From Linux Man Pages
intro - Introduction to user commands
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
interface), 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
peter 0136-7399214
% cp tel tel2
% ls -l
total 3
drwxr-xr-x 2 aeb 1024 Aug 6 23:51 bin
-rw-r--r-- 1 aeb 37 Aug 6 23:52 tel
-rw-r--r-- 1 aeb 37 Aug 6 23:53 tel2
% mv tel tel1
% ls -l
total 3
drwxr-xr-x 2 aeb 1024 Aug 6 23:51 bin
-rw-r--r-- 1 aeb 37 Aug 6 23:52 tel1
-rw-r--r-- 1 aeb 37 Aug 6 23:53 tel2
% diff tel1 tel2
% 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 indi-
cating 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.
Pathnames and the current directory
Files live in a large tree, the file hierarchy. Each has a pathname describing the path from the root of the
tree (which is called /) to the file. For example, such a full pathname might be /home/aeb/tel. Always using
full pathnames would be inconvenient, and the name of a file in the current directory may be abbreviated by only
giving the last component. That is why "/home/aeb/tel" can be abbreviated to "tel" when the current directory is
"/home/aeb".
The command pwd prints the current directory.
The command cd changes the current directory. Try "cd /" and "pwd" and "cd" and "pwd".
Directories
The command mkdir makes a new directory.
The command rmdir removes a directory if it is empty, and complains otherwise.
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 simultaneously. 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" docu-
ment 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 customary 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 forgotten 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.
RELATED
standards(7)
CATEGORY