tmux Script Percentage Issue

August 17, 2018

I created a workspace script for this project that starts a tmux session with a side pane set to 25% of the terminal's width.

I like to use iTerm2 in full-screen when working on software projects. However, the above script starts a tmux session whose side pane seems to be more like 33% of the terminal's width.

I noticed that this issue only occurs when I resize my terminal emulator from it's default dimensions. Also, running the tmux commands directly creates a correct layout. As a result, I had a suspicion that the script is the culprit.


After scouring the internet for hours, I discovered that there was a change in tmux from version 2.5 and 2.6 that may be the cause of my problem:

All new sessions that are unattached (whether with -d or started with no terminal) are now created with size 80 x 24. Whether the status line is on or off does not affect the size of new sessions until they are attached.

I am currently on version 2.7 (tmux -V). In my script, I am also creating a detached session before attaching so that I can set up the initial layout (create panes, send commands, select pane, etc.).

Using this method, I was able to confirm that the changes in version 2.6 breaks my script in respect to percentages.


One simple solution would to be to keep tmux pinned at version 2.5 (explained in the link above). I think this would have been a suitable approach if this issue was a bug. However, according to that change log, this is intended behavior.

According to the tmux man page, the new-session command accepts two options (-x and -y) to set the detached session's size:

If -d is used, -x and -y specify the size of the initial window (80 by 24 if not given).

Unfortunately, this may have broke many tmux scripts when it was released. Fortunately for me, I only had a few scripts to update.

By adding -x $(tput cols) -y $(tput lines) to my new-session command, the layout seems to be fixed.

tmux new-session -d -n "develop" -s $session -x $(tput cols) -y $(tput lines)


The -x and -y options sets the session's initial width and height, respectively.

$() tells the executer to evaluate whatever is inside the parenthesis first.

In our situation, tput writes the following terminal attributes (cols and lines) to standard output. As a result, we are passing in the current width and height of our terminal to the new detached session.

Final Thoughts

My OCD can finally rest whenever I start up this website's tmux workspace.

Many tmux script examples out there are still using new-session without those size options since they were probably written before tmux version 2.6. This caused me many hours of head scratching that I hope to save you from.

Copyright © 2017 - 2018 David Tran