bash: `entr` utility

using entr while writing bash scripts is precious. this small utility listens to the file, you give it, being modified and executes whatever it is set to execute. you can even execute the same file to which entr listens to.

so, in my case, as i am not a pro bash script writer, the usage is:

i also want to note that there may be completely different file_to_execute and file_to_listen.