Control Structure: Bash If then Else

The Bash being a scripting language does tend offer the conditional if else, We shall look at them in the following sections.

Firstly there needs to be a conditional check that has to be performed in order for the corresponding Block of code to be executed.

To break down the semantics of conditional control structures in BASH we need to understand The conditional keyword that performs the validation, the It is represented most commonly as “[“ and very rarely represented as “test” keyword.

It can be better understood by the following demonstration:

vamshi@linux-pc:~/Linux> [ 1 -gt 2 ]
vamshi@linux-pc:~/Linux> echo $?
1
vamshi@linux-pc:~/Linux>
vamshi@linux-pc:~/Linux> [ 1 -lt 2 ]
vamshi@linux-pc:~/Linux> echo $?
0

The [ is synonymous to the command test on the linux kernel.

vamshi.santhapuri@linux-pc:~/Linux> test 1 -gt 2

vamshi.santhapuri@linux-pc:~/Linux> echo $?
1
vamshi.santhapuri@linux-pc:~/Linux> test 1 -lt 2
vamshi.santhapuri@linux-pc:~/Linux> echo $?
0

We Shall now look at the different variations of Conditional controls structures.

  1. if then..fi

    if [ Condition ] ; then
    
    statement1...statementN
    
    fi
  2. if then..else..fi

    if [ Condition ] ; then
    
        If Block statements
    
    ...
    
    else
        else-Block statement
    
    fi
  3. if..then..elif then..elifN then..fi

    if [ Condition ] ; then
    
        If Block statement1
    
    ...
    
    elif [ elif Condition ]; then   # 1st elif Condition
    
        elif Block statement1
    
    
    elif [ elif Condition ]; then    # 2nd elif Condition
    
        elif Block statements
    
    elif [ elif Condition ]; then    # nth elif Condition
    
        elif Block statements
    
    fi

    An else can also be appended accordingly when all the if and elif conditions fail, which we will see in this section .

     

  4. if..then..elif then..elifN then..else..fi

    The “if elif elif else fi” control structure is like multiple test checking control diversion strategy in bash, gives the user the power to write as many test conditions as possible until a test condition is matched leading in the resultant block of code being executed. Writing this multiple elif can be tedious task and the switch case is mostly preferred

    if [ Condition ] ; then
    
        If Block statement
    
    elif [ elif Condition ]; then   # 1st elif Condition
    
        elif Block statement1
    
    elif [ elif Condition ]; then    # nth elif Condition
    
        elif Block statement
    
    ...
    
    else Block statementN # else block while gets control when none of if or elif are true.
    
        else Block statements
    
    fi

    Atleast one of the block statements are executed in this control flow similar to a switch case. The else block here takes the default case when none of the if nor the elif conditions matches up.

  5. Nested if then..fi Control structure Blocks

    Adding to the if..elif..else there is also the nested if block wherein the nested conditions are validated which can be Demonstrated as follows:

    if [ condition ]; then
    
        Main If Block Statements
    
        if [ condition ]; then # 1st inner if condition
    
            1st Inner If-Block statements
    
            if [ condition ]; then # 2nd inner if condition
    
                2nd Inner If-Block statements
              
                if [ condition ]; then 
                    Nth Inner If Block statements 
    
                fi
    
            fi
    
        fi
    
    fi

    This logic of nested ifs are used while dealing with scenarios where the outermost block of statements must be validated before, if the test succeeds then the control flow is passed to the innermost if test statement execution. Thus the name Nested if.

 

Here is the switch case bash script with practical explanation.
We will look at the Exit codes within the BASH in the next sections.

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