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.
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