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Useful Commands - For System Administration

Beginners Level Course: Commands for System Administration

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. My 'df' is going to be a bit confusing because I run a XenServer virtual machine attached to a SAN. Sorry!)

Filesystem           1K-blocks      Used Available Use% Mounted on 
/dev/mapper/VolGroup-lv_root
                      48964432   2838132  43639004   7% / 
tmpfs                  1987488         0   1987488   0% /dev/shm 
/dev/xvda1              495844     52160    418084  12% /boot 
/dev/mapper/VolGroup-lv_home
                      47626600    370604  44836696   1% /home


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


This output isn't very friendly. Try converting things to Megabytes by typing
free -m

Sometimes, if a program is running particularly 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: 193 people_I_owe_money.net


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

If you turn out to be a human, then you may want to set the output to something even more friendly.
Try this:
du -h

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:

top
 


top - 14:11:38 up 12 days, 22:38,  1 user,  load average: 0.00, 0.00, 0.00
Tasks:  97 total,   1 running,  96 sleeping,   0 stopped,   0 zombie
Cpu(s):  0.0%us,  0.0%sy,  0.0%ni,100.0%id,  0.0%wa,  0.0%hi,  0.0%si,  0.0%st
Mem:   3974980k total,  3515800k used,   459180k free,   244404k buffers
Swap:  6209528k total,        0k used,  6209528k free,  2684644k cached

  PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND
    1 root      20   0 19204 1512 1220 S  0.0  0.0   0:01.84 init
    2 root      20   0     0    0    0 S  0.0  0.0   0:00.00 kthreadd
    3 root      RT   0     0    0    0 S  0.0  0.0   0:00.00 migration/0
    4 root      20   0     0    0    0 S  0.0  0.0   0:00.55 ksoftirqd/0
    5 root      RT   0     0    0    0 S  0.0  0.0   0:00.00 migration/0
    6 root      RT   0     0    0    0 S  0.0  0.0   0:02.41 watchdog/0
    7 root      20   0     0    0    0 S  0.0  0.0   1:11.89 events/0
    8 root      20   0     0    0    0 S  0.0  0.0   0:00.00 cgroup
    9 root      20   0     0    0    0 S  0.0  0.0   0:00.00 khelper
   10 root      20   0     0    0    0 S  0.0  0.0   0:00.00 netns
   11 root      20   0     0    0    0 S  0.0  0.0   0:00.00 async/mgr
   12 root      20   0     0    0    0 S  0.0  0.0   0:00.00 pm
   13 root      20   0     0    0    0 S  0.0  0.0   0:00.00 xenwatch
   14 root      20   0     0    0    0 S  0.0  0.0   0:01.47 xenbus
   15 root      20   0     0    0    0 S  0.0  0.0   0:05.50 sync_supers
   16 root      20   0     0    0    0 S  0.0  0.0   0:05.87 bdi-default
   17 root      20   0     0    0    0 S  0.0  0.0   0:00.00 kintegrityd/0
   18 root      20   0     0    0    0 S  0.0  0.0   0:03.82 kblockd/0
   19 root      20   0     0    0    0 S  0.0  0.0   0:00.00 ata/0
   20 root      20   0     0    0    0 S  0.0  0.0   0:00.00 ata_aux
   21 root      20   0     0    0    0 S  0.0  0.0   0:00.00 ksuspend_usbd
   22 root      20   0     0    0    0 S  0.0  0.0   0:00.00 khubd
   23 root      20   0     0    0    0 S  0.0  0.0   0:00.00 kseriod
   24 root      20   0     0    0    0 S  0.0  0.0   0:00.00 md/0
   25 root      20   0     0    0    0 S  0.0  0.0   0:00.00 md_misc/0
   26 root      20   0     0    0    0 S  0.0  0.0   0:00.33 khungtaskd
   27 root      20   0     0    0    0 S  0.0  0.0   0:00.00 kswapd0
   28 root      25   5     0    0    0 S  0.0  0.0   0:00.00 ksmd
   29 root      20   0     0    0    0 S  0.0  0.0   0:00.00 aio/0
   30 root      20   0     0    0    0 S  0.0  0.0   0:00.00 crypto/0
   35 root      20   0     0    0    0 S  0.0  0.0   0:00.00 kthrotld/0
   37 root      20   0     0    0    0 S  0.0  0.0   0:00.00 khvcd
   38 root      20   0     0    0    0 S  0.0  0.0   0:00.00 kpsmoused
   39 root      20   0     0    0    0 S  0.0  0.0   0:00.00 usbhid_resumer
   69 root      20   0     0    0    0 S  0.0  0.0   0:00.00 kstriped
  229 root      20   0     0    0    0 S  0.0  0.0   0:00.00 kdmflush
  231 root      20   0     0    0    0 S  0.0  0.0   0:00.00 kdmflush
  250 root      20   0     0    0    0 S  0.0  0.0   0:13.44 jbd2/dm-0-8
  251 root      20   0     0    0    0 S  0.0  0.0   0:00.00 ext4-dio-unwrit




