How do I read the first line of a file using cat?

Question

How do I read the first line of a file using cat?

Answer 1

I'm surprised that this question has been around as long as it has, and nobody has provided the pre-mapfile built-in approach yet.

IFS= read -r first_line 

...puts the first line of the file in the variable expanded by "$first_line", easy as that.

Moreover, because read is built into bash and this usage requires no subshell, it's significantly more efficient than approaches involving subprocesses such as head or awk.