CRONTAB

Crontab

Scheduling explained

As you can see there are 5 stars. The stars represent different date parts in the following order:

1. minute (from 0 to 59)
2. hour (from 0 to 23)
3. day of month (from 1 to 31)
4. month (from 1 to 12)
5. 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:

1. every minute
2. of every hour
3. of every day of the month
4. of every month
5. 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

Get it? The script is now being executed when the system clock hits:

1. minute: 0
2. of hour: 1
3. of day of month: * (every day of month)
4. of month: * (every month)
5. and weekday: 5 (=Friday)

Execute on workdays 1AM

So if we want to schedule the script to Monday till Friday at 1 AM, we would need the following cronjob:

0 1 * * 1-5 /bin/execute/this/script.sh

Get it? The script is now being executed when the system clock hits:

1. minute: 0
2. of hour: 1
3. of day of month: * (every day of month)
4. of month: * (every month)
5. and weekday: 1-5 (=Monday til Friday)

Execute 10 past after every hour on the 1st of every month

Here's another one, just for practicing

10 * 1 * * /bin/execute/this/script.sh

Fair enough, it takes some getting used to, but it offers great flexibility.

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

But crontab allows you to do this as well:

*/10 * * * * /bin/execute/this/script.sh

Which will do exactly the same. Can you do the the math? ;)

Special words

If you use the first (minute) field, you can also put in a keyword instead of a number:

@reboot     Run once, at startup @yearly     Run once  a year     "0 0 1 1 *" @annually   (same as  @yearly) @monthly    Run once  a month    "0 0 1 * *" @weekly     Run once  a week     "0 0 * * 0" @daily      Run once  a day      "0 0 * * *" @midnight   (same as  @daily) @hourly     Run once  an hour    "0 * * * * 

Leave the rest of the fields empty so this would be valid:

@daily /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 2>&1 >> /var/log/script_output.log

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