Why is this line here and not with the rest of includes?

Newly added letters unsort the list. The new list is missing 'T:'.

What's up with this line?

What's up with this comment? In general, if you look around, there are no comments of the sort in the file.

The style used in the file (and most of the codebase) is "if (condition) {". "{" on a separate lines is inconsistent with your own further uses later.

All variables are supposed to be declared at the beginning of the function.

The first line used "sizeof tagstring", but the second hardcodes 199. Doing snprintf followed by strncat is very odd. This should just snprintf to the target buffer. The argument should be quoted.

style(9) requires parenthesis around the argument, i.e. "sizeof(tagstring)".

popen is a bad idea here. This program execve's out of the code dong the original popen making it impossible to do a proper cleanup with pclose. While likely harmless, it does look weird.

The real problem is the fact that the string goes through the shell which introduces unnecessary complications due to shell parsing.

Instead, the code should fork + exec on its own.

Nothing informative is printed should popen fail.

This execvp plays no role. If it fails, the code drops down to line 247 which contains an idential execvp.

This line now significantly exceeds 80 characters mark.