Cron Job timing

Probably a simple question but i'm having a hard time finding hte answer.  we have a need to set up cron jobs to run at wierd times (every 2 hours, every 6 hours, every 4 days, etc...), and i'm having trouble figureing out how to craft the cron job lines.  i'm new to the cron setup so this might be something simple or i might have unrealistic expectations of cron.  so i guess the question is, how do i create a cron line that says "run this line of code every 4 hours" without having to say " run it at 00, 4, 8, 12, 16, etc..."

any ideas?
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marxyConnect With a Mentor Commented:
     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", "0-4,8-124".

       Step values can be used in conjunction with ranges.  Following a range with "/<number>" 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  aster-
       isk, 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 cronfile.  Per-
       cent-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.

       Note:  The  day of a command's execution can be specified by two fields &#8212; 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.
wesly_chenConnect With a Mentor Commented:

  marxy makes a very good explanation of cron job usage.

Let me make it simple.
> run this line of code every 4 hours
crontab -e
0 */4 * * * <cron job script>

Every 4 days will be kind of tricky.
0 1 */4 * * <cron job script>
may not work as expected since the number of days in each month is different.
So every Friday or Saturday will be more predictable.
0 1 * * 5 <cron job script>   # every Friday 1:00AM

I hope this make you clear.

onsite_techAuthor Commented:
wow, thats a great explination guys, thanks!  the cronjobs are working great now.  thanks again
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