if else construct in shell script

The syntax of if ... else statement is,

if condition
  then
  do something
else
  do something
fi

If the condition of if statement is true then, all commands up to else statement is executed and then control is returned to after fi statement.

If the condition of if condition is false then, else block is executed upto statement fi.

Note that if condition can be nested, suppose within one if another if may be existed, within else another if may be existed but they follow the same construct.

With a simple example I will give an example of if ... else construct. From console script will ask the person age. If age is served over 50 it will display a message if not then else block is executed and another message will shown.

# cat if_else_age.txt

echo -n "Enter you age :"
read age
if test $age -gt 50
    then
    echo "You are an old man"
else
    echo "You are not at least old man"
fi

# chmod +x if_else_age.txt

# ./if_else_age.txt
Enter you age :56
You are an old man

# ./if_else_age.txt
Enter you age :23
You are not at least old man

if -elif -else construct in shell script
Multilevel if -then -else construct can be achieved by if -elif -else construct. The syntax is,

        if condition1
        then
                    if condition1 is true then execute all commands up to elif
                    statement and control return after fi
        elif condition2
        then
                    if condition2 is true then execute all commands up to elif
                    statement and control return after fi
        elif condition3
        then
                    if condition3 is true then execute all commands up to elif
                    statement and control return after fi
        #................Any number of elif can be repeated here
        else
                    if all condition1, condition2, condition3 became false or
                    condition return nonzero value then execute all commands up to
                    fi.
        fi

An example is listed below. User will be prompted to press an age and based on the input with an if -elif -else example it will show a message.

# cat if_elif.sh

echo -n "Enter our age : "
read age
if [[ $age -ge 0 && $age -le 10 ]]
    then
    echo "You are still child"
elif [[ $age -gt 10 && $age -le 50 ]]
    then
    echo "You are young"
elif [[ $age -gt 50 && $age -le 140 ]]
    then
    echo "You are old man"
else
    echo "You pressed invalid age."
fi

# ./if_elif.sh
Enter our age : 34
You are young

# ./if_elif.sh
Enter our age : 87
You are old man

# ./if_elif.sh
Enter our age : 3
You are still child

# ./if_elif.sh
Enter our age : 180
You pressed invalid age.
Related Documents
http://arjudba.blogspot.com/2009/04/checking-condition-in-shell-scipt-by.html

http://arjudba.blogspot.com/2009/04/if-construct-in-shell-script.html

http://arjudba.blogspot.com/2009/04/basic-shell-programming-commands-quote.html