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NAME
intro - introduction to user commands
DESCRIPTION
Section 1 of the manual describes user commands and tools, for example, file manipulation
tools, shells, compilers, web browsers, file and image viewers and editors, and so on.
All commands yield a status value on termination. This value can be tested (e.g., in most
shells the variable $? contains the status of the last executed command) to see whether
the command completed successfully. A zero exit status is conventionally used to indicate
success, and a nonzero status means that the command was unsuccessful. (Details of the
exit status can be found in wait(2).) A nonzero exit status can be in the range 1 to 255,
and some commands use different nonzero status values to indicate the reason why the
command failed.
NOTES
Linux is a flavor 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 favorite 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 indicating that it is ready for the next command. The prompt can be
customized in lots of ways, and one might include stuff like username, machine name,
current directory, time, and so on. 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 differences.
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 telephone 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 giving only 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 foreground, 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 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.
SEE ALSO
standards(7)
COLOPHON
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http://www.kernel.org/doc/man-pages/.