6. Conditionals

Conditionals let you decide whether to perform an action or not, this decision is taken by evaluating an expression.

6.1 Dry Theory

Conditionals have many forms. The most basic form is: if expression then statement where 'statement' is only executed if 'expression' evaluates to true. '2<1' is an expresion that evaluates to false, while '2>1' evaluates to true.xs

Conditionals have other forms such as: if expression then statement1 else statement2. Here 'statement1' is executed if 'expression' is true,otherwise 'statement2' is executed.

Yet another form of conditionals is: if expression1 then statement1 else if expression2 then statement2 else statement3. In this form there's added only the "ELSE IF 'expression2' THEN 'statement2'" which makes statement2 being executed if expression2 evaluates to true. The rest is as you may imagine (see previous forms).

A word about syntax:

The base for the 'if' constructions in bash is this:

 
       if [expression];
       then

       code if 'expression' is true.

       fi

6.2 Sample: Basic conditional example if .. then

     #!/bin/bash
     if [ "foo" = "foo" ]; then
     echo expression evaluated as true
     fi
       

The code to be executed if the expression within braces is true can be found after the 'then' word and before 'fi' which indicates the end of the conditionally executed code.

6.3 Sample: Basic conditional example if .. then ... else

       #!/bin/bash
       if [ "foo" = "foo" ]; then
     echo expression evaluated as true
       else
     echo expression evaluated as false
       fi
       

6.3.1 Sample: Expanded if .. then ... else

       if [ condition ] # true = 0
       then
       # condition is true
       elif [ condition1 ]
       then
       # condition1 is true
       elif condition2
       then
       # condition2 is tru
       else
       # None of the conditions is true
       fi

6.4 Sample: Conditionals with variables

       #!/bin/bash
       T1="foo"
       T2="bar"
       if [ "$T1" = "$T2" ]; then
   echo expression evaluated as true
       else
   echo expression evaluated as false
       fi       

6.5 CASE statements

       expression in
    pattern1|pattern2|pattern3) execute commands ;;
    pattern4) execute commands ;;
    pattern5) execute commands ;;
    *)
       esac

6.5.1 CASE statements Example

       #!/bin/bash

       # if no command line arg given
       # set rental to Unknown
       if [ -z $1 ]
       then
    rental="*** Unknown vehicle ***"
       elif [ -n $1 ]
       then
       # otherwise make first arg as a rental
    rental=$1
       fi

       case $rental in
     "car") echo "For $rental rental is Rs.20 per k/m.";;
     "van") echo "For $rental rental is Rs.10 per k/m.";;
     "jeep") echo "For $rental rental is Rs.5 per k/m.";;
     "bicycle") echo "For $rental rental 20 paisa per k/m.";;
     "enfield") echo "For $rental rental Rs.3  per k/m.";;
     "thunderbird") echo "For $rental rental Rs.5 per k/m.";;
     *) echo "Sorry, I can not get a $rental rental  for you!";;
esac