Poor man's IFTTT in 3 lines of code | Hacker News Reader

Poor man's IFTTT in 3 lines of code

blog.lagentz.com

101 pointsfdorfbauer33 comments

Regarding the watermark generation process, although the bash script works, I immediately reach for my old friend (GNU) make(1) for that kind of task. Here's the Makefile:

    all: watermark
    
    done:=$(wildcard *-BD.pdf)
    src:=$(filter-out $(done),$(wildcard *.pdf))
    marked:=$(patsubst %.pdf,%-BD.pdf,$(src))
    
    watermark: $(marked)
    
    %-BD.pdf: %.pdf
    	./watermark_script.sh $< $@

The done+filter-out mumbo jumbo is there just because without it, -BD.pdf would be matched by .pdf, and therefore would generate -BD-BD.pdf... and so on (this is a loophole that the original bash script has, if parallel runs are timed correctly). Putting the output files in another directory is left ans an exercise for the reader.

The inotify script becomes:

    inotifywait -e create /home/user/pdfs && make -C /home/user/pdfs

There is another loophole here, in that the -BD.pdf are created in that directory, so the script gets run again. It doesn't matter for our makefile, which will do nothing on the second run, but there is the 'parallel runs if timed correctly' issue of the original bash script.

Generally, GNU make has a bunch of very interesting facilities, making it much, much more powerful than its ancestors (BTW, people interested may want to take a look at an implementation of non-recursive make[0], which — while being awesome — also shows up some useful make tricks).

Now, regarding the filesystem events and userland tools (which are awesome), I wish there was a unified front-end for this kind of things (inotify on Linux, fsevents on OSX...)

PS: I wish the comment form supported something like markdown, so that I could escape those wildcards, and more.

[0] http://stackoverflow.com/questions/559216/what-is-your-exper...

I'm sorry if I missed this, but why would you use make instead of bash?

    make figures out which files need updating
    make allows to write pattern rules (.c.o:, or %.o: %.c)
    you don't need to move files out of the 'inbox'
      to ignore them
    regenerating a file is a touch(1) away
    make is descriptive, not imperative (invaluable
      readability when you're coming back on it years later)
    should you ever need a second transformative step or
      a new dependency, it's trivial to insert, and make
      will figure stuff out
    make can trash intermediate results automatically
    make is crazy fast
    make variables are not environment variables
    make has multiple variable assignations, resulting in
      some early/late evaluation system (early with :=,
      late with =, $() is always late unless part of a :=)
    make outputs commands as they're executed (not with a global)
    make outputs nothing when commands are prefixed with @ (not with a global)
    make interrupts itself on a chain failure
    each make recipe line is executed in a distinct shell

You can set bash to do some of those things, or you can write bash code that do some of those, but most of the time you end up just coercing bash into behaving like make.

Of course I would not write everything in make. But knowing what make can do makes it the obvious choice for some tasks, and helps you coming up with a robust task/generator system in a snap, whereas coming up with a task/generator system in bash quickly is already not that easy, and making it robust even less.

"make is crazy fast"

Make calls the shell in order to run programs, correct?

(IOW, does make call execve directly, as the shell does?)

Indeed, execve in exec_command (job.c). That does not make it slow, since it's the very last part of the game, and one you could not really do without.

I use this to compile my LESS scripts already. I was always getting really frustrated with the options with LESS... either compile in the browser, or compile manually. Both were actually a bit of a pain, (caching being the biggest annoyance for browser compilation).

So I used inotifywait...

It would check for any modifications to my "CSS" folders recursively, and it would then run my custom less compilation script [1] ...

     #!/bin/bash
     while inotifywait -e modify -r /var/www/site/public/static/css/base; do
       cd /var/www/site/public/static/css; node compile;
     done

(I also hooked this up to Fab [2], and used a "start"/"stop" script [3] to pause the watch, so that I could manually control when inotify wait would be watching.

[1] https://gist.github.com/3497304 (My Custom LESS compile Script)

[2] http://docs.fabfile.org/en/1.4.3/index.html ( Fabric Command Line Tool )

[3] https://gist.github.com/3497308 (Sample Fab Command )

If it would benefit anyone to know in greater details, I'd be happy to write a blog post about automating compilation using these tools.

I solved this problem using http://leafo.net/lessphp/. Obviously that only works if you are programming in PHP, but I would imagine there are similar solutions available for other languages?

I found this a very intrusive site - I really dislike these animated slide in fade in distractions.

