Crontab Format
Commands are executed by cron 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.
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 "/" 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 to say "every two hours", just 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 crontab file. Percent-signs (%) in the command, unless escaped with backslash (\), will be changed into newline characters, and all data after the first % will be sent to the command as standard input. There is no way to split a single command line onto multiple lines, like the shell's trailing "\".
Note: 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 (i.e., 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:
string meaning
------ -------
@reboot Run once, at startup.
@yearly Run once a year, "0 0 1 1 *".
@annually (same 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 * * * *".
An example of crontab format with commented fields is as follows:
# Minute Hour Day of Month Month Day of Week Command
# (0-59) (0-23) (1-31) (1-12 or Jan-Dec) (0-6 or Sun-Sat)
0 2 12 * * /usr/bin/find
This line executes the "find" command at 2AM on the 12th of every month.
Examples
Here are some more examples of crontab lines. Use the command "crontab -e" to edit your crontab file.
This line executes the "ping" command every minute of every hour of every day of every month. The standard output is redirected to dev null so we will get no e-mail but will allow the standard error to be sent as a e-mail. If you want no e-mail ever change the command line to "/sbin/ping -c 1 192.168.0.1 > /dev/null 2>&1".
* * * * * /sbin/ping -c 1 192.168.0.1 > /dev/null
This line executes the "ping" and the "ls" command every 12am and 12pm on the 1st day of every 2nd month. It also puts the output of the commands into the log file /var/log/cronrun.
0 0,12 1 */2 * /sbin/ping -c 192.168.0.1; ls -la >>/var/log/cronrun
This line executes the disk usage command to get the directory sizes every 2am on the 1st through the 10th of each month. E-mail is sent to the email addresses specified with the MAILTO line. The PATH is also set to something different.
PATH=/usr/local/sbin:/usr/local/bin:/home/user1/bin
MAILTO=user1@nowhere.org,user2@somewhere.org
0 2 1-10 * * du -h --max-depth=1 /
This line is and example of running a cron job every month at 4am on Mondays, and on the days between 15-21. This is because using the day of month and day of week fields with restrictions (no *) makes this an "or" condition not an "and" condition. Both will be executed.
0 4 15-21 * 1 /command
Run on every second Sunday of every month. The test has to be run first because of the issue mentioned in the example above.
0 4 8-14 * * test $(date +%u) -eq 7 && echo "2nd Sunday"
By
Saju Govind,
