How to Use the Cron and Crontab commands

The 'Cron' Command

The cron command starts a process that executes commands at specified dates and times. Regularly scheduled commands can be specified according to instructions found in the crontab files in the directory /var/spool/cron/crontabs. Users can submit their own crontab files via the crontab command.

The 'Crontab' Command

Crontab copies the specified file or standard input if no file is specified, into a directory that holds all users' crontabs.

SYNOPSIS:

The -e option edits a copy of the current users' crontab file or creates an empty file to edit if crontab does not exist.

The-r option removes a user's crontab from the crontab directory.

The -l options lists the crontab file for the invoking user.

Setting up a Crontab job

A crontab file consists of lines of six fields each.The fields are separated by spaces or tabs. The first five are integers that specify the following:

  1. minute (0-59),
  2. hour (0-23),
  3. day of the month (1-31),
  4. month of the year (1-12),
  5. day of the week (0-6 with 0=Sunday).

Each of these patterns may be either an asterisk (meaning all valid values) or a list of elements separated by commas. An element is either a number or two numbers separated by a minus sign ( meaning an inclusive range). Notice the time is in 24 hour format, 0 is midnight and 13 is one in the afternoon.

The sixth field of a line in a crontab file is a string to be executed by the shell at the specified times by the first fife fields. A percent character in this field (unless escaped by \) is translated to a newline character. Only the first line (up to a % or end of line) of the command field is executed by the shell. The other lines are made available to the command as standard input.

Any line beginning with a # is a comment and is ignored.

Example

To illustrate, 0 0 1,15 1 would run a command on the first and fifteenth of each month, as well as on every Monday at exactly midnight. To specify days by only one field, the other field should be set to *. The entry, 0 23 * * 1 would run a command only on Mondays at eleven PM.

A minute specification of 0,30 would indicate the job is to be run on the hour and half hour. Likewise, a day of the month entry of 1,15 would initiate execution on the first and fifteenth of the month.

Make sure you include an explicit path to your programs or scripts that you want to run by crontab. Let's assume we want to execute a Perl program, autoclose.cgi, every day at midnight. Additionally assume the full path to the script is /home/www/yourdirectory/cgi-bin. The full crontab command would be: 0 0 * * * /home/www/yourdirectory/cgi-bin/autoclose.cgi

Execute at midnight, every day of the month, every month of the year, and every day of the week.

If you're not sure of the full path to your script, go to the directory and issue the command:

pwd

This is the "print working directory" command. The requirement for explicit paths also apply to any "require" library file used in a Perl script.

The line:

0 0 1,15 * 1 /home/www/yourdirectory/cgi-bin/autoclose.cgi

runs the command on the first and fifteeth of each month, as well as on every Monday.

The line:

0 0 * * 1 /home/www/yourdirectory/cgi-bin/autoclose.cgi

runs a the command only on Mondays.

Putting it all together

Creating your first crontab job is a three-part process:

  1. Prepare the absolute path to the program or script that you want to run,
  2. Create a text file containing a line to schedule the job as described above for crontab fields,
  3. Upload the text file to your system cron.



When preparing program to run or creating scripts to run remember that crontab jobs are background tasks. There is no terminal attached to a crontab job so there should be no print statements that normally write to the screen. (It is possible to redirect such print statements.) The same consideration hold true for requesting user input.

Let's assume that we want to run this script:

/home/www/yourdirectory/cgi-bin/autoclose.cgi

Next create a file to hold your crontab instructions. Let's call it "cron.txt". Let's also have our script execute at zero minute every hour. Put the following crontab command line in the text file "cron.txt" and save this file as a text file: 0 * * * * /home/www/yourdirectory/cgi-bin/autoclose.cgi

The last step is uploading the contents of your "cron.txt" file to the system crontab spool area. Enter the following at the system prompt.

crontab cron.txt

Now you are done with this project.

If you would like additional detailed information on crontab, enter the following at your Unix prompt.

man crontab