Not only the slide-in nonsense, but also the wiggling "Try Me!" arrow on the right. "You may also like" panels are becoming the Clippy of blogs and news sites.

Didn't see any of it with noscript on.

What's the "try me" pointing to? I tried clicking it and it did nothing, scrolling down it just points to blank right margin all the way ...?

I have no problem with "you may also like" and have found the links useful on occassion; anything animated without user interaction however is generally a turn off for me.

What's the "try me" pointing to?

Some kind of feedback tool about the site.

I used the feedback tool to inform the author that the "Try me!" was annoying enough to cause me to want to leave a nasty note.

So I guess it worked?

Yeah, it's pretty crappy as the article is actually fairly good but it's buried in excess.

Agreed; this was one I hit the Readability bookmarklet for straight away.

(Having said that, I went back afterwards and filled out the feedback form pointing out the annoying bits. It was a good article, and I wanted to repay the favour; I recommend you do the same if you agree.)

I've fixed the page, it's now more focused on the content. Thanks for all the great feedback!

The inotify script likely has a race condition; if the directory is modified while an update is running, it won't pick up the second modification, and will wait in a dirty state until it is modified again. If anyone wants a race-free solution, you should check out my NCD scripting language [1] which has an integrated command to receive directory change notifications (last example in Blockers section is a race-free solution to this problem).

[1] http://code.google.com/p/badvpn/wiki/NCD

After a very brief look, NCD looks very cool. I'm curious about the first complete example. Is it possible to implement a delay of a few seconds before the IP address is reset when the link is lost?

I've never seen why that would be useful. Switches and Ethernet cards aren't that unreliable in my experience, and I've plugged cables into a different switch in less than a few seconds :) I think if you really needed that, the nicest way would be to add it as a feature to net.backend.waitlink(). See source [1] of this command, and source of sleep() [2] to see how to use a timer. It may also be possible to hack it up without C-coding using blocker(), spawn(), sleep() and maybe other control commands.

[1] http://code.google.com/p/badvpn/source/browse/trunk/ncd/modu... [2] http://code.google.com/p/badvpn/source/browse/trunk/ncd/modu...

I've had a situation where an Ethernet connection would occasionally drop link for a very small fraction of a second, which unnecessarily shut down the interface and reset all TCP connections. Maybe a few second delay is too long, but 500ms seems reasonable. Also, maybe I wouldn't want to keep the old address blindly, but trigger a DCHP request if the link is dropped and restored before the timeout.

Also, thanks for the pointers into the source code. It looks like I would want to make the change around line 67 of net_backend_waitlink.c. I highly doubt I'll actually get around to doing it though :).

So you really need this, would that mean you may be able to use my language for something? If so, just say, and I can add this little feature, but I wouldn't like spending time for nothing :)

I haven't seen this problem recently, so don't worry about it. I appreciate the offer, though. Maybe some day I'll look into adding the feature for my embedded Linux systems, if I ever add Ethernet disconnection detection.

In 2006, as inotify first hit the kernel, some friends and I wrote fsniper [1] to accomplish this same task. There's also incron [2] which is similar. In today's parlance we'd call fsniper "opinionated" about its workflow.

[1] http://github.com/l3ib/fsniper [2] http://inotify.aiken.cz/

Someone please tell me what IFTTT is.

http://ifttt.com/wtf

Granted another linux specific method but I'd recommend incrond. Works very well for such tasks. http://www.cyberciti.biz/faq/linux-inotify-examples-to-repli...

I wrote something similar the other day, but failed to realize that having a utility that locks until there are changes (vs one that calls a program when changes happen) is much easier to integrate with bash scripts.

Great use-case for lnotify, haven't thought about that. Feels much cleaner than having cron jobs for conversions like these.

With the listen gem you can write crossplatform scripts (http://rubygems.org/gems/listen).

Hadn't come across listen, that's perfect for what I need. I've used notify on Linux but I wanted something a bit more portable for OS X.

For those of Perl persuasion, there are:

    * File::ChangeNotify (https://www.metacpan.org/module/File::ChangeNotify)
    * Filesys::Notify::Simple (https://www.metacpan.org/module/Filesys::Notify::Simple)

I've had some IFTTT-esque ideas with http://noflojs.org/ as well

Is usersnap.com really facilitating better feedback or is it just annoying people that visit your website?

Used as a feedback tool you are way faster to understand what your visitors mean. Used as a bug reporting tool it's unbeatable: you get exactly what your testers / users see in their browser. Please note that the Try-Me arrow is not part of usersnap - that's maybe the annoying part you are referring to?

Reply on news.ycombinator.com