To set the shell that will be used to run all “Run a Command” scheduled
tasks by default, go to Tools & Settings > Scheduled Tasks >
Settings and select the desired shell from the menu.
Alternatively, you can log in to the server via ssh and run the
following command:
plesk bin server_pref -u -crontab-secure-shell "<shell>"
Where <shell> is the desired shell. For example:
plesk bin server_pref -u -crontab-secure-shell "/bin/bash"