Often a need arises to execute a program on schedule. For example let us imagine that you need to make a copy of your work every midnight. To accomplish this in Linux there is a special program called cron. It is a demon, which means that is starts when computer is booted up and sits silently in the background, executing other programs for you when time comes. Cron has several configuration files, system-wide, and one per each user. By default users do not have crontabs, because nothing is scheduled for them. This are cron configuration file locations:

1. /etc/crontab — system-wide cron configuration file.
2. /var/spool/cron/crontabs/ — directory for storing user configuration files.

Now let us talk about cron configuration file format. If you run cat /etc/crontab you will see this:

# /etc/crontab: system-wide crontab
# Unlike any other crontab you don't have to run the `crontab'
# command to install the new version when you edit this file
# and files in /etc/cron.d. These files also have username fields,
# that none of the other crontabs do.
 
SHELL=/bin/sh
PATH=/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin
 
# m h dom mon dow user  command
17 *    * * *   root    cd / && run-parts --report /etc/cron.hourly
25 6    * * *   root    test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.daily )
47 6    * * 7   root    test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.weekly )
52 6    1 * *   root    test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.monthly )
#

Its syntax is simple enough. Let us take the line

47 6    * * 7   root    test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.weekly

and take it apart:

17                                  # On 17-minute of every hour
*                                   # Every day of month
*                                   # Every month of the year
*                                   # Every weekday
root                                # As user root
cd /                                # Execute command 'cd /'
&&                                  # If 'cd /' completed successfully, then
run-parts --report /etc/cron.hourly # Execute command 'run-parts --report /etc/cron.hourly'

Now I will sum up cron format:

*    *    *    *    *     user            command to be executed
T    T    T    T    T     (for system
|    |    |    |    |      crontab only)
|    |    |    |    |
|    |    |    |    '----- day of week (0 - 6) (0 is Sunday, or use names)
|    |    |    '---------- month of year (1 - 12)
|    |    '--------------- day of month (1 - 31)
|    '-------------------- hour (0 - 23)
'------------------------- min (0 - 59)

This is abbreviated list of possible characters for time specification:

1. Asterisk (*) — all values in the field, e.g. * in minutes field means every minute.
2. Slash (/) — defines increments of ranges. For example, 30-40/3 means run program on 30-th minute and every 3 minutes thereafter until 40-th minute.
3. Percent (%) — in command field, all data after percent will be sent to the command as standard input. Do not bother with this for now.
4. Comma (,) — specify a list, e.g. 30,40 in minutes field means 30th and 40th minute.
5. Hyphen (-) — a range. For example, 30-40 in minutes field means every minute between 30th and 40th minute.
6. L — specify last something, for example it allows you specify last day of month.

Now I will give you more some examples:

# m      h   dom  mon  dow    user            command
# Every minute, every hour, every day of month, every month of year, every day of week
*        *    *    *    *     root            /bin/false
# Every minute in interval of 30-40 minutes, every hour, every day of month, every month of year, every day of week
30-40    *    *    *    *     root            /bin/false
# Every 5 minutes in interval of 30-40 minutes, every hour, every day of month, every month of year, every day of week
30-40/5  *    *    *    *     root            /bin/false
# Every 5 minutes in interval of 0-59 minutes, every hour, every day of month, every month of year, every day of week
*/5      *    *    *    *     user            command to be executed
# Every minute, every hour, last day of month, every month of year, every day of week
*        *    L    *    *     root            /bin/false
# Every minute, every hour, every day of month, every month of year, Monday and Thursday
*        *    *    *    0,3   root          /bin/false

Ok, but how to install a crontab? This is list of the cron commands:

1. crontab -l — print out current crontab.
2. crontab -e — edit crontab for current user.
3. crontab -r — remove crontab for current user.
4. crontab /path/to/file — install crontab for the current user, overwriting existing one in the process.
5. crontab > /path/to/file — save crontab to a file.

Well, that was about using cron system daemon. But there is one more option to schedule program execution. It is via at utility. The difference between them is that cron is designed to run tasks repetitively, many times, and at is designed for scheduling one-time tasks. This are relevant commands:

1. at — executes commands at a specified time.
2. atq — lists pending jobs.
3. atrm — removes a job.
4. batch — executes commands then system in idling.

As if this infodump is not enough, now I will give you a table of possible time specification for at, taken from http://content.hccfl.edu/pollock/unix/atdemo.htm In the examples below assume that the date and time currently is 10:00 AM Tuesday, September 18, 2001

Now you will learn how to add, view and remove at and crontab jobs.

1: echo 'echo Here I am, sitting in ur at, staring at ur date: $(date) | write user1' | at now + 1 minutes
2: atq

Wait for your message to appear, press <ENTER> and type some more:

3: echo '* * * * * echo Here I am, sitting in ur crontab, staring at ur date: $(date) | write user1' > ~/crontab.tmp
4: crontab -l
5: crontab ~/crontab.tmp
6: crontab -l

Now wait for this one message to appear and remove it:

7: crontab -r
8: crontab -l

user1@vm1:~$ echo 'echo Here I am, sitting in ur at, staring at ur date: $(date) | write user1' | at now + 1 minutes
warning: commands will be executed using /bin/sh
job 13 at Thu Jun 28 14:43:00 2012
user1@vm1:~$ atq
14      Thu Jun 28 14:45:00 2012 a user1
user1@vm1:~$
Message from user1@vm1 on (none) at 14:43 ...
Here I am, sitting in ur at, staring at ur date: Thu Jun 28 14:43:00 MSK 2012
EOF
 
user1@vm1:~$ crontab -l
no crontab for user1
user1@vm1:~$ echo '* * * * * echo Here I am, sitting in ur crontab, staring at ur date: $(date) | write user1' > ~/crontab.tmp
user1@vm1:~$ crontab -l
* * * * * echo Here I am, sitting in ur crontab, staring at ur date: $(date) | write user1
user1@vm1:~$
Message from user1@vm1 on (none) at 14:47 ...
Here I am, sitting in ur crontab, staring at ur date: Thu Jun 28 14:47:01 MSK 2012
EOF
 
user1@vm1:~$ crontab -r
user1@vm1:~$ crontab -l
no crontab for user1
user1@vm1:~$

1. Makes at execute the command echo Here I am, sitting in ur at, staring at ur date: $(date) | write user1 when next minute starts.
2. Prints out at job queue.
3. Writes line echo '* * * * * echo Here I am, sitting in ur crontab, staring at ur date: $(date) | write user1 to the file crontab.tmp in your home direcroty.
4. Prints out your current crontab, but there is none currently so it just tells this to you.
5. Loads contents of crontab.tmp to your personal crontab file.
6. Prints out your current crontab. Now there is something in it.
7. Removes your current crontab.
8. Tells you that you do not have a crontab once again.

1. Read man crontab, man at, man write.
2. Make your system tell you current time every 5 minutes.
3. Make your system tell you current hour at the start of every hour.