87

The first noncomment line in a legacy crontab file begins with five asterisks:

* * * * * ([a_command]) >/dev/null 2>&1

The authors are gone, so I do not know their intent. What does all-wildcards mean to (Solaris 8) cron? The betting here is either run once, run continuously, or run never, which is unfortunately broad.

If you are wondering about the comment line preceding this, it is "Do not delete."

Note: This cron file is working. This question is not a duplicate of a question about broken cron files or cron files which require troubleshooting.

Avery Payne
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Thomas L Holaday
  • 1,433

3 Answers3

71

Every minute of every day of every week of every month, that command runs.

man 5 crontab has the documentation of this. If you just type man crontab, you get the documentation for the crontab command. What you want is section 5 of the manual pages which covers system configuration files including the /etc/crontab file. For future reference, the sections are described in man man:

   1   Executable programs or shell commands
   2   System calls (functions provided by the kernel)
   3   Library calls (functions within program libraries)
   4   Special files (usually found in /dev)
   5   File formats and conventions eg /etc/passwd
   6   Games
   7   Miscellaneous  (including  macro  packages and conven‐
       tions), e.g. man(7), groff(7)
   8   System administration commands (usually only for root)
   9   Kernel routines [Non standard]
jokerdino
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Luke has no name
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60

* = always. It is a wildcard for every part of the cron schedule expression.

So * * * * * means every minute of every hour of every day of every month and every day of the week.

 * * * * *  command to execute
 ┬ ┬ ┬ ┬ ┬
 │ │ │ │ │
 │ │ │ │ │
 │ │ │ │ └───── day of week (0 - 7) (0 to 6 are Sunday to Saturday, or use names; 7 is Sunday, the same as 0)
 │ │ │ └────────── month (1 - 12)
 │ │ └─────────────── day of month (1 - 31)
 │ └──────────────────── hour (0 - 23)
 └───────────────────────── min (0 - 59)

The nice drawing above is provided by wikipedia

An other example:

0 * * * * -this means the cron will run always when the minutes are 0 (so hourly)
0 1 * * * - this means the cron will run always at 1 o'clock.
* 1 * * * - this means the cron will run each minute when the hour is 1. So 1:00, 1:01, ...1:59.

Mohammad Faisal
  • 703
12
First star = Minutes: 0-59
Second star = Hours: 0-23
Third star = Day of Month: 0 - 31
Fourth star = Month: 0 - 12
Fifth star = Day of Week: 0 - 6 (0 means sunday)

Say you want to run something every 1st of every month.

0 0 1 * * something.sh
Community
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Michael Burns
  • 296