How do I use the free command?

The free command displays system memory utilization. Here is an example of its output:
total used free shared buffers cachedMem: 255508 240268 15240 0 7592 86188-/+ buffers/cache: 146488 109020Swap: 530136 26268 503868
The Mem: row displays physical memory utilization, while the Swap: row displays the utilization of the system swap space, and the -/+ buffers/cache: row displays the amount of physical memory currently devoted to system buffers.
Since free by default only displays memory utilization information once, it is only useful for very short-term monitoring, or quickly determining if a memory-related problem is currently in progress. Although free has the ability to repetitively display memory utilization figures via its -s option, the output scrolls, making it difficult to easily detect changes in memory utilization.
Tip
A better solution than using free -s would be to run free using the watch command. For example, to display memory utilization every two seconds (the default display interval for watch), use this command:watch free
The watch command issues the free command every two seconds, updating by clearing the screen and writing the new output to the same screen location. This makes it much easier to determine how memory utilization changes over time, since watch creates a single updated view with no scrolling. You can control the delay between updates by using the -n option, and can cause any changes between updates to be highlighted by using the -d option, as in the following command:watch -n 1 -d free
For more information, refer to the watch man page.The watch command runs until interrupted with [Ctrl]-[C]. The watch command is something to keep in mind; it can come in handy in many situations.