As part of cron tasks, it is impossible to implement any non-standard interval or with an indication of waiting for the execution of another command. By itself, crontab can use the schedule only with an accuracy of the minute, and you cannot specify a certain number of seconds. Also, tasks cannot be launched in order of execution, that is, waiting for the previous task to finish and then launching the next one. To implement this, you can use useful bash features.
To run two commands sequentially, you must specify both commands in the task, separating them with the symbol
;... Such an indication will work as a sequence of commands of the form:
command1 ; command2 ; ... ; commandn
The next command will execute in any case, even if the previous one returned an error.
If you need to check for an error or the successful execution of the previous command, then you can use the following divisors:
&&- execute the next command only if the previous one was completed successfully.
||- execute the next command only if the previous NOT was successful and returned an error.
An example of use is as follows:
command1 ; command2 && command3 || command4
The commands will be executed in the following order and conditions:
command2 will be executed anyway, but
command3 will be executed only if
command2 completed without errors.
command4 will only be executed if when executing
command3 an error will be returned.
As mentioned earlier, crontab does not have the ability to customize execution exactly to the second, and therefore you have to resort to various tricks, for example, specifying the sequential execution of several commands using the wait command. For example, the following command will run 30 seconds after it is executed:
sleep 30 ; command1
If you need to run two tasks at the beginning of a minute and at 30 seconds of the same minute, then you should add two tasks with the same interval, but specify one of them
sleep 30 ; command1where
command1 a task to be performed. In this case, the execution of the second task will be delayed by 30 seconds.