Wednesday, June 5, 2013


I can never seem to remember how to use crontab. I found a good resource for it that has lots of examples and good explanation of things. Check it out. This is also a good one.

The highlights that I want to remember are:

crontab -l
This is used to see what is scheduled to be executed and when.

crontab -e
Opens the vi text editor to allow changes to the crontab. In most implementations that I have seen the changes are automatically picked up by crontab
vi remember:
  • ESCAPE then ZZ to save changes
  • ESCAPE then :q to quit and not save changes
  • ESCAPE then i to go into edit mode
Each line in the crontab has the format:
min hr dom month dow cmd

  • minute This controls what minute of the hour the command will run on, and is between '0' and '59'
  • hour This controls what hour the command will run on, and is specified in the 24 hour clock, values must be between 0 and 23 (0 is midnight) 
  • dom This is the Day of Month, that you want the command run on, e.g. to run a command on the 19th of each month, the dom would be 19.
  • month This is the month a specified command will run on, it may be specified numerically (0-12), or as the name of the month (e.g. May)
  • dow This is the Day of Week that you want a command to be run on, it can also be numeric (0-7) or as the name of the day (e.g. sun). 
  • user This is the user who runs the command. 
  • cmd This is the command that you want run. This field may contain multiple words or spaces
Some examples:
30 7 * * * /opt/tomcat/myApp/ "This command is run daily at 7:30 am"
0 * * * * echo "This command is run hourly"

Special Keywords that can be used instead of the 5 part specifiers
@yearly 0 0 1 1 *
@daily 0 0 * * *
@hourly 0 * * * *
@reboot Run at startup.
For example:
@yearly /home/ramesh/red-hat/bin/annual-maintenance

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