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From what I understand, the "traditional" behaviour of terminal emulators is to chop long lines when the window is made narrower, while some terminals rewrap their contents on resize.

Is it possible for a Bash script (or as a last resort, a C program) to detect which of the two behaviours the terminal uses?

forthrin
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  • Passively or actively? I don't think there's a terminfo setting for that, but it should be detectable by displaying stuff, causing a resize and reading back from the screen. – Gilles 'SO- stop being evil' Feb 28 '19 at 20:51
  • @Gilles: Augh! That sounds really messy. So chopping Vs. rewrapping is a built-in non-changeable design decision made by each terminal software developer? – forthrin Mar 01 '19 at 07:53
  • Yes, wrapping isn't a traditional feature. And those two aren't the only possible behaviors, I think some terminals not only remember the content of “chopped” lines but actually do allow setting the cursor position wherever there has been output, even if the target column is larger than the current width. – Gilles 'SO- stop being evil' Mar 01 '19 at 10:41
  • @Gilles: This is interesting to know. Thanks! So if you have some terminal content like an ASCII GUI, it's sort of impossible to gracefully handle resizing of terminal width, because you have no way of telling what your terminal does with lines when you resize? – forthrin Mar 01 '19 at 15:22

1 Answers1

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Why not just set the behavior if you are using bash?

For example, there are a few options:

shopt -s checkwinsize

will tell bash to checkwinsize and act appropriately. The man page notes:

If set, bash checks the window size after each command and, if necessary, updates the values of LINES and COLUMNS.

You can also check the number of columns and lines and then set them:

echo $COLUMNS; echo $LINES

If they are not what you want, you can set them:

COLUMNS=75;
LINES=20;
export COLUMNS LINES;

You can do all of that from within a bash script.

number9
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  • So you mean the terminal can be configured from the command line to either wrap lines or cut lines? I thought this was a built-in, non-changeable thing that varied for each terminal software. As per your suggestion, I tried `shopt -s checkwinsize` and `shopt -u checkwinsize`. My terminal wraps line in both cases. Further ideas? – forthrin Feb 28 '19 at 17:10
  • @forthrin referencing this for detection: https://unix.stackexchange.com/questions/281661/how-can-i-quickly-check-exactly-how-many-columns-my-terminal-has I am wondering, are you on a terminal in X windows (not sure that matters, but that is where I am testing). I get the feeling from your comment, you want to cut text off... see this page for an example: https://superuser.com/questions/188865/how-to-make-bash-not-to-wrap-output – number9 Feb 28 '19 at 17:20
  • I'm on macOS using the built-in Terminal, but I need a solution that works for any UNIX terminal. `tput rmam` makes the cursor return to the start of the same line of you type to the end of the line. However, if I **resize** the window horizontally, the lines are wrapping. It's **this** behaviour I'm interested in detecting (**or** setting, if setting it is even possible.) – forthrin Feb 28 '19 at 17:24
  • @forthrin ahh. I do not have access to a mac to completely verify. That is now an interesting problem though.. I was under the impression shopt -s checkwinsize would take care of the terminal being resized... but in "Terminal" I am not sure if that works. I am assuming you need it to work in Mac "Terminal" also? – number9 Feb 28 '19 at 17:59
  • This answer has nothing to do with the question. `checkwinsize` doesn't give any information about the behavior of the terminal when it's resized. The last part is also wrong: setting `LINES` and `COLUMNS` will not influence the terminal in any way. – Gilles 'SO- stop being evil' Feb 28 '19 at 20:48
  • @Gilles So what do they do? – Hashim Aziz Mar 28 '19 at 07:27
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    @Hashim `LINES` and `COLUMNS` work the other way round: they provide information about the terminal size to applications. – Gilles 'SO- stop being evil' Mar 28 '19 at 12:59