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# Unix - Quick Guide

# Getting Started:

## What is Unix ?

The UNIX operating system is a set of programs that act as a link between the computer and the user.

- Unix was originally developed in 1969 by a group of AT&T employees at Bell Labs, including Ken Thompson, Dennis Ritchie, Douglas McIlroy, and Joe Ossanna.

- There are various Unix variants available in the market. Solaris Unix, AIX, UP Unix and BSD are few examples. Linux is also a flavour of Unix which is freely available.

- Several people can use a UNIX computer at the same time; hence UNIX is called a multiuser system.

- A user can also run multiple programs at the same time; hence UNIX is called multitasking.

## Login Unix:

You can login to the system using login command as follows:

    login : amrood
    amrood's password:
    Last login: Sun Jun 14 09:32:32 2009 from 62.61.164.73
    [amrood]$

## Logging Out:

When you finish your session, you need to log out of the system to ensure that nobody else accesses your files while masquerading as you.

## To log out:

1. Just type logout command at command prompt, and the system will clean up everything and break the connection

# File Management:

In UNIX there are three basic types of files:

1. Ordinary Files: An ordinary file is a file on the system that contains data, text, or program instructions. In this tutorial, you look at working with ordinary files.

2. Directories: Directories store both special and ordinary files. For users familiar with Windows or Mac OS, UNIX directories are equivalent to folders.

3. Special Files: Some special files provide access to hardware such as hard drives, CD-ROM drives, modems, and Ethernet adapters. Other special files are similar to aliases or shortcuts and enable you to access a single file using different names.

## Filename Substitution:

Command | Description
--- | ---
ls -[l] | List Files in Current Directory
ls -[l]a | List Hidden Files
~ | Home Directory
~user | Home Directory of Another User
? | Wild Card, matches single character
* | Wild Card, matches multiple characters

## Filename Manipulation:

Command | Description
--- | ---
cat filename | Display File Contents
cp source destination | Copy source file into destination
mv oldname newname | Move (Rename) a oldname to newname.
rm filename | Remove (Delete) filename
chmod nnn filename | Changing Permissions
touch filename | Changing Modification Time
ln [-s] oldname newname | Creates softlink on oldname
ls -F | Display information about file type.

# Directory Management:

Command | Description
--- | ---
mkdir dirname | Create a new directory dirname
rmdir dirname | Delete an existing directory provided it is empty.
cd dirname | Change Directory to dirname
cd - | Change to last working directory.
cd ~ | Change to home directory
pwd | Display current working directory.

# Environment Setup:

When you type any command on command prompt, the shell has to locate the command before it can be executed. The PATH variable specifies the locations in which the shell should look for commands.

## PS1 and PS2 Variables:

The characters that the shell displays as your command prompt are stored in the variable PS1.

When you issue a command that is incomplete, the shell will display a secondary prompt and wait for you to complete the command and hit Enter again. The default secondary prompt is > (the greater than sign), but can be changed by re-defining the PS2 shell variable:

## Escape Characters:

Escape Sequence | Description
--- | ---
\t | Current time, expressed as HH:MM:SS.
\d | Current date, expressed as Weekday Month Date
\n | Newline.
\s | Current shell environment.
\W | Working directory.
\w | Full path of the working directory.
\u | Current user.s username.
\h | Hostname of the current machine.
\# | Command number of the current command. Increases with each new command entered.
\$ | If the effective UID is 0 (that is, if you are logged in as root), end the prompt with the # character; otherwise, use the $.

## Environment Variables:

Following is the partial list of important environment variables. These variables would be set and accessed as mentioned above:

Variable | Description
--- | ---
DISPLAY | Contains the identifier for the display that X11 programs should use by default.
HOME | Indicates the home directory of the current user: the default argument for the cd built-in command.
IFS | Indicates the Internal Field Separator that is used by the parser for word splitting after expansion.
LANG | LANG expands to the default system locale; LC_ALL can be used to override this. For example, if its value is pt_BR, then the language is set to (Brazilian) Portuguese and the locale to Brazil.
LD_LIBRARY_PATH | On many Unix systems with a dynamic linker, contains a colon-separated list of directories that the dynamic linker should search for shared objects when building a process image after exec, before searching in any other directories.
PATH | Indicates search path for commands. It is a colon-separated list of directories in which the shell looks for commands.
PWD | Indicates the current working directory as set by the cd command.
RANDOM | Generates a random integer between 0 and 32,767 each time it is referenced.
SHLVL | Increments by one each time an instance of bash is started. This variable is useful for determining whether the built-in exit command ends the current session.
TERM | Refers to the display type
TZ | Refers to Time zone. It can take values like GMT, AST, etc.
UID | Expands to the numeric user ID of the current user, initialized at shell startup.

