2.1 The crontab time/date file formatIndex

A crontab is a text file of lines containing times and command lines. Each line starts with a five-field date/time specification. The five blank-separated time and date fields at the start of every crontab line are (from man 5 crontab):

crontab field allowed values
------------- --------------
minute        0-59
hour          0-23
day of month  1-31
month         1-12 (or names, if using a recent version of cron)
day of week   0-7 (0 or 7 is Sun, or use names)

Any field may be a list of numbers, separated by commas (no blanks allowed), or an asterisk (*) indicating “every” or “all”, or a range of two numbers separated by a dash:

00,30 01,13 * * * touch /tmp/foo    # touch every 30 minutes at 1am,1pm
00    08-11 * * * touch /tmp/foo    # touch every hour from 8am to 11am

The minimum resolution of the crontab is one minute, with some systems only running the crontab every 15 minutes.

See more time/date examples below under “Personal crontab Examples”.

2.2 The crontab command line format and current directoryIndex

The command line in a crontab file is any (non-interactive) shell command line. (Don’t try to run an interactive program such as the vim text editor or a GUI program such as Firefox!)

The current working directory of the shell when it runs the personal crontab command line is always your HOME directory, so if you use pathnames in your command line make sure the pathnames are valid relative to your own HOME directory:

* * * * * touch foo etc/bar # touch $HOME/foo and $HOME/etc/bar every minute

Lines that begin with # are comments inside crontab files. They are ignored.

2.3 Personal crontab filesIndex

Most Unix/Linux systems now allow individual users to have private crontab files for commands they want to run as themselves. Users must use the crontab command to manage their personal cron files. Changes take effect when the crontab command saves your personal crontab file and exits.

To edit your personal crontab file, the crontab command uses the text editor specified in the VISUAL or EDITOR environment variables:

$ crontab -l                         # show my personal crontab file
$ crontab -e                         # edit (or create) my crontab
$ crontab -r                         # delete my personal crontab

See man crontab – the crontab command for managing individual user crontab files.

On Ubuntu desktop systems such as the CLS, your selected editor for crontab -e is stored in the file ~/.selected_editor and can be changed with the command select-editor in many cases.

The current working directory of the shell when it runs the personal crontab command line is always your HOME directory, not the directory you were in when you ran the crontab command.

2.4 Personal crontab ExamplesIndex

Below are some examples of personal crontab entries:

*  *  *  *  *  echo "every minute of every day of every month"
30 *  *  *  *  echo "30 minutes after every hour of every day of every month"
30 14 *  *  *  echo "at 14:30 every day of every month"
30 14 15 *  *  echo "at 14:30 on day 15 of every month"
30 14 15 10 *  echo "at 14:30 on day 15 of month 10 (October)"
30 14 *  *  6  echo "at 14:30 every Saturday"
30 0,8,16 * * * echo "at 00:30, 08:30, and 16:30 every day of every month"

Below is an example (a very long line) that sends the current Ottawa temperature to a cell phone every hour. This requires the elinks package and some form of mail or mailx package to be installed and configured to send Internet email. (To find your phone gateway, try http://www.notepage.net/smtp.htm.)

01 * * * * elinks -dump -no-numbering -no-references 'http://weather.gc.ca/rss/city/on-118_e.xml' | fgrep 'Temperature:' | mail -s "Temperature" 6135551212@msg.example.com

See man 5 crontab – (manual section 5) the format of the crontab files (personal and system).

The current working directory of the shell when it runs the personal crontab command line is your HOME directory.

• Feature: Use an SMS Gateway List to find the email address of your SMS enabled phone, and email a test message directly to your phone. (May be subject to length restrictions and incur an arbitrary delay.)
  • If the degree special character between the temperature number and the letter “C” doesn’t display correctly on your phone (e.g. “10°C”), you can try adding either -dump-charset latin1 or -dump-charset ascii to the elinks command line.
  • To find your phone gateway, try http://www.notepage.net/smtp.htm

2.5 The System /etc/crontab FileIndex

There is a system cron configuration file /etc/crontab that can run scheduled commands as any userid (usually as root). You can (as root) use a text editor to edit the system /etc/crontab file. Changes take effect as soon as the file is saved.

* * * * * root      touch /var/log/file   # run as root user
* * * * * syslog    touch /var/adm/file   # run as syslog user
* * * * * www-admin touch /var/http/file  # run as www-admin user

# example line from the system crontab file /etc/crontab (root userid):
17 * * * * root cd / && run-parts --report /etc/cron.hourly

# example line from a personal (non-root) crontab file (no userid):
34 12 * * * echo "It's 12:34 and time to eat lunch" | write idallen

2.6 The System /etc/cron.d DirectoryIndex

Many Unix/Linux systems also have a directory of individual system crontab files under the /etc/cron.d/ directory.

Rather than editing and adding lines to the system /etc/crontab, put those lines in a file and put the file under /etc/cron.d/ for automatic inclusion in the system crontab.

Individual software packages can put their own files here without having to edit the main system crontab file.

2.7 The System /etc/cron.{daily,hourly,weekly,monthly} DirectoriesIndex

Many Unix/Linux systems also have these directories:

/etc/cron.daily/
/etc/cron.hourly/
/etc/cron.weekly/
/etc/cron.monthly/

As the names indicate, programs or scripts placed in these directories will be executed (by the system crontab) at at regular intervals, e.g. daily, hourly, weekly, or monthly.