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0 1 1 * * find /tmp -atime 3 -exec ls -l { } \;

To do it once a week on Monday, restore the third field to an asterisk but specify either 1 or mon as the fifth field:

0 1 * * mon find /tmp -atime 3 -exec ls -l { } \;

To get even more sophisticated, there are ways to specify multiple times in each field. Here, a comma means "run on the 1st and 15th day" of each month:

0 1 1,15 * * find /tmp -atime 3 -exec ls -l { } \;

A hyphen means "run every day from the 1st through the 15th, inclusive":

0 1 1-15 * * find /tmp -atime 3 -exec ls -l { } \;

A slash followed by a 5 means "run every fifth day," which comes out to the 1st, 6th, 11th, and so on:

0 1 */5 * * find /tmp -atime 3 -exec ls -l { } \;

Now we're ready to actually put the entry in our crontab file. Become root (because this is the kind of thing root should do) and enter the crontab command with the -e option for "edit":

rutabaga# crontab -e

By default, this command starts a vi edit session. If you'd like to use XEmacs instead, you can specify this before you start crontab. For a Bourne-compliant shell, enter the command:

rutabaga# export VISUAL=xemacs

For the C shell, enter:

rutabaga# setenv VISUAL xemacs

The environment variable EDITOR also works in place of VISUAL for some versions of crontab. Enter a line or two beginning with hash marks (#) to serve as comments explaining what you're doing, then put in your crontab entry:

# List files on /tmp that are 3 or more days old. Runs at 1:00 AM

# each morning.

0 1 * * * find /tmp -atime 3 -exec ls -l { } \;

When you exit vi, the commands are saved. Look at your crontab entry by entering:

rutabaga# crontab -l

We have not yet talked about a critical aspect of our crontab entry: where does the output go? By default, cron saves the standard output and standard error and sends them to the user as a mail message. In this example, the mail goes to root, but that should automatically be directed to you as the system administrator. Make sure the following line appears in /usr/lib/aliases (/etc/aliases on SUSE, Debian, and RedHat):

root: your-account-name

In a moment, we'll show what to do if you want output saved in a file instead of being mailed to you.

Here's another example of a common type of command used in crontab files. It performs a tape backup of a directory. We assume that someone has put a tape in the drive before the command runs. First, an mt command makes sure the tape in the /dev/qft0 device is rewound to the beginning. Then a tar command transfers all the files from the directory /src to the tape. A semicolon is used to separate the commands; that is standard shell syntax:

# back up the /src directory once every two months.

0 2 1 */2 * mt -f /dev/qft0 rewind; tar cf /dev/qft0 /src

The first two fields ensure that the command runs at 2:00 A.M., and the third field specifies the first day of the month. The fourth field specifies every two months. We could achieve the same effect, in a possibly more readable manner, by entering:

0 2 1 jan,mar,may,jul,sep,nov * mt -f /dev/qft0 rewind; \

tar cf /dev/qft0 /src

The section "Making Backups" in Chapter 27 explains how to perform backups on a regular basis.

The following example uses mailq every two days to test whether any mail is stuck in the mail queue, and sends the mail administrator the results by mail. If mail is stuck in the mail queue, the report includes details about addressing and delivery problems, but otherwise the message is empty:

0 6 */2 * * mailq -v | \

mail -s "Tested Mail Queue for Stuck Email" postmaster

Probably you don't want to receive a mail message every day when everything is going normally. In the examples we've used so far, the commands do not produce any output unless they encounter errors. But you may want to get into the habit of redirecting the standard output to /dev/null, or sending it to a logfile like this (note the use of two > signs so that we don't wipe out previous output):

0 1 * * * find /tmp -atime 3 -exec