Crontab In Linux

Crontab – Quick Reference

1. What is crontab?

 Crontab (CRON TABle) is a file which contains the schedule of cron entries to be run and at specified times.

2.What is a cron job or cron schedule?

Cron job or cron schedule is a specific set of execution instructions specifying day, time and command to execute. crontab can have multiple execution statements.

3. Crontab Commands

export EDITOR=vi ;to specify a editor to open crontab file.

crontab -e    Edit crontab file, or create one if it doesn’t already exist.
crontab -l    crontab list of cronjobs , display crontab file contents.
crontab -r    Remove your crontab file.
crontab -v    Display the last time you edited your crontab file. (This option is only available on a few systems.)

4. Syntax of crontab

The syntax is:

1 2 3 4 5 /path/to/command arg1 arg2

OR

1 2 3 4 5 /root/backup.sh

Where,

• 1: Minute (0-59)
• 2: Hours (0-23)
• 3: Day (0-31)
• 4: Month (0-12 [12 == December])
• 5: Day of the week(0-7 [7 or 0 == sunday])
• /path/to/command – Script or command name to schedule

Easy to remember format:

* * * * * command to be executed
- - - - -
| | | | |
| | | | ----- Day of week (0 - 7) (Sunday=0 or 7)
| | | ------- Month (1 - 12)
| | --------- Day of month (1 - 31)
| ----------- Hour (0 - 23)
------------- Minute (0 - 59)

5. Crontab Examples



1) To run /path/to/command five minutes after midnight, every day, enter:
5 0 * * * /path/to/command
2) Run /path/to/script.sh at 2:15pm on the first of every month, enter:
15 14 1 * * /path/to/script.sh
3) Run /scripts/phpscript.php at 10 pm on weekdays, enter:
0 22 * * 1-5 /scripts/phpscript.php
4) Run /root/scripts/perl/perlscript.pl at 23 minutes after midnight, 2am, 4am …, everyday, enter:
23 0-23/2 * * * /root/scripts/perl/perlscript.pl
5) Run /path/to/unixcommand at 5 after 4 every Sunday, enter:
5 4 * * sun /path/to/unixcommand