A crontab file consists of commands, one command per line, that execute automatically at the time specified by the first five fields of each command line, which are separated by spaces.
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The guidelines for using special characters in crontab time fields are as follows:
Use a space to separate each field
Use a comma to separate multiple values
Use a hyphen to designate a range of values
Use an asterisk as a wildcard to include all possible values
Use a comment mark (#) at the beginning of a line to indicate a comment or a blank line
For example, the following crontab command entry displays a reminder in the user's console window at 4 pm on the first and fifteenth days of every month.
0 16 1,15 * * echo Timesheets Due > /dev/console
Each command within a crontab file must consist of one line, even if that line is very long. The crontab file does not recognize extra carriage returns. For more detailed information about crontab entries and command options, refer to the crontab(1) man page.