How to use perl to modify crontab?

Question

How could I append a cron task in crontab via a perl script?
I thought of the following:

#!/usr/bin/perl  

use strict;  
use warnings;  

`crontab<<EOL  
00 * * * * /home/slynux/download.sh    
EOL`

I don't want to mess up things, so am I on the right track?
Also if I append it, how would I remove it? I am new in Perl

Stack overflow is a place for asking specific questions about programming problems you are having. Please post the code you've written, what it is doing, and the problem you are having. Please provide any relevant source material as well. — xaxxon, Aug 08 '13 at 18:39
@xaxxon:The way you have put it I might as well delete the OP completely. I have not written any code, I am looking for a way to go.See this relevant thread http://stackoverflow.com/questions/18132144/handle-race-condition-between-2-cron-tasks-what-is-the-best-approach?noredirect=1#comment26553722_18132144 — Cratylus, Aug 08 '13 at 18:41
This seems like a bad idea. If you want to schedule a job to run once or twice at a specific time, look into `at(1)` instead of editing and then unediting the crontab. — mob, Aug 08 '13 at 18:41
@Cratylus you posted some code. does that do what you want? if not, what is wrong with it? — xaxxon, Aug 08 '13 at 18:43
The Unix command called `at`. Read the documentation by running `man at` or `man 1 at`. — mob, Aug 08 '13 at 18:43
@xaxxon: I am looking for how would I "undo" this command, if I follow this way — Cratylus, Aug 08 '13 at 18:44
I keep my crontab in a file (under source control), and use `crontab filename` to install it. That guarantees that I can keep track of what changes I've made, and roll back to an earlier crontab if I mess things up. It's hard to see what a Perl script would add to this process. You can certainly use a Perl script to update the file and/or to invoke the `crontab` command; personally, I use an editor and a shell for that. — Keith Thompson, Aug 08 '13 at 18:50
@mob:+1 interesting!So `at` is a one shot scheduling? Could I use `at` to make script reschedule itself via `at`? — Cratylus, Aug 08 '13 at 18:51
@KeithThompson:It is only to avoid the manual modification of crontab and let the script finish their jobs unattended — Cratylus, Aug 08 '13 at 18:52
@mob:Could you please check this if you spare some time http://stackoverflow.com/questions/18132144/handle-race-condition-between-2-cron-tasks-what-is-the-best-approach?noredirect=1#comment26553722_18132144 ? — Cratylus, Aug 08 '13 at 18:53
@Cratylus: You still have to write the script, presumably manually. It's not at all clear what value you're trying to add to the existing process of using the `crontab` command. I'm not saying that what you're trying to do isn't useful, just that you haven't explained it. — Keith Thompson, Aug 08 '13 at 18:56
@KeithThompson:I will write it once to solve a recurrent problem — Cratylus, Aug 08 '13 at 18:58
@KeithThompson:Check this for the why:http://stackoverflow.com/questions/18132144/handle-race-condition-between-2-cron-tasks-what-is-the-best-approach?noredirect=1#comment26553722_18132144 — Cratylus, Aug 08 '13 at 19:07
Apropos daily WTF about a situation where you probably should have used `at`: http://thedailywtf.com/Articles/A-Crony-Joke.aspx — mob, Aug 09 '13 at 16:13

Gilles Quénot · Answer 1 · 2013-08-08T19:18:49.003

6

Quick & dirty way :

#!/usr/bin/perl  

use strict; use warnings;  

`(crontab -l; echo "00 * * * * /home/slynux/download.sh") | crontab -`;

Another (better) approach :

#!/usr/bin/perl  

use strict; use warnings;  

open my $fh, "| crontab -" || die "can't open crontab: $!";
my $cron = qx(crontab -l);
print $fh "$cron\n0 * * * * /home/slynux/download.sh\n";
close $fh;

To remove the crontab line(s) with /home/slynux/download.sh :

#!/usr/bin/perl  

use strict; use warnings;

open my $fh, "| crontab -" || die "can't open crontab: $!";
my $cron = qx(crontab -l);
$cron =~ s!.*/home/slynux/download\.sh.*!!g;
print $fh $cron;
close $fh;

edited Aug 08 '13 at 19:18

answered Aug 08 '13 at 18:41

Gilles Quénot

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This is to append? What is the `-` in the end? And how would I remove it from crontab later? – Cratylus Aug 08 '13 at 18:43
Yes, it's to append. That's why I call `crontab -l` first. The `-` mean _read from stdin_ for the shell. – Gilles Quénot Aug 08 '13 at 19:04
But how can I remove it? – Cratylus Aug 08 '13 at 19:06
run `crontab -e` and remove the line !? – Gilles Quénot Aug 08 '13 at 19:12
You mean remove it manually? But my aim is to remove it programmatically – Cratylus Aug 08 '13 at 19:13
Run `crontab -l`; capture and save the output in a file. To restore the crontab, run `crontab filename`. – Keith Thompson Aug 08 '13 at 19:18
See my edited post, added _remove_ solution. – Gilles Quénot Aug 08 '13 at 19:19
Thank you.1) What is the `|` in the start of `open`? 2)`=~ s!.*/home/slynux/download\.sh.*!!g;` I think removes this but how come you ommited `download`? To reduce typing? – Cratylus Aug 08 '13 at 19:24
The `\n` in `$cron\n` is not required since crontab files are supposed to have the newline. If editing manually, the environment will force you to add a newline at the time of saving (Eg: emacs) – Joel G Mathew Jul 18 '17 at 08:20

score 0 · Answer 2 · answered Aug 09 '13 at 07:17

0

A quick search on metacpan returns Config::Crontab. While I have never used this module, it looks like it would do what you want.

answered Aug 09 '13 at 07:17

mirod

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How to use perl to modify crontab?

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