# Filters & Pipes:

Command | Description
--- | ---
wc [-l] | Word/Line Count
tail [-n] | Displays last n lines from a file
sort [-n] | Sort lines
pr -t | Multicolumn Output
grep "pattern" filename | Searching for a pattern with grep
pg or more | Paginate a file content display.

# Special Variables

Variable | Description
--- | ---
$0 | The filename of the current script.
$n | These variables correspond to the arguments with which a script was invoked. Here n is a positive decimal number corresponding to the position of an argument (the first argument is $1, the second argument is $2, and so on).
$# | The number of arguments supplied to a script.
$* | All the arguments are double quoted. If a script receives two arguments, $* is equivalent to $1 $2.
$@ | All the arguments are individually double quoted. If a script receives two arguments, $@ is equivalent to $1 $2.
$? | The exit status of the last command executed.
$$ | The process number of the current shell. For shell scripts, this is the process ID under which they are executing.
$! | The process number of the last background command.

# Shell Basic Operators

## Arithmetic Operators:

Assume variable a holds 10 and variable b holds 20 then:

Operator | Description | Example
--- | --- | ---
+ | Addition - Adds values on either side of the operator | `expr $a + $b` will give 30
- | Subtraction - Subtracts right hand operand from left hand operand | `expr $a - $b` will give -10
* | Multiplication - Multiplies values on either side of the operator | `expr $a * $b` will give 200
/ | Division - Divides left hand operand by right hand operand | `expr $b / $a` will give 2
% | Modulus - Divides left hand operand by right hand operand and returns remainder | `expr $b % $a` will give 0
= | Assignment - Assign right operand in left operand | a=$b would assign value of b into a
== | Equality - Compares two numbers, if both are same then returns true. | [ $a == $b ] would return false.
!= | Not Equality - Compares two numbers, if both are different then returns true. | [ $a != $b ] would return true.

## Relational Operators:

Assume variable a holds 10 and variable b holds 20 then:

Operator | Description | Example
--- | --- | ---
-eq | Checks if the value of two operands are equal or not, if yes then condition becomes true. | [ $a -eq $b ] is not true.
-ne | Checks if the value of two operands are equal or not, if values are not equal then condition becomes true. | [ $a -ne $b ] is true.
-gt | Checks if the value of left operand is greater than the value of right operand, if yes then condition becomes true. | [ $a -gt $b ] is not true.
-lt | Checks if the value of left operand is less than the value of right operand, if yes then condition becomes true. | [ $a -lt $b ] is true.
-ge | Checks if the value of left operand is greater than or equal to the value of right operand, if yes then condition becomes true. | [ $a -ge $b ] is not true.
-le | Checks if the value of left operand is less than or equal to the value of right operand, if yes then condition becomes true. | [ $a -le $b ] is true.

## Boolean Operators:

Assume variable a holds 10 and variable b holds 20 then:

Operator | Description | Example
--- | --- | ---
! | This is logical negation. This inverts a true condition into false and vice versa. | [ ! false ] is true.
-o | This is logical OR. If one of the operands is true then condition would be true. | [ $a -lt 20 -o $b -gt 100 ] is true.
-a | This is logical AND. If both the operands are true then condition would be true otherwise it would be false. | [ $a -lt 20 -a $b -gt 100 ] is false.

## String Operators:

Assume variable a holds "abc" and variable b holds "efg" then:

Operator | Description | Example
--- | --- | ---
= | Checks if the value of two operands are equal or not, if yes then condition becomes true. | [ $a = $b ] is not true.
!= | Checks if the value of two operands are equal or not, if values are not equal then condition becomes true. | [ $a != $b ] is true.
-z | Checks if the given string operand size is zero. If it is zero length then it returns true. | [ -z $a ] is not true.
-n | Checks if the given string operand size is non-zero. If it is non-zero length then it returns true. | [ -z $a ] is not false.
str | Check if str is not the empty string. If it is empty then it returns false. | [ $a ] is not false.

