Login

In order to start working, you probably first have to open a session by giving your username and password. The program login(1) 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 their own favorite one. The standard one is called sh. See also ash(1), bash(1), chsh(1), csh(1), dash(1), ksh(1), zsh(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
$

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" (see stdout(3)), here the terminal screen.)

The command cp (from "copy") will copy a file.

The command mv (from "move"), on the other hand, 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 alternatively cd and pwd commands and explore cd usage: "cd", "cd .", "cd ..", "cd /", and "cd ~".

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.