Can a bash script be hooked to a file?

Question

I want a script to sleep unless a certain file is modifed/deleted (or a file created in a certain directory, or ...). Can this be achieved in some elegant way? The simplest thing that comes to my mind is a loop that sleeps for some time before checking the status again, but maybe there is a more elegant way?

score 34 · Accepted Answer · answered Jan 07 '11 at 13:20

34

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...

answered Jan 07 '11 at 13:20

shellholic

6,255

1

Most unices have a similar feature. Unfortunately each has its own interface, and many have no shell API. – Gilles 'SO- stop being evil' Jan 07 '11 at 18:40

score 4 · Answer 2 · answered Apr 06 '11 at 08:43

4

There is an API called inotify for C programmers.

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

answered Apr 06 '11 at 08:43

Mikel

57,299
15
134
153

Tremendous, that’s the stuff. Cheers Mikel! – Paul D. Waite Apr 06 '11 at 08:52

eradman · Answer 3 · 2013-07-04T12:17:33.627

3

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

edited Jul 04 '13 at 12:17

answered Jul 04 '13 at 04:33

eradman

354

Intriguing, but weird. So ... it reads a bunch of file names on standard input, and runs the specified command when one of them changes? – tripleee Jul 04 '13 at 05:36
Looks interesting, thanks! Is there any way to have entr pass on the name of the file that has changed as well? – Tobias Kienzler Jul 04 '13 at 07:00
As of the 2.7 release, the special /_ argument (somewhat analogous to $_ in Perl) is replaced with the name of the first file that changed – eradman Dec 06 '14 at 11:50
@TobiasKienzler also, maybe you could use xargs -I % entr <command> % to pass the file name from the previous pipe of find *.ext | or use the special argument as @eradman suggests – alchemy Feb 12 '22 at 22:50

Can a bash script be hooked to a file?

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