Which command is commonly used to activate all swap spaces defined in the system?

The command used to activate all swap spaces defined in the system is swapon -a. This command tells the Linux kernel to enable all swap files and swap partitions specified in the /etc/fstab configuration file. By using the -a option, you are instructing the system to consider all entries in the fstab that are marked for swapping, making it a convenient way to activate multiple swap areas in one go.

When there’s a need to manage memory efficiently in a Linux system, the swapon command plays a critical role in ensuring that the system can utilize swap memory, which essentially acts as an overflow area when RAM is insufficient. This command is essential for maintaining system performance under heavy loads or when physical memory is otherwise exhausted.

The other options are focused on different tasks: swapoff -a disables all swap areas; lsmod displays the status of modules in the Linux kernel; and mount -a is used to mount all filesystems specified in the fstab file. Each of these commands serves its own purpose, but only swapon -a is related to activating swap space.