"free" - Display Free and Used Memory

Linux Tutorials - Herong's Tutorial Examples

∟"free" - Display Free and Used Memory

This section provides a tutorial example on how to display free and used memory information using the 'free' command on Linux systems.

Another important information about your Linux system is the memory. You can use the "free" command to display memory statistics including free and used memory sizes. Here are some examples of "free" command output on my CentOS computer.

1. Display memory information using "free" with default options. The memory counts are displayed in kibibytes (1,024 bytes). Note that a kibibyte (1,024 bytes) is slightly different than a kilobyte (1,000 bytes).

herong$ free
           total     used     free   shared  buff/cache   available
Mem:     7789220  2047412   677256   297448     5064552     5145248
Swap:    8085500        0  8085500

The above output shows that:

The system memory (RAM - Random Access Memory) capacity is 7,789,220 KiB.
Only 2,047,412 KiB of RAM is truely used. So it has about 5,145,248 KiB available for applications to use.
But some (5,064,552 KiB) of available RAM is used as temporary "buff/cache" space. So it has only only 677,256 KiB truely free (not used at all).
The system also configured with a 8,085,500 KiB swap file on the hard disk. It can be used as an extension to the RAM.

2. To control the unit, you can use "free -b | k | m | g" options.

herong$ free -m
           total     used     free   shared  buff/cache   available
Mem:        7606     1999      661      290        4945        5024
Swap:       7895        0     7895

3. Or you can use "free -h" to display counts in human readable formats. And add "-w" to separate "buff" and "cache".

herong$ free -h -w
        total    used    free   shared   buffers   cache   available
Mem:    7.4Gi   2.0Gi   661Mi    290Mi     275Mi   4.8Gi       4.9Gi
Swap:   7.7Gi      0B   7.7Gi

The output columns are defined as:

total - Total installed memory (MemTotal and SwapTotal in /proc/meminfo)
used - Used memory (calculated as total - free - buffers - cache)
free - Unused memory
shared - Memory used (mostly) by tmpfs
buffers - Memory used by kernel buffers (Buffers in /proc/meminfo)
cache Memory used by the page cache and slabs
buff/cache - Sum of buffers and cache
available - Estimation of how much memory is available for starting new applications, without swapping. Unlike the data provided by the cache or free fields, this field takes into account page cache and also that not all reclaimable memory slabs will be reclaimed due to items being in use