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The following commands are frequently used by systems administrators to keep an eye on what's going on with their systems

'last'

The command last will show you the people who have logged into the computer today and the terminals they are/were using.

If you type:

last

You may get something like this:

fred	tty6	Thu Oct 5	16:55 - 20:05	(3:10)
bob	tty1	Thu Oct 5	still logged in	(3:10)
root	tty1	Thu Oct 5	16:23 - 16:43	(0:20)
reboot	system boot	Thu Oct 5 16:22

As you can see, you worked as 'fred' for 3 hours and 10 mins. You are still working as 'bob'. You worked as 'root' for 20 minutes (probably some administration tasks) and you booted your computer at 4:22 PM.

This is a good way to see who's been using the computer if it's networked. For example, if you saw a an entry for 'satan' and you hadn't given the Prince of Darkness permission to login, you could fire off a nasty e-mail to him about mis-use of your server. His address, by the way, is "thedevil@hell.com"

'df'

'df' is a command that you're going to use a lot if you're pressed for hard disk space. Once again, there are many programs that run graphically that will inform you of the space available on your Linux partition. But this is a very good, quick, non-graphic way to keep track of your hard disk space.

If you type df

You may get something like this. (This is actually taken from a system I use for testing versions of Linux. You'll see it's somewhat pressed for space.)

Filesystem	1024-blocks	Used	Available	Capacity	Mounted on
/dev/hda2	481083	408062	48174	89%	/

if you start seeing a 'df' output like this, it's time to get down to your local computer shop and buy a new hard disk. Anyway, 'df' is a good way to keep track of this.

'free'

'free' is a command that you can use if you want to know how much RAM memory you have free on your system. By typing:

free

and you will get something like this

	total	used	free	shared	buffers	cached
Mem:	14452	13904	548	28208	492	7312
-/+ buffers/cache:	6100	8352
Swap:	33260	1556	31704

Sometimes, if a program is running particulary slowly, you may find out that your memory usage is high using this command. Linux's memory management is quite good but a certain program may be "hogging" memory. You could exit that program and then type free again to see if it was the culprit.

'du'

'du' is the way to see how big files are. You can use it on a directory or on a particular file. This is another command I use a lot. It's probably best to use the option du -b (-b for bytes) and it will give you the exact figure in bytes. By default, 'du' shows the closest kilobyte figure. Let's look at a couple of examples:

If I type:

du people_I_owe_money.note

I may get an output like this:

194 people_I_owe_money.note

But instead, if I type:

du -b people_I_owe_money.note

I'll get:

197120 people_I_owe_money.note

As you can see, it's a big file. I owe a lot of people money. On the other hand look at the output for 'people_who_owe_me_money.note':

1 people_who_owe_me_money.note

No, that's not the kilobyte figure. That's the byte figure!

You can also use this on a directory, and it will list the files and subdirectories and give you the byte or kilobyte count, whichever you prefer

top

To show you the use of the 'top' command. Here you will see what processes are running 'top' is a good command to use when you want to see what your system's doing. 'top' is designed to show you how your CPU is being used. It will give you a pretty complete list of everything that's going on in your computer. Here's a sample output of the 'top' command:

9:09am	up 4 min	2 users	load average:	0.12	0.13	0.06
34 processes:	31 sleeping	3 running	0 zombie	0 stopped
CPU states:	11.1% user	0.7% system	0.0% nice	88.4% idle
Mem: 62972K av	50420K used	12552K free	22672K shrd	5312K buff
Swap: 1020116K av	3152K used	1016964K free	27536K cached

PID	USER	PRI	NI	SIZE	RSS	SHARE	STAT	LIB	%CPU	%MEM	TIME	COMMAND
332	root	12	0	4492	4184	1480	R	0	8.3	6.6	0:09	X
350	bob	4	0	13144	11M	7728	S	0	2.7	19.3	0:04	netscape
386	bob	1	0	768	768	596	R	0	0.5	1.2	0:00	top
345	bob	0	0	972	972	704	S	0	0.1	1.5	0:00	FvwmButtons
1	root	0	0	168	168	144	S	0	0.0	0.2	0:02	init
2	root	0	0	0	0	0	SW	0	0.0	0.0	0:00	kflushd
3	root	0	0	0	0	0	SW	0	0.0	0.0	0:00	kpiod
4	root	0	0	0	0	0	SW	0	0.0	0.0	0:00	kswapd
5	root	0	0	0	0	0	SW	0	0.0	0.0	0:00	md_thread
48	root	0	0	136	120	96	S	0	0.0	0.1	0:00	update
137	bin	0	0	300	296	228	S	0	0.0	0.4	0:00	portmap
141	root	0	0	292	264	212	S	0	0.0	0.4	0:00	rpc.ugidd
157	root	0	0	512	508	412	S	0	0.0	0.8	0:00	syslogd
161	root	0	0	692	688	296	S	0	0.0	1.0	0:00	klogd
212	at	0	0	304	276	220	S	0	0.0	0.4	0:00	atd
225	root	0	0	424	416	348	S	0	0.0	0.6	0:00	inetd
258	root	0	0	580	548	428	S	0	0.0	0.8	0:00	lpd