## File Test Operators:

Assume a variable file holds an existing file name "test" whose size is 100 bytes and has read, write and execute permission on:

Operator | Description | Example
--- | --- | ---
-b file | Checks if file is a block special file if yes then condition becomes true. | [ -b $file ] is false.
-c file | Checks if file is a character special file if yes then condition becomes true. | [ -b $file ] is false.
-d file | Check if file is a directory if yes then condition becomes true. | [ -d $file ] is not true.
-f file | Check if file is an ordinary file as opposed to a directory or special file if yes then condition becomes true. | [ -f $file ] is true.
-g file | Checks if file has its set group ID (SGID) bit set if yes then condition becomes true. | [ -g $file ] is false.
-k file | Checks if file has its sticky bit set if yes then condition becomes true. | [ -k $file ] is false.
-p file | Checks if file is a named pipe if yes then condition becomes true. | [ -p $file ] is false.
-t file | Checks if file descriptor is open and associated with a terminal if yes then condition becomes true. | [ -t $file ] is false.
-u file | Checks if file has its set user id (SUID) bit set if yes then condition becomes true. | [ -u $file ] is false.
-r file | Checks if file is readable if yes then condition becomes true. | [ -r $file ] is true.
-w file | Check if file is writable if yes then condition becomes true. | [ -w $file ] is true.
-x file | Check if file is execute if yes then condition becomes true. | [ -x $file ] is true.
-s file | Check if file has size greater than 0 if yes then condition becomes true. | [ -s $file ] is true.
-e file | Check if file exists. Is true even if file is a directory but exists. | [ -e $file ] is true.

# Shell Decision Making

## The if...fi statement:

    if [ expression ]
    then
       Statement(s) to be executed if expression is true
    fi

## The if...else...fi statement:

    if [ expression ]
    then
       Statement(s) to be executed if expression is true
    else
       Statement(s) to be executed if expression is not true
    fi

## The if...elif...fi statement:

    if [ expression 1 ]
    then
       Statement(s) to be executed if expression 1 is true
    elif [ expression 2 ]
    then
       Statement(s) to be executed if expression 2 is true
    elif [ expression 3 ]
    then
       Statement(s) to be executed if expression 3 is true
    else
       Statement(s) to be executed if no expression is true
    fi

## The case...esac Statement:

    case word in
      pattern1)
         Statement(s) to be executed if pattern1 matches
         ;;
      pattern2)
         Statement(s) to be executed if pattern2 matches
         ;;
      pattern3)
         Statement(s) to be executed if pattern3 matches
         ;;
    esac

# Shell Loop Types:

## The while Loop:

    while command
    do
       Statement(s) to be executed if command is true
    done

## The for Loop:

    for var in word1 word2 ... wordN
    do
       Statement(s) to be executed for every word.
    done

## The until Loop:

    until command
    do
       Statement(s) to be executed until command is true
    done

## The select Loop:

    select var in word1 word2 ... wordN
    do
       Statement(s) to be executed for every word.
    done

# Shell Loop Control:

## The break statement:

    break [n]

## The continue statement:

    continue [n]

# Shell Substitutions:

The shell performs substitution when it encounters an expression that contains one or more special characters.

## Command Substitution:

The command substitution is performed when a command is given as:

    `command`

## Variable Substitution:

Here is the following table for all the possible substitutions:

Form | Description
--- | ---
${var} | Substitue the value of var.
${var:-word} | If var is null or unset, word is substituted for var. The value of var does not change.
${var:=word} | If var is null or unset, var is set to the value of word.
${var:?message} | If var is null or unset, message is printed to standard error. This checks that variables are set correctly.
${var:+word} | If var is set, word is substituted for var. The value of var does not change.

# Redirection Commands:

Following is the complete list of commands which you can use for redirection:

Command | Description
--- | ---
pgm > file | Output of pgm is redirected to file
pgm < file | Program pgm reads its input from file.
pgm >> file | Output of pgm is appended to file.
n > file | Output from stream with descriptor n redirected to file.
n >> file | Output from stream with descriptor n appended to file.
n >& m | Merge output from stream n with stream m.
n <& m | Merge input from stream n with stream m.
<< tag | Standard input comes from here through next tag at start of line.
| | Takes output from one program, or process, and sends it to another.

# Shell Man Pages Help:

This quick guide lists commands, including a syntax and brief description. For more detail, use:

    $man command

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