bash: `entr` utility

using entr when writing bash scripts is precious. this little utility listens for the file you give it to be modified and executes whatever it is set to execute. you can even execute the same file that entr is listening to.

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

$ entr -sc "source <file_to_execute>" <<< $( echo <file_to_listen> )

i would also like to point out that there can be completely different file_to_execute and file_to_listen.