Writing to the same file with awk and truncate - linux

Question

My system is Arch Linux and my window manager is DWM. I use dash as my shell interpreter.
I have written this extension shell script for my timer.
xev -root |
awk -F'[ )]+' '/^KeyPress/ { a[NR+2] }
NR in a {
if ($8 == "Return") {
exit 0;
} else if ($8 == "BackSpace") {
system("truncate -s-1 timer.txt");
} else if (length($8) == 1) {
printf "%s", $8;
fflush(stdout);
}
system("pkill -RTMIN+3 dwmblocks");
}' | tee timer.txt
The timer itself sits in dwmblocks status bar. I want to name my timers first and then let it start. But I don't think that's that important.
The purpose of this script - I want to input characters into the root window of DWM and have them appear in my status bar instantly. So, xev produces the key pressed information, then awk takes that information, finds the exact key (from all the information that xev outputs) and checks. If the key is "Return", awk exits (job done). If key is "BackSpace" awk calls truncate from the system. If it's a regular character key, then awk outputs it to timer.txt with tee (I could use "> timer.txt" too, I think, but I want to see the output in my terminal for debugging.
After every relevant keypress (single character) I fflush stdout. After all of that I finally call pkill so that dwmblocks knows that it should update. (dwmblocks issues cat operation on the file)
Okay, "Return" and character input works fine. But there's a problem with "BackSpace". I've read about it a bit (I'd say I'm still a Unix newbie even though I've been using Linux for two years now) and I found out that writing to the same file from different processes is bad news. Still. Could it be done somehow? The fact is that truncate only writes to the file when awk, doesn't, so, maybe, it wouldn't be that big of a deal?
This exact script worked earlier yesterday but now it doesn't. At first, I tried using sed instead of truncate and truncate seemed to let me delete characters from timer.txt but now truncate seems to not work anymore too. Well, it kinda works. I can input my characters and then I can delete them. BUT. After pressing Backspace I can not enter any more characters. If I try to enter a character Backspace stops working too.
So yeah. I'd have several questions. First - what the hell is the problem? As I've said, it used to work and now it doesn't. Am I wandering into undefined behavior in this script?
Second - could this be done - meaning - could I somehow write and delete from the same file. Maybe with some other tool, not awk?
Thanks in advance.

Answer 1

This probably isn't an answer but it's too much to go in a comment. I don't know the details of most of the tools you mention, nor do I really understand what it is you're trying to do but:
A shell is a tool to manipulate files and processes and schedule calls to other tools. Awk is a tool to manipulate text. You're trying to use awk like a shell - you have it sequencing calls to truncate and pkill and calling system to spawn a subshell each time you want to execute either of them. What you should be doing, for example, is just:
shell { truncate }
but what you're actually doing is:
shell { awk { system { shell { truncate } } } }
Can you take that role away from awk and give it back to your shell? It should make your overall script simpler, conceptually at least, and probably more robust.
Maybe try something like this (untested):
#!/usr/bin/env bash
while IFS= read -r str; do
case $str in
Return ) exit 0 ;;
BackSpace ) truncate -s-1 timer.txt ;;
? ) printf "%s" "$str" | tee -a timer.txt ;;
esac
pkill -RTMIN+3 dwmblocks
done < <(
xev -root |
awk -F'[ )]+' '/^KeyPress/{a[NR+2]} NR in a{print $8; fflush()}'
)
I moved the write to timer.txt inside the loop to make sure tees not trying to write to it while you're truncating it - that may not be necessary.