Crontab

When editing crontabs, you must edit the root cron. I found that editing the user cron file, will not run the script which you desire. Today I've read that Crontab is a file/process which acts to automate script files insync with time. The best example of what a Cronjob is, are monthly backups that people perform incase data becomes corrupt or they get a virus. Rather than doing it manually, or downloading some process heavy software to do it for you, you edit the Crontab file, and specify script/s that you want to run.

Below will be the list of code you will use + an example in which you can see if you have set up a Cronjob correctly. Yes, I'm one of those people whom enjoy peace of mind - knowing something works is better than thinking something works.

Here's what you need - but don't execute them yet, keep reading:

$ sudo crontab -e # To access the root cron file
$ sudo touch /etc/cron.d/ # To refresh the cron process with your new settings

The first time you access the crontab file, it will ask you what editor you wish to use - I use nano. I'm puzzled about the location of the crontab file as I originally thought /tmp files were deleted on shutdown. Anywho...the default crontab might look like this:

# Edit this file to introduce tasks to be run by cron.
#
# Each task to run has to be defined through a single line
# indicating with different fields when the task will be run
# and what command to run for the task
#
# To define the time you can provide concrete values for
# minute (m), hour (h), day of month (dom), month (mon),
# and day of week (dow) or use '*' in these fields (for 'any').#
# Notice that tasks will be started based on the cron's system
# daemon's notion of time and timezones.
#
# Output of the crontab jobs (including errors) is sent through
# email to the user the crontab file belongs to (unless redirected).
#
# For example, you can run a backup of all your user accounts
# at 5 a.m every week with:
# 0 5 * * 1 tar -zcf /var/backups/home.tgz /home/
#
# For more information see the manual pages of crontab(5) and cron(8)
#
# m h  dom mon dow   command

The information you need to know about the Cronfile, and editing, it are in the Cronfile itself. But here's a quick run down.

You add a cronjob by using the following syntax:

* * * * * /path/to/crontest.sh

Notice there are 5 positions. From left to right they specify:

1. Minute, 0 - 59
2. Hour, 0 - 23
3. Day of month, 1 - 31
4. Month, 1 - 12
5. Day of week, 0 (Sunday) - 6 (Saturday)

So cronjobs are on an annual cycle. The asterisks example is basically telling the computer that the cronjob is to be run every minute.

Testing Crontab/Cronjobs
Here's a quick way to test crontab.

1) Open terminal and move to a directory of your choice.

2) Make a new file and call it "crontest.sh".

3) Give crontest.sh user executable permissions. Or 755, which also enables write-perms:

$ sudo chmod u+x crontest.sh

4) In crontest.sh, add this script. This is just a script that echos the date, given the format you desire.

#!/bin/bash

date +"%d-%m-%Y" >> /path/to/the/storage/file.txt # Date ID, >> tells terminal to append to end of file

Save and exit.

5) Open up the crontab file:

$ sudo crontab -e

6) Look at the current time and edit the crontab file with a few minutes added to the current time. So, if the time is 17:40, make the crontab entry look like this:

...
# For more information see the manual pages of crontab(5) and cron(8)
#
# m h  dom mon dow   command
45 17 * * 1-5 /path/to/crontest.sh

Close nano and save the changes.

7) Now refresh crontab:

sudo touch /etc/cron.d/

Giving your password where required.

8) Sit and wait. When 17:45 comes around, the script will be run. Check the /path/to/the/storage/file.txt file and see if the date is there. If yes, then your method is good. If no, then check that permissions are correct; check that the code and all pathways are right.

Hope you enjoy this HowTo. I'm probably going to make a cronjob to append CPU temps to a file such that I can graph them and see if certain parameters are better/worse for my CPU performance.

Here are two sites which I found helpful:
http://klenwell.com/press/2010/11/cron-d/
http://kevin.vanzonneveld.net/techblog/article/schedule_tasks_on_linux_using_crontab/

Enjoy, Ciao!