Execute a user-specified command if a certain amount of time has passed.
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Execute a user-specified command if a certain amount of time has passed.

If the specified command exits successfully (ie, with an exit code of zero) the current timestamp is saved in a file. Every time the script runs, it checks the timestamp stored in the file. If the timestamp is greater than a user-specified period, the specified command is executed.

See source for configuration.


I want to perform daily backups of a laptop to a USB drive using rsnapshot. I only want to perform the backup once per day. The drive is only plugged in when I'm at my desk. By calling this script every hour via cron, there is a high likelyhood of completing a backup each day, but I'm guaranteed to never complete more than one backup per day.

@hourly backitup.sh -l ~/.rsnapshot-monthly -b "/usr/bin/rsnapshot monthly" -p MONTHLY
@hourly backitup.sh -l ~/.rsnapshot-weekly -b "/usr/bin/rsnapshot weekly" -p WEEKLY
@hourly backitup.sh -l ~/.rsnapshot-daily -b "/usr/bin/rsnapshot daily"

While the script was written to execute backups, it can be used to call any command.


The desired period may be specified either in seconds or as DAILY, WEEKLY or MONTHLY.

Note that the latter options will result in different behaviour than using the equivalent seconds. For instance, a period of DAILY may result in the command being executed twice in a 24-hour period (but on separate calendar days), where a period of 86400 will guarantee the command will never be executed more than once in a 24-hour period.