On some systems, use of cron is limited to the root user. In any case, let's look at a useful command you might want to run as root and show how you'd specify it as a crontab entry. Suppose that every day you'd like to clean old files out of the /tmp directory, which is supposed to serve as temporary storage for files created by lots of utilities.

Notice that cron never writes anything to the console. All output and error messages are sent as an email message to the user who owns the corresponding crontab. You can override this setting by specifying MAILTO=address in the crontab file before the jobs themselves.

ls -l filename

But how do you know which filename to specify? You have to place the command inside a find command, which lists all files beneath a directory and performs the operation you specify on each one.

Figure 8-1. Sample cron entry

If you want a command to run every day at 1:00 A.M., specify the minute as 0 and the hour as 1. The other three fields should be asterisks, which mean "every day and month at the given time." The complete line in crontab is:

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

Because you can do a lot of fancy things with the time fields, let's play with this command a bit more. Suppose you want to run the command just on the first day of each month. You would keep the first two fields, but add a 1 in the third field:

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 {} \;

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

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

and 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 Emacs instead, you can specify this before you start crontab. For a Bourne-compliant shell, enter the command:

rutabaga# export VISUAL=emacs

For the C shell:

rutabaga# setenv VISUAL emacs

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

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.

# 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

0 6 */2 * * mailq -v | \
          mail -s "Tested Mail Queue for Stuck Email" postmaster

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

In this entry, we redirect the standard output, but allow the standard error to be sent as a mail message. This can be a nice feature because we'll get a mail message if anything goes wrong. If you want to make sure you don't receive mail under any circumstances, redirect both the standard output and the standard error to a file:

0 1 * * * find /tmp -atime 3 -exec ls -l {} \; >> /home/mdw/log 2>&1

When you save output in a log file, you get the problem of a file that grows continuously. You may want another cron entry that runs once a week or so, just to remove the file.

Only Bourne shell commands can be used in crontab entries. That means you can't use any of the convenient extensions recognized by bash and other modern shells, such as aliases or the use of ~ to mean "my home directory." You can use $HOME, however; cron recognizes the $USER, $HOME, and $SHELL environment variables. Each command runs with your home directory as its current directory.

Some people like to specify absolute pathnames for commands, like /usr/bin/find and /bin/rm, in crontab entries. This ensures that the right command is always found, instead of relying on the path being set correctly.

If a command gets too long and complicated to put on a single line, write a shell script and invoke it from cron. Make sure the script is executable (use chmod +x) or execute it by using a shell, such as:

0 1 * * * sh runcron

The choice of a user also affects file ownership: a crontab file for news should run files owned by news, and so on. In general, make sure utilities are owned by the user in whose name you create the crontab file.

As root, you can edit other users' crontab files by using the -u option. For example:

tigger # crontab -u news -e