free - Memory Monitoring

$ free –m
$ free –c 5 –s 3
$ free -m

      total used free shared buffers cached
Mem :  1003  981   22      0      91    688
-/+ buffers/cache: 201 802
Swap:  1058    0 1058

As you can see, my system has 1 GB of ram and 981 MB are in use leaving 22MB free. If you look at the cached column, it shows 688 MB free. This is a good thing as cached memory is basically free memory. This is where programs a user may have used earlier and then quit are stored, just on the off chance that the user might start up the program again. On the other hand, if the user starts up a new program, this cache can be replaced for the new program that is running. It should be mentioned that the caching works not just for recently loaded programs but also for data, i.e. recently used files and directories. Program loading is just a special case of loading a file.

The -/+ buffers/cache section is will show you what is really going on. In my example, it shows that only 201 MB are in use and that 802 MB are free. The rest is just cached. 

What a user really needs to worry about is that last line. If you start seeing the swap file go into use that means that you are out of free ram and you are now using space on your hard disk to help out. If this starts happening, the best thing to do is run the top command and see what is taking up all the memory. Then, if it is an unneeded program, shut it down.