A Brief Overview of UNIX Operating System

Using UNIX Commands

UNIX commands are executable files and built-in utility programs
All UNIX commands are case sensitive. Most commands are in lower case.

Commands typically have the syntax:

 
     command  option(s)   argument(s)

Options
1. Modify the way that a command works
2. Usually consist of a hyphen followed by a single letter
3. Some commands accept multiple options which can usually be grouped together after a single hyphen
4. A small number of commands require each option to be given separately
5. Should be separated from the command name by a space
Arguments
1. Most commands are used together with one or more arguments
2. Some commands assume a default argument if none is supplied
3. Arguments are optional for some commands and required by others
4. In most cases, multiple arguments should be separated from each other by a space. They should be separated from the command name and/or options by a space also.
Examples
1. To use a command with no argument: the "date" command displays the current time and date.
```
     date 
```
2. To use a command with a single argument: the "cd" command changes to the directory of its argument, newdir
```
     cd newdir 
```
3. To use a command with both an option and an argument: the wc command counts the number of words, characters, and lines in a file. By using a different option you can choose what is counted.
```
     wc -w file1     counts the words in file1 
     wc -c file1     counts the characters in file1 
     wc -l file1     counts the lines in file1 
          
```
4. To use a command with several arguments: the cat command takes the names of three files as arguments. It prints file1 followed by file2 and then file3 on the screen.
```
     cat file1 file2 file3 
```
5. To use a command with multiple options and default argument: the ls command provides a long (l) listing of all (a) files and also shows the file size (s). Because no argument is specified, the default argument (current directory) will be used.
```
     ls -als 
```
Multiple commands can be entered on one line if you separate each with a semicolon. For example: this command line contains two commands. The first, cd newdir changes the current directory to the directory newdir. The second, ls -l produces a long listing of the contents of the newdir directory.
```
     cd newdir ; ls -l  
```
Commands can continue beyond one line by using a backslash (\) character. For example: this command copies the file called recipe from kelly's home directory to the current directory and renames it kelly.recipe. It then sends this file to the printer ps3 with the request that 6 copies be printed.
```
     cp /users/kelly/recipe  kelly.recipe ; \  
     lpr -Pdip #6m kelly.recipe
     
```
Go back to "Using UNIX Commands" exercise

Special Characters

The UNIX shell interprets a number of characters in special ways. These characters are most often used with UNIX commands - as arguments and for other means. The following list contains most of UNIX's special characters


     NEWLINE   - initiates command execution 
     ;         - separates commands on same line 
     ( )       - groups commands or identifies a function 
     &         - executes a command in the background 
     |         - pipe 
     >         - redirects standard output 
     >>        - appends standard output 
     <         - redirects standard input 
     *         - wildcard for any number of characters in a 
                 file name 
     ?         - wildcard for a single character in a file 
                 name 
     \         - quotes the following character 
     '         - quotes a string preventing all substitutions 
     "         - quotes a string allowing variable and 
                 command substitution 
     `         - performs command substitution 
     [ ]       - denotes a character class in a file name 
     $         - references a variable 
     { }       - command grouping within a function 
     .         - executes a command (if at beginning of line) 
     #         - begins a comment 
     :         - null command

Examples
1. Use the * character in file names to match any number of characters. The following command:
```
     ls  *.txt 
```
  Will match the following files:
```
     chapter1.txt   doc.txt  memo.txt  a.txt 
```
  Will not match the following files:
```
     doctxt   txt.memo  
```
2. Use the ? character in file names to match any single character. The following command:
```
     ls  ???.txt 
```
  Will match the following files:
```
     one.txt   doc.txt  two.txt 
```
  Will not match the following files:
```
     chap1.txt   doctxt  
```
3. Use the [ ] characters in file names to match any character within a range of characters. The following command:
```
     ls  chapter[1-3].txt 
```
  Will match the following files:
```
     chapter1.txt   chapter2.txt  chapter3.txt 
```
  Will not match the following files:
```
     chap1.txt   chapter4.txt  
```
Other uses for special characters and examples will be discussed later.
Go back to "Special Characters" Exercise

Terminal Control Keys

Several key combinations on your keyboard usually have a special effect on the terminal.
These "control" (CTRL) keys are accomplished by holding the CTRL key while typing the second key. For example, CTRL-c means to hold the CTRL key while you type the letter "c".

