Subscribe
Apr
12
In my first article, I provided some basic understanding of how scheduling on Linux operating system works and also gave some examples of how to schedule jobs using cron at different date and times. If you haven’t read it, please read Scheduling for Absolute Beginner first.
This article will provide some additional detail that I may have missed and it will mostly give examples of many different neat tricks you can do in a crontab file. For example, you can write single line cron entries to gather system statistics and email you details without even writing a script in a separate file.
Some more basics
There are a couple of different ways to use cron. In the /etc directory you will probably find some sub directories called cron.hourly, cron.daily, cron.weekly and cron.monthly. If you place a script into one of those directories it will be run either hourly, daily, weekly or monthly, depending on the name of the directory.
If you want more flexibility than this, you can edit /etc/crontab file which is the main config file. On a default Linux distro, the crontab will look something like this:
root@andalus:/etc# cat /etc/crontab
SHELL=/bin/sh
PATH=/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin
HOME=/
MAILTO=hugefatass007@gmail.com
# run-parts
01 * * * * root run-parts /etc/cron.hourly
02 4 * * * root run-parts /etc/cron.daily
22 4 * * 0 root run-parts /etc/cron.weekly
42 4 1 * * root run-parts /etc/cron.monthly
The first part sets the variables SHELL, PATH, MAILTO and HOME.
SHELL is the ’shell’ cron runs under. If unspecified, it will default to
the entry in the /etc/passwd file.
PATH contains the directories which will be in the search path for cron script.
If you have a shell script in /opt/bin, then you should add that to the PATH variable.
MAILTO is who gets mailed the output of each command. If no one is specified, then the output will be mailed to the owner of the process that produced the output.
HOME is the home directory that is used for cron. If unspecified, it will default to the entry in the /etc/passwd file.
** You can also set the above variables if you decide to create your own crontab file and entries.
Controlling Access to cron
You can choose to restrict cron usage by the use of /etc/cron.allow and /etc/cron.deny files. To stop a user using cron, just put their name in cron.deny, to allow a user put their name in the cron.allow. If you wanted to prevent all users from using cron, you could add the line ALL to the cron.deny file:
root@andalus # echo ALL >> /etc/cron.deny
If you want to deny only user MrBabyMan to stop using cron to automate so many of his digg posts, then you can add a line MrBabyMan to the cron.deny file:
root@andalus # echo MrBabyMan >>/etc/cron.deny
Location of crontab files
The individual crontab file is stored as text file in /var/spool/cron/crontabs but maybe different into your distro. However, normally they’re all under /var/spool/cron base.
Examples, Tips & Tricks
We’re done with providing basic information. Now to the actual part of the article that’ll give you many different example usage of crontab. You’ll start feeling comfortable using cron in no time.
#Run backup.sh at 11:59 Monday, Tuesday, Wednesday, Thursday and Friday.
30 11 * * 1-5 /usr/local/bin/backup.sh
Basic Entries
#Run every minute
* * * * * command
#Run every 10 minutes
*/10 * * * * command
#Every weekday, touch file, the easy way
0 6 * * mon-fri touch ~/file
#One last example, every hour between 6-10 on the hour and 4
0 6-10,4 * * * touch ~/file
#Backup your crontab every morning at 6AM
0 6 * * * /bin/crontab -l > /root/cronjobs.txt 2>/dev/null
#Run your command at :05 and :45 past the hour, every hour, Monday through Friday
5,35 * * * 1-5 command
#Remove all of the files in you /tmp directory every morning at 4:45 AM
45 4 * * * rm /tmp/*
#Run command at 7:00am each weekday [mon-fri]
00 07 * * 1-5 command
#Run command on 1st of each month, at 5:30pm
30 6 1 * * command
#Run command at 8:00am,10:00am,12:00pm and 2:00pm every day
00 8,10,12,14 * * * command
#Run command every 5 minutes during market hours
*/5 6-13 * * mon-fri command
#Run command every 3-hours
0 7-23/3 * * * command
Disable email alerts in crontab
If you’re wondering why you keep getting an email each time your Cron job runs, it’s because crontab notifies you that it has completed (or not) the job you requested it to run. After everything is running smoothly, you may want to disable this feature, or redirect the output to a log file instaed of an email.
To disable, simply place >/dev/null 2>&1 after your command.
#This’ll write the email to no where and you’ll no longer be receiving annoying cron emails.
45 4 * * * rm /tmp/* >/dev/null 2>&1
#This writes the email no where but keeps the output of the command in rmtmp.log file
45 4 * * * rm /tmp/* > /home/MrBabyMan/cronlogs/rmtmp.log >/dev/null 2>&1
Note: One > means replace the current log with a new one, while (two) >> means append the current output to the end of the current log.
Tips & Tricks
#The above will zip your access-log file into access-log.mmddyy.zip format.
30 0 * * * /usr/bin/zip -q access-log.`/bin/date +\%m\%d\%y`.zip access-log
#Run disk usage command and email result
0 2 * * * du -h –max-depth=1 / | mail -s “Disk Usage” mrfatass@gmail.com
#Send list of users logged on to server via email
59 11 * * * last | grep date ‘+%a %b %d’ > /tmp/users.out ; mail -s “Users Logged Today” hugefatass007@gmail.com
Note: You can take this idea and email out many different types of alerts to your email at anytime.
Special Entries
There are several special entries, some which are just shortcuts, that you can use instead of specifying the full cron entry. The most useful of these is probably @reboot which allows you to run a command each time the computer gets reboot. You can alert yourself when server is back online after a reboot. Also becomes useful if you want to run certain services or commands at start up.
The complete list of special entries are:
| Entry | Description | Equivalent To |
|---|---|---|
| @reboot | Run once, at startup. | None |
| @yearly | Run once a year | 0 0 1 1 * |
| @annually | (same as @yearly) | 0 0 1 1 * |
| @monthly | Run once a month | 0 0 1 * * |
| @weekly | Run once a week | 0 0 * * 0 |
| @daily | Run once a day | 0 0 * * * |
| @midnight | (same as @daily) | 0 0 * * * |
| @hourly | Run once an hour | 0 * * * * |
For the people who still feel like they need a hand with setting up cron, I found a cool website that’ll generate crontab entries based on date/time you specifiy. You can pay a visit to the following:
- http://htmlbasix.com/crontab.shtml
- http://www.thepcmanwebsite.com/crontab_generator.shtml
- https://www.psek.com/support/scripts/croncreate.php
- http://www.robertplank.com/cron/
Please comment if I’ve missed something or if you’d like to contribute a tip/trick etc.
Comments
2 Comments so far
Good writeup, thanks!
Could you also explain how the “at” scheduler works with “chron?”
I believe this could help the people who don’t run the box 24/7.
Nice work!
This is a complete brief on the subject. I rather refer to this single page than do a desperate search on a bunch of man pages