Why does pkill not work with rofi?

Question

I want to toggle rofi an application launcher with one keybinding. I made a shell script,

#!/bin/bash

rofi -dpi 1 -modi drun -show drun -show-icons

And bound it to some key. This works but to toggle toggle it back to not running, I tried:

pkill -0 rofi || rofi -dpi 1 -modi drun -show drun -show-icons

That won't let rofi start to begin with. I tested pkill emacs || emacs and that does toggle emacs. Could someone tell me how can I fix it for rofi? And, what does the argument "-0" do? I could not find it in the manual.

I did find this,

I spent a decent chunk of time beating my head against this, and then I realized that rofi stores its pid. We can access the pid file via the config, which in turn gives us access to all the information we need. Before I get to the exciting stuff, though, it's important to mention safety. It's a really good idea to limit your process count (somehow) in case you create a runaway script. Speaking from experience, it could be half an hour before you can free up enough memory to switch to another tty and kill everything.

if [[ 10 -lt $(pgrep -c -f "$0") ]]; then
    pkill -f rofi
    pkill -f "$0"
    exit 1
fi

I am not sure if this is exactly what I want or what does that code even do.

Answer 1

tl;dr: As it is, your code says "if pkill doesn't find a rofi process that we can kill, then run rofi". It doesn't actually say to kill it if it's already running, just to crudely check if it's running and if not, start it. If you really want it to kill it every time, remove -0.

---

-0 kills with signal 0, which just checks whether the calling process has the authority to kill the process, and the process exists. It doesn't actually kill anything.

From man 3 kill:

If sig is zero (the null signal), error checking is performed but no signal is actually sent. The null signal can be used to check the validity of pid.

Passing the signal this way is documented here in man pkill (somewhat unclearly as -signal, which really means -[signal]):

-signal --signal signal: Defines the signal to send to each matched process. Either the numeric or the symbolic signal name can be used.

If you want to kill the process, use pkill without -0 to use the default signal (SIGTERM), or pass another signal name or number to use that instead.

As a side note, this method of handling application lifecycle is pretty basic, and can fail for a number of reasons (eg. kill isn't synchronous, so there's no guarantee that the previous rofi will be gone before the next one appears if you replace -0 with a fatal signal). For that reason it would ideally be better to have the application itself, or a service manager, handle this more reliably.