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.
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.
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
%
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.
The command pwd prints the current directory.
The command cd changes the current directory. Try "cd /" and "pwd" and "cd" and "pwd".
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).
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.