see: crontab(5) 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 ‘‘Note’’ below). cron(8) examines cron entries once every minute. The time and date fields are: field allowed values ----- -------------- minute 0-59 hour 0-23 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 ‘‘/’’ specifies skips of the number’s value through the range. For example, ‘‘0-23/2’’ can be used in the hours field to spec‐ ify command execution every other hour (the alternative in the V7 stan‐ dard 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 charac‐ ters, 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 cur‐ rent 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 * * * *". EXAMPLE CRON FILE # use /bin/bash to run commands, instead of the default /bin/sh SHELL=/bin/bash # mail any output to ‘paul’, no matter whose crontab this is MAILTO=paul # # 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" EXAMPLE SYSTEM CRON FILE This has the username field, as used by /etc/crontab. # /etc/crontab: system-wide crontab # Unlike any other crontab you don’t have to run the ‘crontab’ # command to install the new version when you edit this file # and files in /etc/cron.d. These files also have username fields, # that none of other the crontabs do. SHELL=/bin/sh PATH=/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin # m h dom mon dow user command 42 6 * * * root run-parts --report /etc/cron.daily 47 6 * * 7 root run-parts --report /etc/cron.weekly 52 6 1 * * root run-parts --report /etc/cron.monthly # # Removed invocation of anacron, as this is now handled by a # /etc/cron.d file