I have heard crontab is a good choice, but how do I write the line and where do I put it on the server?
6 Answers
270
votes
Here's a good tutorial on what crontab is and how to use it on Ubuntu. Your crontab line will look something like this:
00 00 * * * ruby path/to/your/script.rb
(00 00 indicates midnight--0 minutes and 0 hours--and the *s mean every day of every month.)
Syntax: mm hh dd mt wd command mm minute 0-59 hh hour 0-23 dd day of month 1-31 mt month 1-12 wd day of week 0-7 (Sunday = 0 or 7) command: what you want to run all numeric values can be replaced by * which means all
41
votes
from the man page
linux$ man -S 5 crontab
cron(8) examines cron entries once every minute.
The time and date fields are:
field allowed values
----- --------------
minute 0-59
hour 0-23
day of month 1-31
month 1-12 (or names, see below)
day of week 0-7 (0 or 7 is Sun, or use names)
...
# run five minutes after midnight, every day
5 0 * * * $HOME/bin/daily.job >> $HOME/tmp/out 2>&1
...
It is good to note the special "nicknames" that can be used (documented in the man page), particularly "@reboot" which has no time and date alternative.
# Run once after reboot.
@reboot /usr/local/sbin/run_only_once_after_reboot.sh
You can also use this trick to run your cron job multiple times per minute.
# Run every minute at 0, 20, and 40 second intervals
* * * * * sleep 00; /usr/local/sbin/run_3times_per_minute.sh
* * * * * sleep 20; /usr/local/sbin/run_3times_per_minute.sh
* * * * * sleep 40; /usr/local/sbin/run_3times_per_minute.sh
To add a cron job, you can do one of three things:
add a command to a user's crontab, as shown above (and from the crontab, section 5, man page).
- edit a user's crontab as root with
crontab -e -u <username> - or edit the current user's crontab with just
crontab -e - You can set the editor with the
EDITORenvironment variableenv EDITOR=nano crontab -e -u <username>- or set the value of EDITOR for your entire shell session
export EDITOR=vimcrontab -e
- Make scripts executable with
chmod a+x <file>
- edit a user's crontab as root with
create a script/program as a cron job, and add it to the system's anacron
/etc/cron.*lydirectories- anacron /etc/cron.*ly directories:
- /etc/cron.daily
- /etc/cron.hourly
- /etc/cron.monthly
- /etc/cron.weekly
- as in:
- /etc/cron.daily/script_runs_daily.sh
chmod a+x /etc/cron.daily/script_runs_daily.sh-- make it executable
- See also the anacron man page:
man anacron - Make scripts executable with
chmod a+x <file> - When do these cron.*ly script run?
- For RHEL/CentOS 5.x, they are configured in
/etc/crontabor/etc/anacrontabto run at a set time - RHEL/CentOS 6.x+ and Fedora 17+ Linux systems only define this in
/etc/anacrontab, and define cron.hourly in/etc/cron.d/0hourly
- For RHEL/CentOS 5.x, they are configured in
- anacron /etc/cron.*ly directories:
Or, One can create system crontables in
/etc/cron.d.- The previously described crontab syntax (with additionally providing a user to execute each job as) is put into a file, and the file is dropped into the /etc/cron.d directory.
- These are easy to manage in system packaging (e.g. RPM packages), so may usually be application specific.
- The syntax difference is that a user must be specified for the cron job after the time/date fields and before the command to execute.
- The files added to
/etc/cron.ddo not need to be executable. - Here is an example job that is executed as the user
someuser, and the use of/bin/bashas the shell is forced.
File: /etc/cron.d/myapp-cron
# use /bin/bash to run commands, no matter what /etc/passwd says
SHELL=/bin/bash
# Execute a nightly (11:00pm) cron job to scrub application records
00 23 * * * someuser /opt/myapp/bin/scrubrecords.php
6
votes
Quick guide to setup a cron job
Create a new text file, example: mycronjobs.txt
For each daily job (00:00, 03:45), save the schedule lines in mycronjobs.txt
00 00 * * * ruby path/to/your/script.rb
45 03 * * * path/to/your/script2.sh
Send the jobs to cron (everytime you run this, cron deletes what has been stored and updates with the new information in mycronjobs.txt)
crontab mycronjobs.txt
Extra Useful Information
See current cron jobs
crontab -l
Remove all cron jobs
crontab -r
5
votes
Sometimes you'll need to specify PATH and GEM_PATH using crontab with rvm.
Like this:
# top of crontab file
PATH=/home/user_name/.rvm/gems/ruby-2.2.0/bin:/home/user_name/.rvm/gems/ruby-2.2.0@global/bin:/home/user_name/.rvm/rubies/ruby-2.2.$
GEM_PATH=/home/user_name/.rvm/gems/ruby-2.2.0:/home/user_name/.rvm/gems/ruby-2.2.0@global
# jobs
00 00 * * * ruby path/to/your/script.rb
00 */4 * * * ruby path/to/your/script2.rb
00 8,12,22 * * * ruby path/to/your/script3.rb
1
votes
You can execute shell script in two ways,either by using cron job or by writing a shell script
Lets assume your script name is "yourscript.sh"
First check the user permission of the script. use below command to check user permission of the script
ll script.sh
If the script is in root,then use below command
sudo crontab -e
Second if the script holds the user "ubuntu", then use below command
crontab -e
Add the following line in your crontab:-
55 23 * * * /path/to/yourscript.sh
Another way of doing this is to write a script and run it in the backgroud
Here is the script where you have to put your script name(eg:- youscript.sh) which is going to run at 23:55pm everyday
#!/bin/bash
while true
do
/home/modassir/yourscript.sh
sleep 1d
done
save it in a file (lets name it "every-day.sh")
sleep 1d - means it waits for one day and then it runs again.
now give the permission to your script.use below command:-
chmod +x every-day.sh
now, execute this shell script in the background by using "nohup". This will keep executing the script even after you logout from your session.
use below command to execute the script.
nohup ./every-day.sh &
Note:- to run "yourscript.sh" at 23:55pm everyday,you have to execute "every-day.sh" script at exactly 23:55pm.
