Linux

To find out how much swap space has been allocated and is currently being used, use either the swapon or top commands on Linux:

% /sbin/swapon -s
Filename         Type         Size     Used    Priority
/dev/hda7        partition  128484    28      -1
    
% top
jcownie@pc2: top
(null) 1:29pm up 4:28, 1 user, load average: 0.00, 0.00, 0.00
52 processes: 50 sleeping, 2 running, 0 zombie, 0 stopped
CPU states: 1.1% user, 0.4% system, 0.0% nice, 98.4% idle
Mem: 127904K a, 116512K used, 11392K free, 36020K shrd, \
            3632K buff
Swap: 128484K av,     28K used, 128456K free       79804K 
cached
... remainder of "top" listing removed ...

You can use the mkswap(8) command to create swap space. The swapon(8) command tells Linux that it should use this space.