Episode: 2003
Title: HPR2003: Using the Incron file watching daemon
Source: https://hub.hackerpublicradio.org/ccdn.php?filename=/eps/hpr2003/hpr2003.mp3
Transcribed: 2025-10-18 13:10:29

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This is HPR episode 2003 entitled, using the Incron file watching demon and in part of the series bash-crypting.
It is hosted by me using it and in about 12 minutes long.
The summary is, I briefly introduce the Incron file watching demon and give an example of how I use it.
This episode of HPR is brought to you by an honesthost.com.
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Hey HackerPupWook Radio, this is Be Easy once again signing in.
Coming in with another episode, this time I'm going to be talking about the file watching demon called Incron.
Now I said I would do something with this one before and now I'm going to do it because it's a pretty
cool utility and I don't use it all the time but when I do it's always very useful and I can see
how it has lots of applications, situations where the haven't thought of. So with no further ado,
here we go. So from the man page, Incron or more specifically Incron tab, which related to
Cron, Incron tab is a file that contains instructions to the Incron D Damon of the general form.
Run this command on these files, events. There are two categories of tables, system tables with
fruit privileges and user tables with user privileges. And so instead of reading the entire
rest of the man page, I'll just let you read it. But if you're installing it, it's usually just
called Incron and whatever repository you're looking at. As far as I know, it's only available for
Linux. I'm not exactly sure about BSD. I could probably look that up. But it is a very useful tool.
So one thing they can do with it and something I use it for is to automatically have tasks happen
when a file is either created, modified, deleted, moved, changed status. And they're the format
goes the path, the mask and the command. Now the mask is usually what you want to do.
And it's usually the event symbols. And so the event symbols are in access, which means when
the file is accessed in a tribe, which is when metadata about it, the file change, like permissions
or D timestamps, and close right and no right and create and delete and delete shelf and modify
and move shelf and move from and move to and open. And then there's in all events and and don't
follow in one shot and in only there. And those are all separated. All those different words are
separated with underscores. But the one I'm going to talk about right now is Increate. And so
that means when a new file is in the directory, do something to it. And what I use it for
in this example is I have a Raspberry Pi 2 with PanDoc installed. And I've talked about PanDoc
before. It's a utility that is used to create multiple types of documents from other types
of documents. For instance, from markdown to HTML or to PDF or from from wiki.no, you know,
the wiki.no to PDF or HTML or even stuff like reveal.js slides or slidey.js slides.
I use it all the time. And I have lots of other computers that I don't necessarily want install
PanDoc on because PanDoc itself is not that big. But if it's depending on what distribution you
download, you download it with sometimes you have to install all of Haskell with it. And other times
you have to also install if you want to use PDFs, you always have to install text live. And the
minimum amount of text live is like a half a gig. So if you're hard up on hard drive space,
you might not want such a heavy program. Like I have two computers that I use all the time. Both
of them have 32 gig SSDs. I know I could put faster ones in bigger ones in there. I have no
reason to. But anyway, so after that, there is some wild cards that you can use to identify the
files and the directories that the wild cards are dollar sign at, which is the watch file system.
So if you're going to refer to the path of it in the command that you're writing, it is dollar sign
at symbol or on the American keyboard is the shift to. And then you also have the dollar sign
pound or dial sign, you know, the little tic-tac-toe looking thing, which is the shift three. And that is
the event related file name. So if you want to the full the full path of a file, it would be
dollar sign at slash dollar sign pound. And then there's also dollar sign percent and dollar sign
ampersand and those for or for event tags. And I haven't used event tags. I haven't had to use
for them yet. So I'm not going to talk about them because I'm not exactly sure how they work and
how best to use them. And the main page it has to be examples, but it's kind of a generic example.
I want to go over a specific example of how I use it. So in particular, I have a couple of folders
on my Raspberry Pi inside of a folder and on the inside of the user's home folder. There's a
directory called docs inside of docs. There's all these other folders. One for letters, one for
notes, one for things I will turn to PDFs, one for reveal.js slides, another one for standard
operating procedures, and another one for validation protocols, some of the things I do for work.
