Can my MIT/BSD licensed, open source project, or a closed source project I build and sell, execute a command, where the command is a program released under the GPL?
I would not be distributing any version of the GPL'd program, either source code or binary. I would be telling the customer/end user that they need to have the package/binary installed prior to running the program.
Yes. If your program does not contain any code - source or object - from a GPL'd work, then it is not subject to the GPL.
I am not a lawyer, nor do i retain any, but Oracle does. Lots of them, rather (in)famously. Oracle distributes version of many of its products that run on Linux. The Linux kernel and most of its utilities are GPL'd, and its products certainly call them. If Oracle's lawyers think that's okay, then i think it's more than likely that it's okay.
I don't think there's a problem as long as you don't include GPL code in the same executable. Calling external tools in the command line doesn't necessarily break your license.
In practice, doing what you suggested allows the customer/end user to replace the existing command by some custom solution at any moment.
PS: I'm not a lawyer, so you should probably talk to one.
