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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 - The system clock daemon SYNOPSIS
/usr/sbin/cron DESCRIPTION
The cron daemon runs shell commands at specified dates and times. Commands that are to run according to a regular or periodic schedule are found within the crontab files. Commands that are to run once only are found within the at files. You submit crontab and at file entries by using the crontab and at commands. Because the cron process exits only when killed or when the system stops, only one cron daemon should exist on the system at any given time. Normally, you start the cron daemon from within a run command file. During process initialization and when cron detects a change, it examines the crontab and at files. This strategy reduces the overhead of checking for new or changed files at regularly scheduled intervals. The cron command creates a log of its activities. The cron daemon must be started from the system startup scripts because it must begin execution without a login user ID set. The cron daemon starts each job with the following process attributes stored with the job by the invoking process: Login user ID Effective and real user IDs Effective and real group IDs Supplementary groups It also establishes the following attributes from the authentication profile of the account associated with the login user ID of the invok- ing process: Audit control and disposition masks Kernel authorizations DIAGNOSTICS
The at and batch programs will refuse to accept jobs submitted from processes whose login user ID is different from the real user ID. FILES
Specifies the command path. Main cron directory Directory containing the crontab files. List of allowed users. List of denied users His- tory information for cron Queue description file for at, batch, and cron RELATED INFORMATION
Commands: at(1), crontab(1), rc0(8), rc2(8), rc3(8) Files: queuedefs(4) delim off cron(8)
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