The cron system allows you to schedule UNIX programmes to run at specified intervals without manual intervention.
Users are each assigned a ‘crontab’ which is administered via the ‘crontab’ programme using the ‘vi’ editor (or your default editor if you have set a different one).
The format of the crontab file is explained in the cron main page which you can read by typing the following at the UNIX prompt:
man 5 crontab
To view your crontab, type:
To edit your crontab, type:
You are now using the VI editor. Generally, you type escape and i to insert text. When you are done, press escape and :wq to exit and save the crontab.
To delete a line, move to that line and press escape and dd. You may find it helpful to prepare your crontab in an editor first and then paste it into the terminal.
Blank lines and leading spaces and tabs are ignored. Lines whose first non-space character is a pound sign (#) are comments, and are ignored. Note that comments are not allowed on the same line as cron commands, since they will be taken to be part of the command. Similarly, comments are not allowed on the same line as environment variable settings.
The first line of your crontab should be a MAILTO command. Enter a command as follows:
This will ensure that you receive copies of all the errors that occur when your programmes run.
The format of a cron command is as follows:
- Each line has five time and date fields, followed by a user name (if this is the system crontab file), followed by a command. Commands are executed by cron(8) when the minute, hour, and month of year fields match the current time, and when at least one of the two day fields (day of month, or day of week) match the current time (see below). cron(8) examines cron entries once every minute. The time and date fields are:
field allowed values
day of month 1-31
month 1-12 (or names, see below)
day of week 0-7 (0 or 7 is Sun, or use names)
A field may be an asterisk (*), which always stands for ‘first-last’.
Ranges of numbers are allowed. Ranges are two numbers separated with a hyphen. The specified range is inclusive. For example, 8-11 for an ‘hours’ entry specifies execution at hours 8, 9, 10 and 11.
Lists are allowed. A list is a set of numbers (or ranges) separated by commas. Examples: ‘1, 2, 5, 9’, ‘0-4,8-12’.
Step values can be used in conjunction with ranges. Following a range with a slash (‘/’) specifies skips of the number's value through the range. For example, ‘0-23/2’ can be used in the hours field to specify command execution every other hour (the alternative in the V7 standard is ‘0, 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22’). Steps are also permitted after an asterisk, so if you want execution every two hours use ‘*/2’.
Names can also be used for the ‘month’ and ‘day of week’ fields. Use the first three letters of the particular day or month (case doesn't matter). Ranges or lists of names are not allowed.
The ‘sixth’ field (the rest of the line) specifies the command to be run. The entire command portion of the line, up to a newline or % character, will be executed by /bin/sh or by the shell specified in the SHELL variable of the cronfile. Percent-signs (‘%’) in the command, unless escaped with slash (‘/’), will be changed into newline characters, and all data after the first % will be sent to the command as standard input.
Advanced settings and troubleshooting
The day of a command's execution can be specified by two fields -- day of month, and day of week. If both fields are restricted (ie, aren't *), the command will be run when either field matches the current time. For example, ‘30 4 1, 15 * 5’ would cause a command to be run at 4:30 am on the 1st and 15th of each month, plus every Friday.
Instead of the first five fields, one of eight special strings may appear:
@reboot Run once, at startup
@yearly Run once a year, ‘0 0 1 1 *’
@annually Sames as @yearly
@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
@hourly Run once an hour, ‘0 * * * *’
Example Cron File
# use /bin/sh to run commands, overriding the default set by cron SHELL=/bin/sh
# mail any output to `paul', no matter whose crontab this is
# run five minutes after midnight, every day
5 0 * * * $HOME/bin/daily.job >> $HOME/tmp/out 2>&1
# run at 2:15pm on the first of every month -- output mailed to paul
15 14 1 * * $HOME/bin/monthly
# run at 10 pm on weekdays, annoy Joe
0 22 * * 1-5 mail -s "It's 10pm" joe%Joe,%%Where are your kids?%
23 0-23/2 * * * echo "run 23 minutes after midn, 2am, 4am ..., everyday"
5 4 * * sun echo "run at 5 after 4 every sunday"
You may find more information at http://en.tldp.org/LDP/lame/LAME/linux-admin-made-easy/using-cron.html.