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+# tmux tricks #1: battery status indicator
+
+[tmux](https://github.com/tmux/tmux/wiki) is a program that allows
+you to create multiple virtual terminals in the same shell and keep
+your terminal sessions open even after the shell exits. A common
+use case for both of these features is connecting to a remote server:
+with *multiplexing* you can use multiple terminals with a single
+SSH connection, and the session persistence feature leaves the
+remote programs running even when you disconnect from the server.
+Having multiple virtual terminals open on the same screen and easily
+switching between windows is also useful when working locally.  In
+fact, tmux is the first command I use pretty much every time I use
+my [netbook](../2022-09-10-netbooks) - even though
+[one shell is all I need](../2023-02-25-job-control).
+
+Since tmux is quite a complex program, I never learned the details
+of how it works. I learned how to do the couple of things I need,
+and use the default settings for everything. But sometimes I do
+feel the need to improve my workfow a bit, and what better excuse
+for tinkering with a software tool than writing about it?
+
+In this post I have one simple goal: display the battery level in
+tmux's status bar. I am going to focus on OpenBSD, because this is
+the system where I need this: on my Linux laptop I use my window
+manager status bar to display this information, but on my underpowered
+OpenBSD laptop I rarely use a graphical environment, and I rely on
+tmux as a desktop environment.
+
+## tmux commands
+
+tmux can be controlled using a variety of key bindings, all prefixed by
+a "prefix" key combination, which by default is `Ctrl+B`. For example,
+`Ctrl+B C` creates a new window and `Ctrl+B %` splits the screen
+vertically.
+
+Each key binding calls a *tmux command*. Not all commands are bound to
+a key combination, and there are other ways to call them, including:
+
+* Typing them in the tmux command line, invoked by pressing `Ctrl+B :`.
+* Calling them from any shell as `tmux command ...`.
+* Adding them to the tmux configuration file (by default `~/.tmux.conf`)
+  to be called on startup.
+
+## The tmux status bar
+
+tmux has a status bar, which can be any size between 0 (disabled)
+and 5 lines long. The default is 1 line with date, time and hostname
+displayed on the right, and some information on the current session
+on the left.
+
+My goal is to customize the status indicators on the right. After
+consulting the [man page](https://man.openbsd.org/tmux) for a while,
+I figured out that the command I need is:
+
+```
+set status-right "Hello, World!"
+```
+
+To show the output of a command one can use the `#()` notation:
+
+```
+set status-right "#(date +'%H:%M')"
+```
+
+This example is actually a bit redundant, because the string is
+passed through [strftime](https://man.openbsd.org/strftime) before
+displaying. So one could simply use `set status-right "%H:%M"` to
+specify a date in the HH:MM format.
+
+The status bar by default refreshes (and so re-runs the commands
+in `#()`) every 15 seconds, but this can be changed with the
+`status-interval` command.
+
+## Battery information
+
+The next step is getting the battery status from a command. On Linux
+I wrote a little [script](https://git.tronto.net/scripts) to automate
+this, but I don't have one (yet!) for OpenBSD. On this OS, this
+information can be retrieved with `apm`:
+
+```
+$ apm
+Battery state: high, 91% remaining, 367 minutes life estimate
+AC adapter state: not connected
+Performance adjustment mode: manual (1000 MHz)
+```
+
+I am only interested in the `91%` part, so I should pass this through `grep`
+
+```
+$ apm | grep -o '[^ ]*%'
+91%
+```
+
+This command means "print the strings that consist of a % symbol
+preceded by any number of non-space characters". To learn more about
+`grep`, check out [my blog post](../2023-08-20-grep).
+
+## Putting it all together
+
+We can get our status line to show the battery status with this
+tmux command:
+
+```
+set status-right "#(apm | grep -o '[^ ]*%%')"
+```
+
+Cool! But did you notice the double `%%`? That's because the string
+is formatted with `strftime`, as I mentioned before, so we must
+escape the `%`.  Tricky!
+
+## More status info
+
+We reached our goal, but in doing so we also got rid of all the information
+that was already part of the status. I would like to have at least some of
+it back.
+
+One way to solve this is to use the `-a` flags for the `set` command,
+like this:
+
+```
+set -a status-right " #(apm | grep -o '[^ ]*%%')"
+```
+
+This will append the new string to the current status, instead of
+replacing it. Alternatively, we could declare exactly all the stuff
+that we want there:
+
+```
+set status-right "#(apm | grep -o '[^ ]*%%') | %Y-%m-%d | %H:%M"
+```
+
+But what if one wants even more information there? I don't want to have
+a cluttered status line, but for example being able to see at a glance if
+I am connected to wifi or not would be convenient.
+
+A simple solution is writing a shell script that prints out all this
+information, save it somewhere in the `$PATH`, and then use
+`set status-right "#(status_script)"`. I already have such a script
+for Linux, and now I added one for OpenBSD in my
+[scripts repository](https://git.tronto.net/scripts).
+
+## Configuration file
+
+Finally, we can have our status bar set on startup by adding the
+following line to our configuration file:
+
+```
+set -g status-right "#(status_script)"
+```
+
+...wait a minute, that is the `-g` flag about? Witout it, I get the
+following error on startup:
+
+```
+/home/sebastiano/.tmux.conf:1 no current session
+```
+
+Apparently, commands run *without* `-g` only apply to the current
+"session", but the configuration file is sourced *before* a session
+is created. The `-g` is used to apply this command globally. My
+understanding of this is still superficial, but for now I am happy
+that the battery status is there.  Maybe I'll learn about sessions
+at some point, and I'll write a new blog post about them :)