ps
'ps' will give you a list of the processes running on your system.

Just typing ps will give you the processes you're running as a user. It may look like this:
ps
 
PID TTY          TIME CMD
22987 pts/0    00:00:00 ps
29552 pts/0    00:00:00 bash
If you happen to forget what your name is, you can type ps u. This is the user mode and your user name will appear in the first column. Actually, there's more than that.


There will be other columns about memory usage, the time you started running the processes and others.

You can see other users processes if you type ps -au. If you're not using a networked computer, you will see yours and root's processes. If you're into detective work, you can type just ps -a and try to guess who's using the process.


The information that you'll probably be most interested in is the column that shows the "process ID" or "PID". We'll get into why these are so important in the next part of the lesson.

kill
"kill" is a very explicit word. It implies 'death'. In the last part of this lesson we talked about process IDs or "PIDs". Well, with the command 'kill' plus a PID, you can terminate a program. In other words, you kill the program. You will probably only use this with troublesome processes; programs that may not let you exit regularly. A good example of this is when you try a new program that hasn't got all the bugs worked out of it yet. If the 'exit' button doesn't work, you can 'kill' it.

To do this, first, you would type ps and you would get this output that we talked about before.

PID TTY STAT TIME COMMAND
293 2 S 0:00 -bash
422 2 S 0:00 sh /usr/X11R6/bin/startx
437 2 S 0:00 tee /home/bob/.X.err
438 2 S 0:00 xinit /home/bob/.xinitrc --
441 2 S 0:01 /usr/X11R6/bin/evilaliens

Let's say you're trying a new game that was just ported to Linux. The game is called "Evil and Nasty Alien Visitors". The name of the "binary" or program itself is called evilaliens. (the last one on my 'ps' example) Now you can't exit the program either. The aliens just took over Oklahoma and you swore you got them all. You're playing in your x-windows enviroment and you have an x-terminal open (probably the one you typed 'ps' into). You would just type:
kill and the PID number, in my example, 441.

That is: 
kill 441
and the program disappears, along with all those nasty aliens.

Here's a little trick. If you don't want to do two steps, 'ps' and 'kill' and you know the name of the "binary" or program itself, as I mentioned before, you can just type:
killall evilaliens
and that should also do the trick.

Using 'kill' as root
Remember that when you work as root, you are the all-powerful master of the universe. (or at least the computer). When you use 'kill' as root, you are the 007 on Her Majesty's Linux Service. You have license to 'kill' the process you desire. If you're working as 'root' and you need to use the 'kill' command, it's a good idea to run ps -au and look closely at your PIDs. You don't want to end up killing some process that you need.


Another way to kill a process.
Try typing this: 
find *.
(Kind of an absurd thing to do, but good as an example) It will just start finding everything. If you have done something like this by mistake, you can use the keys 
CTR+ C
 to stop the 'find' process. In these cases, there's no need to use the 'kill' command.