Scheduling Jobs with crontab

This final chapter will introduce the details on how to schedule shell scripts to execute on a regular timetable.   The cron daemon is the system task that runs scripted jobs on a pre-determined schedule.  The crontab command is used to tell the cron daemon what jobs the user wants to run and when to run those jobs.

Each Linux user can have their own crontab file, if allowed by the System Administrator.  The administrator controls use of crontab by including users in the cron.deny file to disallow use of crontab.

crontab Options

As shown in Table 10.1, the crontab command has several options with different purposes.

Table 10.1: crontab options and purposes

crontab also accepts a file name and will use the specified file to create the crontab file.  Many users prefer to use this option rather than the crontab -e command because it provides a master file from which the crontab is built, thus providing a backup to the crontab.   The following example specifies a file called mycron.tab to be used as the input for crontab.

$ crontab mycron.tab

Here's how you would use the crontab ?l command to view the current cron entries for the logged in user.

$ crontab -l

Now if we wanted to delete all the entries in the crontab we can use the ?r option.

$ crontab -r

The Format of the crontab File

The crontab file consists of a series of entries specifying what shell scripts to run and when to run it.  It is also possible to document crontab entries with comments.  Lines which have a pound sign (#) as the first non-blank character are considered comments.  Blank lines are completely ignored.  Comments cannot be specified on the same line as cron command lines.  Comments must be kept on their own lines within the crontab.

There are two types of command lines that can be specified in the crontab: environment variable settings and cron commands.  The following sections will provide more detail on these two types of crontab entries.

Environment variable settings

Each environment variable line consists of a variable name, an equal sign (=), and a value.  Values that contain spaces need to be enclosed within quotes.  The following are some examples of environment variable settings:

color = red
title = ?My Life in a Nutshell?

It is important to remember that variable names are case sensitive and that system variables are usually defined with upper case names, while user defined variables are defined with lower case names.

crontab Command Lines

Each crontab command line is comprised of six positional fields specifying the time, date and shell script or command to be run.  The format of the crontab command line is described in Table 10.2 below:

Table 10.2: crontab command line format

Each of these fields can contain a single number, a range of numbers indicated with a hyphen (such as 2-4), a list of specific values separated by commas (like 2,3,4) or a combination of these designations separated by commas (such as 1,3-5).  Any of these fields may also contain an asterisk (*) indicating every possible value of this field.  This can all get rather confusing so let's take a look at a few examples.

The next several examples are all part of the same crontab file.  We have broken it up in order to explain each entry individually.

# Use the Korn Shell for all shell scripts

SHELL=/bin/ksh

This sets the default shell for these cron scripts by setting the SHELL environment variable.

#**********************************************************
# Run the Weekly file cleanup task at 6:00AM every Monday
# and send any output to a file called cleanup.lst in the
# /tmp directory
#**********************************************************
00 06 * * 1 /home/terry/cleanup.ksh > /tmp/cleanup.lst

This entry will run the script cleanup.ksh at 0 minutes past the hour, 6 am, every day of the month, every month of the year, but only on Mondays.  This illustrates that for a crontab to execute all of the conditions specified must be met, so even though we've said every day of the month by making the third field a wildcard, the day also has to meet the final condition that the day is a Monday.

#**********************************************************
# Run the Weekly Management Report every Monday at 7:00 AM
# and save a copy of the report in my /home directory
#**********************************************************
00 07 * * 1 /home/terry/weekly_mgmt_rpt.ksh wprd > /home/terry/weekly_mgmt_rpt.lst

This entry is very similar but will execute at 7:00am.  Since the hour is in 24 hour format (midnight is actually represented as 00) we know the 07 represents 7:00 a.m.  This entry again will only be run once a week.

#**********************************************************
# Weekly Full Backup - run every Sunday at 1:30AM
#**********************************************************
30 01 * * 0 /home/terry/full_backup.ksh wprd > /tmp/full_backup.lst

Here we have specified this script to be run at 30 minutes past the hour, the first hour of the day, but only on Sundays.  Remember that in the day of the week column Sunday can be represented by either 0 or 7.

#**********************************************************
# Nightly Incremental Backup - run Monday-Saturday at 1:30AM
#**********************************************************
30 01 * * 1-6 /home/terry/incr_backup.ksh  > /tmp/incr_backup.lst

In this crontab entry we see the same indication for hour and minute as the last entry but we have specified a range for the day of the week.  The range 1-6 will cause the incr_backup.ksh to be executed at 1:30 every morning from Monday through Saturday.

#**********************************************************
# Low disk space alert ... run every 15 minutes, sending
# alerts to key individuals via e-mail
#**********************************************************
00,15,30,45 * * * * /home/terry/free_space.ksh > /tmp/free_space.lst

This entry has minutes separated by a comma indicating that it should be run at each of the indicated times.  Since all the other fields are wildcards (*) the entry will be run on the hour (00), 15 minutes past the hour, 30 minutes past the hour and 45 minutes past the hour.

#**********************************************************
# Lunch Time Notification - run Monday-Friday at Noon -
# sends a message to all users indicating it's lunch time
#**********************************************************
00 12 * * 1-5 /home/terry/lunch_time.ksh wprd > /tmp/lunch_time.lst

This lunch reminder is set up to run at 12:00 p.m. Monday through Friday only.

The most important thing to remember is that a crontab entry will execute every time all of its conditions are met.  To take the last entry as an example, any time it is 00 minutes past the hour of 12 on any day of the month and any month of the year and the day of the week is between Monday and Friday inclusive (1-5) this crontab will be executed.

You will use wildcards in most crontab entries but be careful where you use them.  For instance, if we mistakenly placed a * in the minute position of the last crontab example above we would end up running the script for ever minute of the 12:00 hour instead of just once at the beginning of the hour.  I don't think anyone needs that many reminders to go to lunch, do you?

As mentioned above, the day-of-week field accepts either zero or seven as a value for Sunday.  Any of the time/date fields can also contain an asterisk (*) indicating the entire range of values.  Additionally, month and day-of-week fields can contain name values, consisting of the first three letters of the month, as indicated in Table 10.3 below.

When numbers are used, the user can specify a range of values separated by a hyphen or a list of values separated by commas.  In other words, specifying 2-5 in the hour field means 2AM, 3AM, 4AM and 5AM, while specifying 2,5 means only 2AM and 5AM.

We've talked an awful lot about how to specify the date and time in the crontab but what about the command?  Well, most folks will write shell scripts to execute with their crontab entries but you can actually just execute a command from the crontab as well.  Either way make sure you put the absolute path to your command in the crontab.