> 
> This shows the grep command in the output:
> 
> 
> # ps aux | grep  tmux
> root        9032  0.0  0.0  23476  5632 pts/1    S+   Sep05   0:04 sudo tmux new -s w00
> root        9041  0.0  0.0  23476  1560 pts/9    Ss   Sep05   0:00 sudo tmux new -s w00
> root        9042  0.0  0.0  10916  3712 pts/9    S+   Sep05   0:00 tmux new -s w00
> root        9045  0.0  0.0  14236  4920 ?        Ss   Sep05   1:14 tmux new -s w00
> root      748047  0.0  0.0   9212  2560 pts/10   S+   17:59   0:00 grep --color=auto --directories=skip tmux
> 
> 
> 
> BUT, changing what's being grep'd for by making it [t]mux:
> 
> # ps aux | grep  [t]mux
> root        9032  0.0  0.0  23476  5632 pts/1    S+   Sep05   0:04 sudo tmux new -s w00
> root        9041  0.0  0.0  23476  1560 pts/9    Ss   Sep05   0:00 sudo tmux new -s w00
> root        9042  0.0  0.0  10916  3712 pts/9    S+   Sep05   0:00 tmux new -s w00
> root        9045  0.0  0.0  14236  4920 ?        Ss   Sep05   1:14 tmux new -s w00
> 
> 
> Now grep's output is NOT shown.
> 
> 
> And I cannot figure out why not?!?
> 
> 
> I understand that [t] is a character range (of one character), but without explicit anchoring, why does that work?
> 
> 
> 
> Anyone able to explain this?
> 
> 
> (Also, https://www.youtube.com/@yousuckatprogramming channel is highly recommended - 3 to 4 minute videos, usually about `bash` topics, where there's always something I learn.
> 
> Then check the comments for user "extrageneity", who expands on the topic with a Uni CS level comment.)
> 
> 
> Thanks,
> 
> rb
> 
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