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cron and crontab

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Old Unix and Linux 04-04-2004   -   Original Discussion by Perderabo
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cron and crontab

We have hundreds of threads involving cron and crontab. Rather than creating a post with dozens of links, I thought I'd just write up a tutorial with only a few selected links.

The Basics

cron is a daemon that runs periodic tasks. crontab is name of textfile that is used to control cron. And crontab is also the name of program used to modify the file called crontab. Every user could conceivably have a crontab file. These are often stored in /var/spool/cron/crontabs/. If my user name is perderabo, then my real crontab is /var/spool/cron/crontabs/perderabo. When cron was first written there was a single crontab called /etc/crontab and only root could modify it. Now that everyone can use cron, each crontab gets the name of the user who owns it. Because there are so many possible crontabs, cron can't monitor them all for changes. That's why you must use the crontab program. In addition to modifying the crontab file, it also lets cron know about the change. If you simply edit the file in /var/spool/cron/crontabs/, cron will not notice the change.

The crontab command

root can always use the crontab command. Other users may be locked out. A file, usually at /usr/lib/cron/cron.deny contains a list of users who are prohibited from using cron. If that file doesn't exist, /usr/lib/cron/cron.allow may list users who can use cron. If neither file exists, only root can use cron. To let everyone use cron, create an empty cron.deny file.

But be careful with that crontab command! We must have a dozen threads from folks who accidently did a "crontab -r" which removes your crontab completely. There is no easy way to recover from that. Rather than simply using "crontab -e" to edit your file, some of our members suggest
crontab -l > mycrontab
vi mycrontab
crontab < mycrontab

The Format of a crontab entry

A typically crontab entry might be:
15 18 * * 1-5 /some/script
This says to run /some/script at 18:15 on Monday through Friday.

The first five fields are:
minute (0-59)
hour (0-23)
day of the month(1-31)
month of the year (1-12)
day of the week (0-6 with 0 = Sunday)

Each field can be an asterisk meaning all values, or a single integer, several integers separated by commas, or two integers separated by a hypen to indicate a range.

With some versions of cron, day of the week is 1-7 with 7 = Sunday. Many versions of cron accept either 0 or 7 as Sunday.

Crontab entry for the first Friday of a Month, or every other Tuesday, or Last Saturday of the Month

Consider this entry:
0 0 1-7 * 5 /some/script
You might hope that will run /some/script during the first minute of the first Friday of the month. Unfortunately, it will run the script on each day of the first week of the month and on every Friday. See this thread to see a solution to this problem.
crontab entry to run every last day of the month

It works from the command line but not in crontab

This is perhaps our number one complaint with cron. When you login to Unix, startup scripts setup your environment. You can see your environment with the commands "printenv" or "env". On the other hand, cron sets up only a sparse environment (See Problem with crontab for the details.)

I think that the best solution is to write shell scripts that set up their own environment and run these via cron. That's what worked in not running in cron.

In ping from cron, the user used an absolute path rather than redefining the PATH environment variable.

In this thread: Cron problem?, the user sourced the .profile file to set up the environment and it worked. But that technique caused a problem in stty: tcgetattr: Not a typewriter And I also think that it sets you up for a mysterious problem. Change .profile and your cronjobs may suddenly fail.

A final reason is using the per cent sign in a crontab command as in this post.

More cron Tutorials

And finally, Need tut on Cron info and Cron Jobs have some links to other cron tutorials.
The Following 7 Users Say Thank You to Perderabo For This Useful Post:
CarloM (06-19-2014), G1GPL (07-06-2011), orange47 (01-11-2012), saikiran_1984 (09-27-2010), Simtech (08-31-2011), TheTransporter (07-23-2011), VijaySolaris (04-03-2012)
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