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Top Forums Shell Programming and Scripting Script that will look the same as Cron Post 303037677 by drysdalk on Friday 9th of August 2019 10:24:12 AM
Old 08-09-2019
Hi,

In that case, you'd really be into the territory of literally re-inventing crontab. For example, you could define an input file format that consisted of runtimes and their associated commands, and have your script read that file and run the commands when the runtimes arrived - in other words, to do pretty much exactly what the crond daemon does.

I've not got time right now to write a full example unfortunately, but if you defined your input format as something like:

Code:
Mon,10,00,/usr/local/bin/foo.sh
Fri,17,00,/usr/local/bin/bar.sh

and had your script read this file line-by-line, breaking out the variables in each line into the days, hour, minutes and commands, you could then run those commands via the same kind of infinite-loop approach as you've seen in the examples given thus far.

If you absolutely don't have access to crontab, and can't get the maintainer of the server to put in the cron entries for you, then this would more or less be the only way you'd get a general-purpose task scheduler going. But you'd probably want to be sure that the maintainer of the server was happy with you doing this, since presumably they've blocked access to cron to prevent people running commands on a set schedule, which is what you'd end up doing anyway here.
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CRON(8) 						      System Manager's Manual							   CRON(8)

NAME
cron - daemon to execute scheduled commands (Vixie Cron) SYNOPSIS
cron DESCRIPTION
Cron should be started from /etc/rc or /etc/rc.local. It will return immediately, so you don't need to start it with '&'. Cron searches /var/spool/cron for crontab files which are named after accounts in /etc/passwd; crontabs found are loaded into memory. Cron also searches for /etc/crontab and the files in the /etc/cron.d/ directory, which are in a different format (see crontab(5)). Cron then wakes up every minute, examining all stored crontabs, checking each command to see if it should be run in the current minute. When execut- ing commands, any output is mailed to the owner of the crontab (or to the user named in the MAILTO environment variable in the crontab, if such exists). Additionally, cron checks each minute to see if its spool directory's modtime (or the modtime on /etc/crontab) has changed, and if it has, cron will then examine the modtime on all crontabs and reload those which have changed. Thus cron need not be restarted whenever a crontab file is modified. Note that the Crontab(1) command updates the modtime of the spool directory whenever it changes a crontab. SEE ALSO
crontab(1), crontab(5) AUTHOR
Paul Vixie <paul@vix.com> 4th Berkeley Distribution 20 December 1993 CRON(8)
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