I have a program that I'd like to run, deluding it to believe that my terminal has less columns than it does, because that enables its native truncation, which I need.
I tried resize from xterm as suggested on askubuntu, however, that only works properly within xterm and can hardly be scripted.
I need something that I can use within a script to get the output of a taskwarrior command.
Exporting a different value for COLUMNS doesn't seem to change anything unfortunately.
The community answer is on the right track.
You can use tmux to create a detached session with the geometry that you want (column width, assume some number of rows).
tmux new-session -d -s task-session -x WIDTH -y HEIGHT
Then run your command.
tmux send-keys 'task' ENTER
Do not pipe command output to a file.
Dump the tmux pane to file:
tmux capture-pane -b temp-buffer -S -
tmux save-buffer -b temp-buffer output.txt
tmux delete-buffer -b temp-buffer
tmux man page is fairly well-written, but there's a lot there. I've updated my answer with actual commands.
Commented
Oct 24, 2020 at 18:50
buffer invalid error trying this code. But I found that capture-pane has a -p flag to print the buffer to stdout. The command that worked for me: tmux capture-pane -t ${tmux_session}.0 -pe -S -500.
Commented
Nov 19, 2022 at 3:08
The POSIX standard is, indeed, that the value of the COLUMNS environment variable supersedes other ways of obtaining the terminal size.
The standard describes one of the variable's purposes as "display[ing] data in an area arbitrarily smaller than the terminal or window" amongst other things, which matches what you want.
Those other ways are not standardized, but they are commonly an ioctl() of TIOCGWINSZ against a terminal device open file descriptor.
Your program does this; albeit that it just assumes without isatty() checks that its standard output is a terminal device.
Unfortunately, there are programs that do not respect the COLUMNS environment variable and yours is sadly one of them.
Instead it provides is own idiosyncratic non-standard mechanism, the detection and defaultwidth settings in its configuration file.
Adding to the sadness is that these operate the other way around to the COLUMNS environment variable.
They are a fallback, not an override.
You have to turn detection off, so that the program stops using TIOCGWINSZ and falls back to the default.
You have to then set a value for defaultwidth, which is 80 if unset and 65536 with detection forced off if zero.
Further addition to the sadness is that turning detection off applies to both width and height, unlike the POSIX standard COLUMNS and LINES variables which operate independently of each other.
So you have to concern yourself with a defaultheight as well, which is 24 if unset and 65536 with detection forced off if zero.
You could, alternatively, leave detection on and fool your program with a pseudo-terminal instead.
tmux is a complex way to achieve this, with all that key stuffing and intermediary file use.
Using Bernstein ptybandage and stty columns (against the internal pseudo-terminal) is simpler, takes one line, and outputs to standard output in real time as your program is running:
ptybandage sh -c 'stty columns 40 ; exec task'
ptybandage. djbwares.ptybandage. nosh Guide. Softwares.
ptybandage is superceded by execlineb which is part of the execline package in the AUR
The following does not yet work, but I suspect that you could make it work with some modifications. The commands create a new tmux session and modify the size of the pane before executing taskwarrior:
tmux new -d -s s1
tmux split -v
tmux resize-pane -t s1 -y 5
tmux send-keys "task > output.txt" ENTER
sleep 1
tmux kill-session -t s1
cat output.txt