Sorting down processes by memory usage
Use the following command:
ps aux --sort -rss
Check here for more Linux process memory usage
A quick and dirty method is to just pipe the output of ps aux to the sort command:
$ ps aux | sort -rn -k 5,6
Example
$ ps aux | sort -rn -k 5,6
...
root 1584 0.0 0.0 22540 1236 ? S 07:04 0:01 hald-addon-storage: polling /dev/sr0 (every 2 sec)
root 1575 0.0 0.0 22536 872 ? S 07:04 0:00 /usr/libexec/hald-addon-generic-backlight
root 1574 0.0 0.0 22536 880 ? S 07:04 0:00 /usr/libexec/hald-addon-leds
root 1565 0.0 0.0 22536 876 ? S 07:04 0:00 /usr/libexec/hald-addon-rfkill-killswitch
saml 2507 0.0 0.0 22232 500 ? S 07:05 0:00 dbus-launch --sh-syntax --exit-with-session
root 1671 0.0 0.0 22156 936 ? Ss 07:04 0:00 xinetd -stayalive -pidfile /var/run/xinetd.pid
...
This doesn't handle for the column headers which get mixed in with the output, but it's easy to remember on the command line, and is an acceptable way to do what you want when manually viewing this type of output.
Example
root 1791 0.0 0.0 4140 536 tty2 Ss+ 07:04 0:00 /sbin/mingetty /dev/tty2
USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND
root 996 0.0 0.0 0 0 ? S 07:04 0:01 [kdmflush]
root 982 0.0 0.0 0 0 ? S 07:04 0:00 [kvm-irqfd-clean]
More tips
An additional tip would be to pipe the entire output to another command such as less. This allows you to look at the information a page at a time and also use the arrow keys and page up/down keys to scroll back and forth through the output.
$ ps aux | sort -rn -k 5,6 | less
If your output is wrapping a lot you can also utilize the -S switch to less, which will force all the output to stay on a single line instead. You can then use your arrow keys to move left/right/up/down to see all of it.
$ ps aux | sort -rn -k 5,6 | less -S
Sorting within ps
Certain versions of ps provide the ability to use --sort. This switch can then take keys that are either prefixed with a + or a - to denote the sort order...least to greatest or greatest to least.
Examples
vsz,-rss
$ ps aux --sort=vsz,-rss | head -5
USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND
root 2 0.0 0.0 0 0 ? S 07:03 0:00 [kthreadd]
root 3 0.0 0.0 0 0 ? S 07:03 0:00 [ksoftirqd/0]
root 4 0.0 0.0 0 0 ? S 07:03 0:01 [migration/0]
root 5 0.0 0.0 0 0 ? S 07:03 0:00 [watchdog/0]
+vsz,+rss
$ ps aux --sort=+vsz,+rss | head -5
USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND
root 2 0.0 0.0 0 0 ? S 07:03 0:00 [kthreadd]
root 3 0.0 0.0 0 0 ? S 07:03 0:00 [ksoftirqd/0]
root 4 0.0 0.0 0 0 ? S 07:03 0:01 [migration/0]
root 5 0.0 0.0 0 0 ? S 07:03 0:00 [watchdog/0]
-vsz,-rss
$ ps aux --sort=-vsz,-rss | head -5
USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND
root 1832 0.0 0.0 2088924 3312 ? Sl 07:04 0:00 /usr/sbin/console-kit-daemon --no-daemon
saml 3517 0.2 1.2 2073196 100492 ? Sl 07:06 0:34 /home/saml/.dropbox-dist/dropbox
saml 3516 0.0 0.8 2071032 67388 ? Sl 07:06 0:07 /home/saml/.dropbox-dist/dropbox
saml 2657 0.1 0.7 1580936 57788 ? Sl 07:05 0:27 nautilus
Even if ps do not reflect the actual memory used, this command is pretty helpful.
ps -eo size,pid,user,command --sort -size | awk '{ hr=$1/1024 ; printf("%13.2f Mb ",hr) } { for ( x=4 ; x<=NF ; x++ ) { printf("%s ",$x) } print "" }'