In any one of those, there might be a I will throw a markdown file because as I've said,
another episode, I write almost exclusively in markdown. And I have developed these really
specific templates that all the documents that I want to write go into. And they, if it's a note,
I always want to output it in a certain format. If it's a standard operating procedure, I always
want it in the same format. And it's annoying to have to reformat those. And I can use templates and
then copy and paste stuff in there. But it's anyone working with where processes knows that,
copying paste and then updating the styles is not very fast. If you want to do it really quickly,
and you want to do it consistently the same way every time. So instead, I also have one for
letters. If I want to write a letter that needs to be mailed and that's a professional format
at one, I have one for that too, which uses, which is a PDF, which inserts a signature and a whole
bunch of other stuff. So it's pretty cool. But what I do is I write the pen doc command in
a.sh file. So in the shell command, and all I have to do inside of my cron tab. So to use,
to make a new cron tab job, you have to get to the cron table in cron table, which is just like
cron. So if I keep on mixing up, that's why it's very similar to cron. So if you ever have used
cron before, which is the kind of the scheduler. Instead of going cron tab dash A, you do
in cron tab dash A. And that will bring you to your your cron your in contact. So for instance,
on my first line of my in cron table, I have slash home slash user slash docs slash PDF. So in that
directory, the next command is in underscore create. And then I say slash home slash user mkpdf.sh
And then I have dollar sign at slash dollar sign pound, which if I read it all the way out, it's
in the director of slash home slash users slash docs slash PDF. When a file is created, run the
command mkpdf.sh on the file of the full path. And so what that file looks like is very simply,
it's a bash script that has file name and I get the base name of the file. So you know,
so my mkpdf takes one argument, which is the file name. So I get the base name of the file. I change
the I change to that md to dot PDF. I run a markdown command that tells about the geometry of the
page and the fonts and anything else and and the template that I use. And then I say dash O, put it
into the slash home slash pie slash docs slash done directory and then remove that original file.
I probably could move that original file to a completed folder so that if everyone's
to do it again, I could and maybe that's something I'll do in the future. But for now, I just remove it
so that when I go back and look at the directory, I can tell that it's done. Because if you don't,
you could look in the done folder to see if it's done. But it's for me, it's just easier to look
in the pending folder to see how many have already completed and how many still need to be completed.
And so, you know, it's a pretty simple thing. So after I throw a file in there, the job starts to
run and if you do a tail F on the slash bar slash syslog, you can see that there's an event
in the syslog that says in crowns running and it says the command name and the file name. And then
so you know that it's working. Or if you open up top or each top, you'll see that the command starts
in and then it'll start running. And so what I usually do is I have the root directory, so the
home directory of that user on the Raspberry Pi mounted to my different workstations.
And when I have a document that I want to throw into one of those folders, I just open it up,
drag it from where I'm working on and drag it in there, or you know, control C and then control V
in the other directory. Depending on what I'm doing and how complicated it is, not big the file is,
come back a couple seconds or a couple minutes later. Usually no longer than 60 seconds.
And then pick up the file in the drop off spot in the dead folder and email it, continue to work
on if there's post processing that I want to do on it or anything else that my job might acquire.
It's really convenient. It allows it's doing, it's kind of the Unix philosophy of have, you know,
do one job really well. So that Raspberry Pi is pretty much only doing a couple of little
medial tasks like running Pandak, running a couple of little things on in-crond and crond.
That's all days in-crond and crond jobs. And you know, it's very useful. There's, if you think
about the nature of this command, it's very powerful. And if you've ever used Windows Systems,
and you've ever tried to use a Demon or Watcher on Windows Systems, it's not a straightforward
and to get something that is this straightforward and the simple, you should cost a pretty good amount
of money. So I really appreciate having a command like this in my, just a simple install command
the way from any Linux distribution I have. And I appreciate your time once again on Hacker Public
Radio. This is easy signing off.
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