How can I monitor the memory usage?

I recommend combining the previous answers

watch -n 5 free -m

Note that Linux likes to use any extra memory to cache hard drive blocks. So you don't want to look at just the free Mem. You want to look at the free column of the -/+ buffers/cache: row. This shows how much memory is available to applications. So I just ran free -m and got this:

             total       used       free     shared    buffers     cached
Mem:          3699       2896        802          0        247       1120
-/+ buffers/cache:       1528       2170
Swap:         1905         62       1843

I know that I'm using 1528 MB and have 2170 MB free.

Note: To stop this watch cycle you can just press Ctrl+C.

I think htop is the best solution.

• sudo apt-get install htop

This way you will notice what programs is using most RAM. and you can easily terminate one if you want to. Here's a screenshot!

If you looking for a nice breakdown of the memory used by each running process, then I might recommend checking out ps_mem.py (found here at pixelbeat.org).

I know in the comments above, you mentioned wanting a one-line snapshot from free, but I figured others might find this useful.

Example output:

user@system:~$ sudo ps_mem.py
[sudo] password for user:
 Private  +   Shared  =  RAM used       Program

  4.0 KiB +   7.5 KiB =  11.5 KiB       logger
  4.0 KiB +   8.0 KiB =  12.0 KiB       mysqld_safe
  4.0 KiB +  10.0 KiB =  14.0 KiB       getty
  4.0 KiB +  42.0 KiB =  46.0 KiB       saslauthd (5)
 48.0 KiB +  13.0 KiB =  61.0 KiB       init
 56.0 KiB +  27.5 KiB =  83.5 KiB       memcached
 84.0 KiB +  26.5 KiB = 110.5 KiB       cron
120.0 KiB +  50.0 KiB = 170.0 KiB       master
204.0 KiB + 107.5 KiB = 311.5 KiB       qmgr
396.0 KiB +  94.0 KiB = 490.0 KiB       tlsmgr
460.0 KiB +  65.0 KiB = 525.0 KiB       rsyslogd
384.0 KiB + 171.0 KiB = 555.0 KiB       sudo
476.0 KiB +  83.0 KiB = 559.0 KiB       monit
568.0 KiB +  60.0 KiB = 628.0 KiB       freshclam
552.0 KiB + 259.5 KiB = 811.5 KiB       pickup
  1.1 MiB +  80.0 KiB =   1.2 MiB       bash
  1.4 MiB + 308.5 KiB =   1.7 MiB       fail2ban-server
888.0 KiB +   1.0 MiB =   1.9 MiB       sshd (3)
  1.9 MiB +  32.5 KiB =   1.9 MiB       munin-node
 13.1 MiB +  86.0 KiB =  13.2 MiB       mysqld
147.4 MiB +  36.5 MiB = 183.9 MiB       apache2 (7)
---------------------------------
                        208.1 MiB
=================================

 Private  +   Shared  =  RAM used       Program

The only part I don't like is the fact that the script claims to require root privileges. I haven't had an opportunity yet to see exactly why this is the case.

Use the free command. For example, this is the ouput of free -m:

             total       used       free     shared    buffers     cached
Mem:          2012       1666        345          0        101        616
-/+ buffers/cache:        947       1064
Swap:         7624          0       7624

free -m | grep /+ will return only the second line:

-/+ buffers/cache:        947       1064

The watch command may be useful. Try watch -n 5 free to monitor memory usage with updates every five seconds.

You can do it using cat /proc/meminfo.

MemTotal:        4039160 kB
MemFree:          309796 kB
MemAvailable:    3001052 kB
Buffers:          345636 kB
Cached:          2341288 kB
SwapCached:            8 kB
Active:          1725160 kB
Inactive:        1551652 kB
Active(anon):     538404 kB
Inactive(anon):    70076 kB
Active(file):    1186756 kB
Inactive(file):  1481576 kB
Unevictable:          32 kB
Mlocked:              32 kB
SwapTotal:       4194300 kB
SwapFree:        4194044 kB
Dirty:                 0 kB
Writeback:             0 kB
AnonPages:        589988 kB
Mapped:           255972 kB
Shmem:             18596 kB
Slab:             374888 kB
SReclaimable:     310496 kB
SUnreclaim:        64392 kB
KernelStack:        6976 kB
PageTables:        26452 kB
NFS_Unstable:          0 kB
Bounce:                0 kB
WritebackTmp:          0 kB
CommitLimit:     6213880 kB
Committed_AS:    3589736 kB
VmallocTotal:   34359738367 kB
VmallocUsed:           0 kB
VmallocChunk:          0 kB
HardwareCorrupted:     0 kB
AnonHugePages:         0 kB
ShmemHugePages:        0 kB
ShmemPmdMapped:        0 kB
CmaTotal:              0 kB
CmaFree:               0 kB
HugePages_Total:       0
HugePages_Free:        0
HugePages_Rsvd:        0
HugePages_Surp:        0
Hugepagesize:       2048 kB
DirectMap4k:      147392 kB
DirectMap2M:     4046848 kB

I just found this solution and it works very well, creating a graphical interface through the CLI:

# fixed link
git clone https://github.com/aristocratos/bashtop
cd bashtop
bash bashtop

For visual monitoring of overall RAM usage, if you use Byobu, it will keep your memory usage in the lower right-hand corner of the terminal and will run while you are in any terminal session.

As you can see from screenshot, my virtual machine has a 1h3m uptime, 0.00 load, has 2.8GHz (virtual) processor and 994MB (21%) of the RAM available on the system.

Single line solution and output:

free -m | grep "Mem:"

Here is an example of the expected output:

Mem:           3944         652         302          18        2990        2930

I would use Cacti. This will graph your memory usage etc over a period of time, and you will be able to check on usage using your web browser.