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 canchange 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:
01**5/bin/execute/this/script.sh
Get it? The
script is now being executed when the system clock hits:
·
minute:
0
·
of
hour:
1
·
of
day of month:
* (every day of month)
·
of
month:
* (every
month)
·
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:
01**1-5/bin/execute/this/script.sh
Get it? The
script is now being executed when the system clock hits:
·
minute:
0
·
of
hour:
1
·
of
day of month:
* (every day of month)
·
of
month:
* (every
month)
·
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
For 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 * * * *"
Leaving the
rest of the fields empty, 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>>/var/log/script_output.log2>&1
Explained
Although CronTabs are useful,
sometimes you just want to run a command at a given time once and not have it
reoccurring. The commands `at` and `batch` can do just that.
The `at`
command can be used to run a command based on a specific time. Lets say you
wanted to run a command 1 day in advance or even 1 hour from your current time,
`at` would be the tool to use.
`batch` on the
other hand runs commands based on the systems load average. By default it will
run when the load average is less then 0.8.
Installation
Before using
the command we need to make sure the `at` package is installed. We can do this
as follows:
This will
display a list of packages that start with the word at. If you do not have the
at package installed you can install it by using the rpm command or yum.
Installing the
at package also installed the batch binary.
`at`
Usage
Lets say you
wanted to schedule at to run a specific command at a future time. You could do
this by specifying the at command followed by a future time: at
Then press
enter, and you will be prompted to enter a command. For this example, I’m going
to output the current date and time to a file in the root directory.
Then you could
press CTRL-D to issue the one time cron, or you could press enter to issue
another command for the same scheduled task.
You could also
specify a number of days to wait until running the given command by doing the
following:
Then you will
be prompted for your command. After you type the command you want to issue,
press CTRL-D to set your schedule.
To view a list
of items in the at queue you can type the following:
This would
list all of the tasks you have scheduled via at. You will also see a list of
numbers in front of each task. Each number specifies the job number for that
task. You can use that job number in order to remove the scheduled task from
running.
For my
example, the job number we scheduled was set to 5. In order to remove the
scheduled task you type the command atrm followed by the job number.
`batch`
Usage
The batch
utility does not allow any command line options. In order to use batch, you
simply just type the word batch.
Press enter
and then type the command you wish to issue.
After issuing
your command press CTRL-D to apply it. It will then be run the next time your
server load is less then 0.8.