Dynamically trim stdout line width in Bash

Question

Lately, I have been experimenting with the ps command, and sometimes long paths wrap to the next line (or two) and make it hard to read. I want to pipe the ps output into another program to limit the output to x number of characters.

Here is what I have so far, but it doesn't work quite right:

ps aux | cut -c1-$(stty size | cut -d' ' -f2)

$(stty size | cut -d' ' -f2) evaluates to 167, but doesn't seem to be valid input for cut.

Is there a way to get this type of syntax to work in bash?

Answer 1

The following works:

ps aux | cut -c1-$(stty size </dev/tty | cut -d' ' -f2)

This also works:

v=$(stty size | cut -d' ' -f2) ; ps aux | cut -c1-$v

The problem seems to be that stty needs to have the tty on its standard input in order to function. The above two approaches solve that.

There is still another option. While stty's stdin and stdout are both redirected in the above commands, its stderr is not: it still points to a terminal. Strangely enough, stty will also work if it is given stderr as its input:

ps aux | cut -c1-$(stty size <&2 | cut -d' ' -f2)

Answer 2

Some shells like zsh, bash or mksh automatically set the $COLUMNS variable to the width of the terminal, so you don't need to invoke stty here.

All the implementations of ps I tried that support that non-standard (BSD-type) syntax query the terminal width by themselves. I'm surprised yours doesn't. I expect it will look at the content of the COLUMNS environment variable though.

So you could do:

export COLUMNS; ps aux

If not,

ps aux | cut -c"1-$COLUMNS"

You can also tell your terminal not to wrap lines:

tput rmam
ps aux

Answer 3

You can also handle ps output a little better.

ps --width ${n:-$COLUMNS} ${opts} #set ps terminal width

ps -ww ${opts} #no word wrap

ps -o ${only_interesting_output} ${opts} #trim output

That will tell ps to parse its output to your specifications as necessary.

Of course, if you don't word wrap, though, then you've got the problem of missing info. Do you really need all of it for all processes? Open it in a pager if so:

ps ww ${opts} | $PAGER

If not, specify what you do want to see:

ps -o args= -p $pid

Alternatively you can explicitly inform ps of your terminal --width:

man ps

...

w Wide output. Use this option twice for unlimited width.

-w Wide output. Use this option twice for unlimited width.

--width n Set screen width.

The --width thing works exactly to your requested specs without having to involve any additional filters or ancillary processes (which will likely only clutter your -aux output even more). And with $COLUMNS as shown above and as Stephane points out it will even work dynamically.

It's probably worth noting, though, that I find people often try to add these kinds of unnecessary filters so they can accomodate a |pipe through another filter to parse output which is also pretty likely to be unnecessary. Of course, by people I mostly mean me.

The -output operand I mention above allows you to filter which columns ps displays, and when you add the =assigment you can even name the column as you please. I leave the assignment empty and hand it a target -process $pid so the only output from ps at all is the $pid command name and its args at invocation. And -o barely scratches the surface of how you can define what ps will or won't display. This is the direction I would recommend you take, especially via:

man ps

... 

To see every process on the system using BSD syntax:

ps ax

ps axu

To print a process tree:

ps -ejH

ps axjf

To get info about threads:

ps -eLf

ps axms

To get security info:

ps -eo euser,ruser,suser,fuser,f,comm,label

ps axZ

ps -eM

To see every process running as root (real & effective ID) in user format:

ps -U root -u root u

To see every process with a user-defined format:

ps -eo pid,tid,class,rtprio,ni,pri,psr,pcpu,stat,wchan:14,comm

ps axo stat,euid,ruid,tty,tpgid,sess,pgrp,ppid,pid,pcpu,comm

ps -Ao pid,tt,user,fname,tmout,f,wchan

Print only the process IDs of syslogd:

ps -C syslogd -o pid=

Print only the name of PID 42:

ps -p 42 -o comm=