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Startup a fresh GTK emacs with no config with emacs -Q. Then run M-x shell. Then at the prompt:

$ ps -ef

Long commands will be truncated to the frame width, like this:

root      526315       2  0 15:08 ?        00:00:00 [kworker/6:2-eve

So far, this matches normal terminal behavior. But if you do:

$ ps -ef | grep kworker

In emacs shell-mode it will still be truncated, but in a non-emacs terminal (e.g. xterm) the line will not be truncated. Why is this? How do I get it to match normal terminal behavior?

I've noticed shell-mode will respect $COLUMNS, so you can set it to a large value as a temporary fix, but this is unnecessary in a regular terminal where piping will prevent truncation regardless of the $COLUMNS value, and shell-mode always assigns $COLUMNS back to the frame width on the next command.

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In bash, export -p | grep COLUMNS shows me that this variable is exported in my *shell* buffer, but not in my regular terminal.

export -n COLUMNS in my *shell* buffer therefore has the effect you're after.

Original answer follows...


The man page for my version of ps says:

If ps can not determine display width, as when output is redirected (piped) into a file or another command, the output width is undefined (it may be 80, unlimited, determined by the TERM variable, and so on). The COLUMNS environment variable or --cols option may be used to exactly determine the width in this case. The w or -w option may be also be used to adjust width.

Where the latter options are:

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

And indeed for me, ps -wwef | grep <whatever> has the desired effect in a shell buffer.

My ps also support any of the following set a specific screen width:

  • --cols <N>
  • --columns <N>
  • --width <N>
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  • this is a good workaround, but doesn't explain why the emacs behavior is different. when I invoke ps ... | grep .. bash is still responsible for doing the piping between ps and grep, so I wouldn't expect ps to behave differently Commented Apr 18 at 21:25
  • They'll be making decisions based on the environment they observe, and something in those differences will be driving that decision. I don't know what, specifically. As the chosen width is explicitly "undefined" in that situation, though, neither of the two outcome is incorrect.
    – phils
    Commented Apr 18 at 22:27
  • In my case at least, I suspect the decision is driven by the fact that in the two different scenarios I get a different result from the following: echo $COLUMNS | sh -c 'cat - && echo and && echo $COLUMNS'
    – phils
    Commented Apr 19 at 1:25
  • export -n COLUMNS in my *shell* buffer has the effect you're after. export -p | grep COLUMNS shows that it is exported in my *shell* buffer, but not in my regular terminal.
    – phils
    Commented Apr 19 at 1:35
  • I observe that if you change COLUMNS in the shell buffer, then hit enter a couple of times, the old value magically comes back Commented Apr 19 at 16:55

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