Brief Introduction To Using Cron in Linux

Cron is a system daemon used to execute desired tasks (in the background) at designated times.

A crontab is a simple text file with a list of commands meant to be run at specified times. It is edited with a command-line utility. These commands (and their run times) are then controlled by the cron daemon, which executes them in the system background.

crontab -e : opens an editor to edit the crontab file

The data that need to be entered in this file should follow the following format:

minute (0-59), hour (0-23, 0 = midnight), day (1-31), month (1-12), weekday (0-6, 0 = Sunday), command

Examples

01 04 1 1 1 /usr/bin/somedirectory/somecommand

The above example will run /usr/bin/somedirectory/somecommand at 4:01am on January 1st plus every Monday in January

01 04 * * * /usr/bin/somedirectory/somecommand

The above example will run /usr/bin/somedirectory/somecommand at 4:01am on every day of every month

45 04 * * * /usr/sbin/chkrootkit && /usr/bin/updatedb

The double-ampersand (&&) can also be used in the “command” section to run multiple commands consecutively. The above example will run chkrootkit followed by updatedb at 4:45am daily - providing you have all listed apps installed. If chkrootkit fails, updatedb will NOT be run

Setting up environment variable

Cron job will not use the environment variables that are set up in the .bash_profile file or the bashrc file in the home directory of the user. If you are running a shell script with cron, type the following line before executing any other command in your shell script:

source /home/johndoe/.bash_profile

The above line will make all the user environment variables available to the cron job.

Debugging cron jobs

/var/log/cron has logs of all cron jobs

Any output to stdout written during the runtime of your cronjob goes to /var/spool/mail/johndoe

Your crontab file is in the folder /var/spool/cron/johdoe

• Miscellaneous