- Display system memory:free- Display memory in Bytes/KB/MB/GB:free -[b|k|m|g]- Display memory in human readable units:free -h- Refresh the output every 2 seconds:free -s 
free displays the total amount of free and used physical and swap memory in the system, as well as the buffers and caches used by the kernel. The information is gathered by parsing /proc/meminfo. The displayed columns are:
B = bytes K = kibibyte M = mebibyte G = gibibyte T = tebibyte P = pebibyte
If unit is missing, and you have exbibyte of RAM or swap, the number is in tebibytes and columns might not be aligned with header.
The value for the shared column is not available from kernels before 2.6.32 and is displayed as zero.
ps(1), slabtop(1), top(1), vmstat(8).