How can I see the amount of RAM and swap? How do you know how much RAM is currently being used by the system and how much is available to run new applications? For this, there is in linux free utility.
Description of the free utility
Linux free utility shows free and used memory on the system. Data is obtained by parsing /proc/meminfo.
When linux free utility launched without keys, the will display the following information:
$ free
total used free shared buff/cache available
Mem: 3871612 3444088 123092 4800 304432 199308
Swap: 3145724 977220 2168504The output contains information about the physical memory Mem and the swap file Swap. The columns contain the following parameters:
total – total installed memory
used – used memory (calculated as total – free – buffers – cache)
free – unused memory
shared – memory used by (mostly) tmpfs
buff / cache – memory used by kernel buffers and page cache and slabs
available – available memory for launching new applications (excluding swap)
By default, all values of the listed parameters are specified in kibybytes (2 to the power of 10 = 1024).
Linux free utility: output format
The linux free utility can accept the following options to change output format:
-b, –bytes – display the amount of memory in bytes
-k, –kibi – display the amount of memory in kibibytes (this is the default)
-m, –mebi – display the amount of memory in mebibytes
-g, –gibi – display the amount of memory in gibibytes
–tebi – display the amount of memory in tebibytes
–pebi – display the amount of memory in pebibytes
For example, if we want to see the memory values in mebibytes, we use the -m switch:
$ free -m
total used free shared buff/cache available
Mem: 3780 3361 117 4 302 196
Swap: 3071 1015 2056Likewise, there are the options –kilo, –mega, –giga, –tera, –peta. However, unlike those described above, which use a multiplier of 1024 during the conversion, in these keys this multiplier will be 1000.
–si – uses kilo, mega, giga, etc. (power of 1000) instead of kibi, mebi, gibi (power of 1024)
-h, –human – display memory values in human-readable form. The utility will automatically select the most suitable prefix (B, K, M, G, T, P with a multiplier of 1024):
$ free -h
total used free shared buff/cache available
Mem: 3,7Gi 3,2Gi 109Mi 4,0Mi 352Mi 239Mi
Swap: 3,0Gi 1,0Gi 2,0Gi-w, –wide – switch output to extended mode, which provides more than 80 characters per line. In this mode, the buffers and cache parameters are divided into 2 columns:
free -h -w
total used free shared buffers cache available
Mem: 3,7Gi 3,2Gi 107Mi 4,0Mi 145Mi 203Mi 232Mi
Swap: 3,0Gi 1,0Gi 2,0Gi-l, –lohi – additionally displays statistics for the lowest and highest values:
$ free -h -l
total used free shared buff/cache available
Mem: 3,7Gi 3,2Gi 115Mi 4,0Mi 347Mi 240Mi
Low: 3,7Gi 3,6Gi 115Mi
High: 0B 0B 0B
Swap: 3,0Gi 1,0Gi 2,0Gi-t, –total – additionally displays data on the total value of all memory (Mem + Swap):
$ free -h -t
total used free shared buff/cache available
Mem: 3,7Gi 3,2Gi 114Mi 4,0Mi 343Mi 234Mi
Swap: 3,0Gi 1,0Gi 2,0Gi
Total: 6,7Gi 4,3Gi 2,1Gi-s, –seconds delay – continually refresh data with a delay specified in seconds. It is allowed to use microseconds after the dot. For example, let’s set the data update to 2.5 seconds:
$ free -h -s 2.5
total used free shared buff/cache available
Mem: 3,7Gi 3,2Gi 126Mi 4,0Mi 338Mi 241Mi
Swap: 3,0Gi 1,0Gi 2,0Gi
total used free shared buff/cache available
Mem: 3,7Gi 3,2Gi 126Mi 4,0Mi 338Mi 241Mi
Swap: 3,0Gi 1,0Gi 2,0Gi
total used free shared buff/cache available
Mem: 3,7Gi 3,2Gi 125Mi 4,0Mi 338Mi 240Mi
Swap: 3,0Gi 1,0Gi 2,0GiEvery 2.5 seconds, a new output from the free command with updated data appears on the screen. To stop the command execution press Ctrl + C.
-c, –count count – display new data a certain number of times. It is required to specify the -s parameter with a delay time (if the switch is omitted, the default value of 1 second will be used).
Let’s see the change in used memory with a difference of 60 seconds:
$ free -h -c2 -s60
total used free shared buff/cache available
Mem: 3,7Gi 3,2Gi 113Mi 4,0Mi 339Mi 229Mi
Swap: 3,0Gi 1,0Gi 2,0Gi
total used free shared buff/cache available
Mem: 3,7Gi 3,3Gi 212Mi 4,0Mi 339Mi 229Mi
Swap: 3,0Gi 1,0Gi 2,0GiIn this case, after 60 seconds and the second output, the free utility will automatically finish its work.
-V, –version – view the current version of the free utility:
$ free -V
free from procps-ng 3.3.15–help – displays brief help on using the utility:
$ free --help
Usage:
free [options]
Options:
-b, --bytes show output in bytes
--kilo show output in kilobytes
--mega show output in megabytes
--giga show output in gigabytes
--tera show output in terabytes
--peta show output in petabytes
-k, --kibi show output in kibibytes
-m, --mebi show output in mebibytes
-g, --gibi show output in gibibytes
--tebi show output in tebibytes
--pebi show output in pebibytes
-h, --human show human-readable output
--si use powers of 1000 not 1024
-l, --lohi show detailed low and high memory statistics
-t, --total show total for RAM + swap
-s N, --seconds N repeat printing every N seconds
-c N, --count N repeat printing N times, then exit
-w, --wide wide output
--help display this help and exit
-V, --version output version information and exit
For more details see free(1).You can find even more useful Linux utilities on this page.