LINUX How to use crontab

Crontab

The crontab (cron derives from chronos, Greek for time; tab stands for table) command, found in Unix and Unix-like operating systems, is used to schedule commands to be executed periodically. To see what crontabs are currently running on your system, you can open a terminal and run:
sudo crontab -l
To edit the list of cronjobs you can run:
sudo crontab -e
This wil open a the default editor (could be vi or pico, if you want you can
change the default editor) to let us manipulate the crontab. If you save
and exit the editor, all your cronjobs are saved into crontab. Cronjobs are
written in the following format:  
* * * * * /bin/execute/this/script.sh 

Scheduling explained

As you can see there are 5 stars. The stars represent different date parts in the following order:
  • minute (from 0 to 59)
  • hour (from 0 to 23)
  • day of month (from 1 to 31)
  • month (from 1 to 12)
  • day of week (from 0 to 6) (0=Sunday)

Execute every minute

If you leave the star, or asterisk, it means every. Maybe that’s a bit unclear. Let’s use the the previous example again:
* * * * * /bin/execute/this/script.sh
They are all still asterisks! So this means execute /bin/execute/this/script.sh:
  • every minute
  • of every hour
  • of every day of the month
  • of every month
  • and every day in the week.
In short: This script is being executed every minute. Without exception.

Execute every Friday 1AM

So if we want to schedule the script to run at 1AM every Friday, we would need the following cronjob:
0 1 * * 5 /bin/execute/this/script.sh

Neat scheduling tricks

What if you’d want to run something every 10 minutes? Well you could do this:
0,10,20,30,40,50 * * * * /bin/execute/this/script.sh 
Or crontab allows you to do this as well: 
*/10 * * * * /bin/execute/this/script.sh

Storing the crontab output

By default cron saves the output of /bin/execute/this/script.sh in the user’s mailbox (root in this case). But it’s prettier if the output is saved in a separate logfile. Here’s how: 
*/10 * * * * /bin/execute/this/script.sh >> /var/log/script_output.log 2>&1

Explained

Linux can report on different levels. There’s standard output (STDOUT) and standard errors (STDERR). STDOUT is marked 1, STDERR is marked 2. So the following statement tells Linux to store STDERR in STDOUT as well, creating one datastream for messages & errors:
 2>&1
Now that we have 1 output stream, we can pour it into a file. Where > will
overwrite the file, >> will append to the file. In this case we’d like to
to append:
>> /var/log/script_output.log

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