Crontab: Difference between revisions

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>/dev/null 2>&1
>/dev/null 2>&1
</pre>
</pre>
{|
! style="text-align:right;"|>
|is for redirect
|-
! style="text-align:right;"|/dev/null
|is a black hole where any data sent, will be discarded
|-
! style="text-align:right;"|2
|is the file descriptor for Standard Error
|-
! style="text-align:right;"|&
|is the symbol for file descriptor (without it, the following 1 would be considered a filename)
|-
! style="text-align:right;"|1
|is the file descriptor for Standard Out
|}
Therefore '''>/dev/null 2>&1''' is redirect the output of your program to /dev/null. Include both the Standard Error and Standard Out.
[[Category : Linux]]
[[Category : Linux]]

Revision as of 09:52, 15 May 2016

Crontab is the equivelant of the scheduler in Windows.

Crontab Commands

Command Description
crontab -e Edit your crontab file, or create one if it doesn't already exist.
crontab -l List your crontab file.
crontab -r Remove your crontab file.
crontab -v Display the last time you edited your crontab file

Crontab File

A crontab file has five fields for specifying day , date and time followed by the command to be run at that interval.

   *   *   *   *   *  command to be executed
   -   -   -   -   -
   |   |   |   |   |
   |   |   |   |   +----- day of week (0 - 6) (Sunday=0)
   |   |   |   +------- month (1 - 12)
   |   |   +--------- day of month (1 - 31)
   |   +----------- hour (0 - 23)
   +------------- min (0 - 59)

Example

of Specific Minutes

3,5,10-15,30,55-60 * * * * /command/to/run.sh

of every 4 hours

0 */4 * * * /command/to/run.sh

at boot

@reboot /command/to/run.sh

Disable Email Notifications

By default cron jobs sends a email to the user account executing the cronjob. If this is not needed put the following command At the end of the cron job line .

>/dev/null 2>&1
> is for redirect
/dev/null is a black hole where any data sent, will be discarded
2 is the file descriptor for Standard Error
& is the symbol for file descriptor (without it, the following 1 would be considered a filename)
1 is the file descriptor for Standard Out

Therefore >/dev/null 2>&1 is redirect the output of your program to /dev/null. Include both the Standard Error and Standard Out.