intro
DESCRIPTION
Linux is a flavour of Unix, and as a first approximation
all user commands under Unix work precisely the same under
Linux (and FreeBSD and lots of other Unix-like systems).
Under Linux there are GUIs (graphical user interfaces),
where you can point and click and drag, and hopefully get
work done without first reading lots of documentation. The
traditional Unix environment is a CLI (command line inter
face), where you type commands to tell the computer what
to do. That is faster and more powerful, but requires
finding out what the commands are. Below a bare minimum,
to get started.
Login
In order to start working, you probably first have to
login, that is, give your username and password. See also
login(1). The program login now starts a shell (command
interpreter) for you. In case of a graphical login, you
get a screen with menus or icons and a mouse click will
start a shell in a window. See also xterm(1).
The shell
One types commands to the shell, the command interpreter.
It is not built-in, but is just a program and you can
change your shell. Everybody has her own favourite one.
The standard one is called sh. See also ash(1), bash(1),
csh(1), zsh(1), chsh(1).
A session might go like
knuth login: aeb
Password: ********
% date
Tue Aug 6 23:50:44 CEST 2002
% cal
August 2002
Su Mo Tu We Th Fr Sa
1 2 3
4 5 6 7 8 9 10
11 12 13 14 15 16 17
18 19 20 21 22 23 24
25 26 27 28 29 30 31
% ls
bin tel
% ls -l
total 2
drwxrwxr-x 2 aeb 1024 Aug 6 23:51 bin
-rw-rw-r-- 1 aeb 37 Aug 6 23:52 tel
% cat tel
maja 0501-1136285
% rm tel1
% grep maja tel2
maja 0501-1136285
%
and here typing Control-D ended the session. The % here
was the command prompt - it is the shell's way of indicat
ing that it is ready for the next command. The prompt can
be customized in lots of ways, and one might include stuff
like user name, machine name, current directory, time,
etc. An assignment PS1="What next, master? " would change
the prompt as indicated.
We see that there are commands date (that gives date and
time), and cal (that gives a calendar).
The command ls lists the contents of the current directory
- it tells you what files you have. With a -l option it
gives a long listing, that includes the owner and size and
date of the file, and the permissions people have for
reading and/or changing the file. For example, the file
"tel" here is 37 bytes long, owned by aeb and the owner
can read and write it, others can only read it. Owner and
permissions can be changed by the commands chown and
chmod.
The command cat will show the contents of a file. (The
name is from "concatenate and print": all files given as
parameters are concatenated and sent to "standard output",
here the terminal screen.)
The command cp (from "copy") will copy a file. On the
other hand, the command mv (from "move") only renames it.
The command diff lists the differences between two files.
Here there was no output because there were no differ
ences.
The command rm (from "remove") deletes the file, and be
careful! it is gone. No wastepaper basket or anything.
Deleted means lost.
The command grep (from "g/re/p") finds occurrences of a
string in one or more files. Here it finds Maja's tele
phone number.
Path names and the current directory
Files live in a large tree, the file hierarchy. Each has
a path name describing the path from the root of the tree
(which is called /) to the file. For example, such a full
path name might be /home/aeb/tel. Always using full path
names would be inconvenient, and the name of a file in the
current directory may be abbreviated by only giving the
The command find (with a rather baroque syntax) will find
files with given name or other properties. For example,
"find . -name tel" would find the file "tel" starting in
the present directory (which is called "."). And "find /
-name tel" would do the same, but starting at the root of
the tree. Large searches on a multi-GB disk will be time-
consuming, and it may be better to use locate(1).
Disks and Filesystems
The command mount will attach the filesystem found on some
disk (or floppy, or CDROM or so) to the big filesystem
hierarchy. And umount detaches it again. The command df
will tell you how much of your disk is still free.
Processes
On a Unix system many user and system processes run simul
taneously. The one you are talking to runs in the fore
ground, the others in the background. The command ps will
show you which processes are active and what numbers these
processes have. The command kill allows you to get rid of
them. Without option this is a friendly request: please go
away. And "kill -9" followed by the number of the process
is an immediate kill. Foreground processes can often be
killed by typing Control-C.
Getting information
There are thousands of commands, each with many options.
Traditionally commands are documented on man pages, (like
this one), so that the command "man kill" will document
the use of the command "kill" (and "man man" document the
command "man"). The program man sends the text through
some pager, usually less. Hit the space bar to get the
next page, hit q to quit.
In documentation it is custumary to refer to man pages by
giving the name and section number, as in man(1). Man
pages are terse, and allow you to find quickly some for
gotten detail. For newcomers an introductory text with
more examples and explanations is useful.
A lot of GNU/FSF software is provided with info files.
Type "info info" for an introduction on the use of the
program "info".
Special topics are often treated in HOWTOs. Look in
/usr/share/doc/howto/en and use a browser if you find HTML
files there.
Linux 2002-08-06 INTRO(1)
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