The most common control keys are listed below:


     CTRL-u   - erase everything you've typed on the command 
                line 

     CTRL-c   - stop/kill a command 

     CTRL-h   - backspace (usually) 

     CTRL-z   - suspend a command 

     CTRL-s   - stop the screen from scrolling 

     CTRL-q   - continue scrolling 

     CTRL-d   - exit from an interactive program (signals end 
                of data)

Go back to "Terminal Control Keys" Exercise

Changing Your Password

Your password is important; it stops other users from gaining access to your account. Never give your password to anyone
You should change your initial password very soon after your first login
To change your password: enter the command passwd and then respond to the prompts by entering your old password followed by your new one. You are then asked to retype your password for confirmation.
Note that what you type will not appear on the screen for security reasons. For example:
```
     passwd  
     Old password: - enter your current password 
     New password: - enter your new password 
     Retype new password: - re-enter your new password 
     
```
If you make a mistake, the message: Mismatch - password unchanged. is displayed and your password remains unchanged. Try again.
Guidelines for creating a password
Don't
- choose a word from a dictionary
- use personal information such as your name, make of car, pet, sweetheart, etc.
- tell anyone your passwd
- write your password down anywhere
- keep the password you were given when you first got your userid
Do
- choose an obscure password preferably between 6 and 8 characters in length, of mixed case and containing at least one non-alphanumeric character such as ! or @
- change your password at regular intervals
Keep in mind that there are people who think it's fun to "crack" passwords and get into other people's accounts. There are plenty of easily available "cracking" programs which make guessing easy passwords a cinch.
Begin the "Changing Your Password" Exercise

Getting Information

The "man" command
- The "man" command man gives you access to an on-line manual which potentially contains a complete description of every command available on the system. In practice, the manual usually contains a subset of all commands.
- man can also provide you with one line descriptions of commands which match a specified keyword
- The online manual is divided into sections:
```
     Section  Description
     -------  -----------
        1     User Commands
        2     System Commands
        3     Subroutines
        4     Devices
        5     File Formats
        6     Games
        7     Miscellaneous
        8     System Administration
        l     Local Commands
        n     New Commands
     
```
- Examples of using the man command:
  1. To display the manual page for the cp (copy files) command:
```
     man cp
```
    --More--23% at the bottom left of the screen means that only 23% of the man page is displayed. Press the space bar to display more of it or type q to quit.
  2. By default, the man page in section 1 is displayed if multiple sections exist. You can access a different section by specifying the section. For example:
```
     man 8 telnetd 
```
  3. Keyword searching: use the -k option followed by the keyword. Two examples appear below.
```
     
     man -k mail 
     man -k 'copy files' 
               
```
  4. To view a one line description of what a command does:
```
     whatis more
```
    will display what the "more" command does: more, page (1) - browse or page through a text file
who - shows who is on the system
```
     who
     who am i
     
```
finger - displays information about users, by name or login name
```
     finger doe 
     finger userid 
     
```
Go back to "Getting Information" Exercise

Logging Off The System

To finish using a UNIX system you must go through a process known as "logging out" or "logging off".
To logout enter the command logout or exit. If this does not, work press Ctrl-d.
If you have a .logout file in your home directory, the system will execute any commands contained there.
Problems
1. If you see the message There are stopped jobs it is a warning which tells you that there is a job which you suspended earlier (Ctrl-z) or a job running in the background (discussed later).
  The simplest way to accomplish the logout is to just issue the logout command again. Your suspended/background jobs will be terminated and you will be logged off the system.
2. If you see the message Not login shell it means you are working in another "shell" that has been started from within your "login shell" (shells will be discussed later).
  To logout, issue the exit command and then try to logout again.
Begin the "Logging Off the System" Exercise

This concludes the brief overview of UNIX.