How to run a program with only one CPU core?

It is not terminator that is doing the "spreading" on all the CPU of a given process. Linux itself (the kernel) is doing this. A task (process) is scheduled as available to run on all CPUs by defaults; if it uses threads it can uses more than one CPU at a time.

To restrict a process to a specific CPU, you use the command taskset.

taskset --cpu-list 1,2 my_command 

This command forces my_command to run just on CPUs #1 or #2.

To learn more, type man taskset or search for "linux CPU affinity" (first hit here).

Though another answer has already given the literal answer I would explore the possibility this is not being done the right way. Instead you should run a script which you do not want to slow down the computer (such as non interactive scripts) using the nice command.

To do this simply prefix the command you wish to run with nice, for example: nice command_to_run This will cause the program to be de-prioritized below other tasks and will not slow down your machine. I often use this technique when doing a long compile which would otherwise slow down my PC to a crawl, this has the advantage of allowing the program to use all cores when the machine is not busy but will quickly stop it using them when you run something else.