Can a bash script be hooked to a file?

On linux, you can use the kernel's inotify feature. Tools for scripting can be found there: inotify-tools.

Example use from wiki:

#!/bin/sh

EVENT=$(inotifywait --format '%e' ~/file1) # blocking without looping
[ $? != 0 ] && exit
[ "$EVENT" = "MODIFY" ] && echo 'file modified!'
[ "$EVENT" = "DELETE_SELF" ] && echo 'file deleted!'
# etc...

There is an API called inotify for C programmers.

There are some tools that use it, e.g. incron and inotify-tools.

Indeed there is: entr(1) will run arbitrary commands when files change, and also provides an auto-reload option for restarting application servers.

edit: some examples

Rebuild if sources files change

$ find *.c | entr make

Launch and auto-reload test server if files change

$ ls *.py | entr -r python main.py

Providing a the agument +/path/to/fifo allows more complex scripting by instructing entr to write the name of each file that changes to a named pipe. The following will convert Markdown files in the current directory to HTML as they're edited

$ ls *.md | entr +/tmp/notify &
$ while read F
> do
>   markdown2html $F
> done < /tmp/notify