if, for, while, case in linux shell

1. if condition

if condition which is used for decision making in shell script, If given condition is true then command1 is executed.
Syntax:

 if condition
 then
  command1 if condition is true or if exit status
  of condition is 0 (zero)
  ...
  ...
 fi

2. test command or [ expr ]
test command or [ expr ] is used to see if an expression is true, and if it is true it return zero(0), otherwise returns nonzero for false.
Syntax: 
test expression OR [ expression ]

Mathematical Operator in  Shell Script	Meaning	Normal Arithmetical/ Mathematical Statements	But in Shell
			For test statement with if command	For [ expr ] statement with if command
-eq	is equal to	5 == 6	if test 5 -eq 6	if [ 5 -eq 6 ]
-ne	is not equal to	5 != 6	if test 5 -ne 6	if [ 5 -ne 6 ]
-lt	is less than	5 < 6	if test 5 -lt 6	if [ 5 -lt 6 ]
-le	is less than or equal to	5 <= 6	if test 5 -le 6	if [ 5 -le 6 ]
-gt	is greater than	5 > 6	if test 5 -gt 6	if [ 5 -gt 6 ]
-ge	is greater than or equal to	5 >= 6	if test 5 -ge 6	if [ 5 -ge 6 ]

For string Comparisons use

Operator	Meaning
string1 = string2	string1 is equal to string2
string1 != string2	string1 is NOT equal to string2
string1	string1 is NOT NULL or not defined
-n string1	string1 is NOT NULL and does exist
-z string1	string1 is NULL and does exist

Shell also test for file and directory types

Test	Meaning
-s file	Non empty file
-f file	Is File exist or normal file and not a directory
-d dir	Is Directory exist and not a file
-w file	Is writeable file
-r file	Is read-only file
-x file	Is file is executable

Logical Operators

Logical operators are used to combine two or more condition at a time

Operator	Meaning
! expression	Logical NOT
expression1  -a  expression2	Logical AND
expression1  -o  expression2	Logical OR

3. if...else...fi
If given condition is true then command1 is executed otherwise command2 is executed.
Syntax:

           if condition
           then
                       condition is zero (true - 0)
                       execute all commands up to else statement

           else
                       if condition is not true then
                       execute all commands up to fi
           fi

4. Multilevel if-then-else
Syntax:

           if condition
           then
                       condition is zero (true - 0)
                       execute all commands up to elif statement
           elif condition1 
           then
                       condition1 is zero (true - 0)
                       execute all commands up to elif statement  
           elif condition2
           then
                       condition2 is zero (true - 0)
                       execute all commands up to elif statement          
           else
                       None of the above condtion,condtion1,condtion2 are true (i.e. 
                       all of the above nonzero or false)
                       execute all commands up to fi
           fi

5. for Loop
Syntax:

            for { variable name } in { list }
            do
                     execute one for each item in the list until the list is
                     not finished (And repeat all statement between do and done)
            done

6. while loop

Syntax:

           while [ condition ]
           do
                 command1
                 command2
                 command3
                 ..
                 ....
            done

7. The case Statement
The case statement is good alternative to Multilevel if-then-else-fi statement. It enable you to match several values against one variable. Its easier to read and write.
Syntax:

           case  $variable-name  in
                pattern1)   command
                                ...
                                ..
                                command;;
                pattern2)   command
                                ...
                                ..
                                command;;
                patternN)   command
                                ...
                                ..
                                command;;
                *)             command
                                ...
                                ..
                                command;;
           esac