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"If you can save a timestamp file between runs, you can check the date of it instead of relying solely on the current date.
If your find command supports fractional values for -mtime (or has -mmin) (GNU find has both, POSIX does not seem to require either), you could ‘throttle’ the cron jobs with with find and touch.
Or, if you have a stat command that supports showing file dates as “seconds since the epoch” (e.g. stat from Gnu coreutils, also other implementations), you could make your own comparison using date, stat, and the shell's comparison operators (along with touch to update a timestamp file). You also might be able to use ls instead of stat if it can do the formatting (e.g. ls from GNU fileutils).
Below is a Perl program (I called it n-hours-ago) that updates a timestamp file and exits successfully if the original timestamp was old enough. Its usage text shows how to use it in a crontab entry to throttle a cron job. It also describes adjustments for “daylight savings” and how to handle ‘late’ timestamps from previous runs.
#!/usr/bin/perl use warnings; use strict; sub usage { printf STDERR <<EOU, $0; usage: %s <hours> <file>
If entry at pathname <file> was modified at least <hours> hours ago, update its modification time and exit with an exit code of 0. Otherwise exit with a non-zero exit code.
This command can be used to throttle crontab entries to periods that are not directly supported by cron.
34 2 * * * /path/to/n-hours-ago 502.9 /path/to/timestamp && command
If the period between checks is more than one ""day"", you might want to decrease your <hours> by 1 to account for short ""days"" due ""daylight savings"". As long as you only attempt to run it at most once an hour the adjustment will not affect your schedule.
If there is a chance that the last successful run might have been launched later ""than usual"" (maybe due to high system load), you might want to decrease your <hours> a bit more. Subtract 0.1 to account for up to 6m delay. Subtract 0.02 to account for up to 1m12s delay. If you want ""every other day"" you might use <hours> of 47.9 or 47.98 instead of 48.
You will want to combine the two reductions to accomodate the situation where the previous successful run was delayed a bit, it occured before a ""jump forward"" event, and the current date is after the ""jump forward"" event.
EOU }
if (@ARGV != 2) { usage; die ""incorrect number of arguments"" } my $hours = shift; my $file = shift;
if (-e $file) { exit 1 if ((-M $file) * 24 < $hours); } else { open my $fh, '>', $file or die ""unable to create $file""; close $fh; } utime undef, undef, $file or die ""unable to update timestamp of $file""; exit 0;"
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