How can I check if swap is active from the command line?

in linux, you can use

• cat /proc/meminfo to see total swap, and free swap (all linux)
• cat /proc/swaps to see which swap devices are being used (all linux)
• swapon -s to see swap devices and sizes (where swapon is installed)
• vmstat for current virtual memory statistics

in Mac OS X, you can use

• vm_stat to see information about virtual memory (swap)
• ls -lh /private/var/vm/swapfile* to see how many swap files are being used.

in Solaris, you can use

• swap -l to see swap devices/files, and their sizes
• swap -s to see total swap size, used & free
• vmstat to see virtual memory statistics

On some systems, "virtual memory" refers only to disk-backed memory devices, and on other systems, like Solaris, Virtual Memory can refer to any user process address space, including tmpfs filesystems (like /tmp) and shared memory space.

Try swapon -s for a list of current swap devices and other relevant information.

swapon --show

This is a bit better than swapon -s as it provides human friendly size units such as:

NAME      TYPE      SIZE   USED PRIO
/dev/dm-1 partition 7.5G 563.8M   -2

man swap says:

-s, --summary

Display swap usage summary by device. Equivalent to "cat /proc/swaps". This output format is DEPRECATED in favour of --show that provides better control on output data.

--show[=column...]

Display a definable table of swap areas. See the --help > output for a list of available columns.

Tested in Ubuntu 18.04, util-linux 2.31.1.