free : display free and used memory
[SYNOPSIS] free OPTION
OPTION
-b show output in bytes
-k show output in kibibytes
-m show output in mebibytes
-g show output in gibibytes
-h show human-readable output
-l show detailed low and high memory statistics
-L show output on a single line
-t show total for RAM + swap
-v show committed memory and commit limit
-s N repeat printing every N seconds
# free -s 3
updates information every 3 seconds (press Ctrl+C to abort)
-c N repeat printing N times, then exit
-w wide output