Loop Control in Shell Script - BunksAllowed

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    Loop Control in Shell Script

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    Most languages have the concept of loops. If we want to perform a task repetitively with a little bit of change, the loop is the solution to write the logic. Different types of loops are used in this scripting language. The following examples demonstrate the loops with some examples.

    for loop

    There are two types of syntaxes of for loop. In the first example, the loop is iterated based on the value of i. The first part is an initialization, the second part is condition checking and the last part is increment or decrement.

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    #!/bin/bash
    for (( i=1; i<=5; i++ ))
    do
    echo "$i"
    done
    echo $x
    XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

    In the following example, the loop iterates over the elements starting from 0 to 5. In the first iteration, i will be assigned 0, in the next iteration i will be 1, and so on.


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    #!/bin/bash
    for i in 0 1 2 3 4 5
    do
    echo "$i"
    done
    echo $x
    XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

    nested for loop

    Similar to the if-else block, for loop can also be nested as shown below.


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    #!/bin/bash
    echo "Stars"
    for (( i=1; i<=5; i++ ))
    do
    for (( j=1; j<=i; j++ ))
    do
    echo -n " *"
    done
    echo ""
    done
    XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

    while loop

    Any one of for or while loops can be used to perform repetitive tasks. The syntax of the while loop is shown in the following example.


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    #!/bin/bash
    i=5
    while test $i != 0
    do
    echo "$i"
    i=`expr $i - 1`
    done
    XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

    nested while loop

    Similar to for loop nesting, a while loop can also be nested as shown below.


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    #!/bin/bash
    x=0
    while [ $x -lt 10 ] ;
    do
    y=$x
    while [ $y -ge 0 ] ;
    do
    echo "$y \c"
    y=`expr $y - 1`
    done
    x=`expr $x + 1`
    echo $x
    done
    XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

    until loop

    There is another loop, until, which can be used instead of the while loop. But the difference is that the loop will run until the condition is true, whereas the while loop runs until the condition is false.


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    #!/bin/bash
    x=1;
    until [ $x -ge 10 ]
    do
    echo $x
    x=`expr $x + 1`
    done
    XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

    select loop



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    #!/bin/bash
    XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

    break and continue statement

    Script to find an average of numbers.

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    #!/bin/bash
    #
    avg=0
    temp_total=0
    number_of_args=$#
    if [ $# -lt 2 ] ; then
    echo -e "Opps! I need at least 2 command line args\n"
    echo -e "Syntax: $0: number1 number2 ... numberN\n"
    echo -e "Example:$0 5 4\n\t$0 56 66 34"
    exit 1
    fi
    for i in $*
    do
    temp_total=`expr $temp_total + $i`
    done
    avg=`expr $temp_total / $number_of_args`
    XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

    The sum of digits of a number

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    #!/bin/bash
    if [ $# -ne 1 ]
    then
    echo "Usage: $0 number"
    echo " I will find the sum of all digits for given number"
    echo " For eg. $0 123, I will print 6 as sum of all digit (1+2+3)"
    exit 1
    fi
    n=$1
    sum=0
    sd=0
    while [ $n -gt 0 ]
    do
    sd=`expr $n % 10`
    sum=`expr $sum + $sd`
    n=`expr $n / 10`
    done
    echo "Sum of digit for number is $sum"
    XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

    Factorial of a Number using Shell Script

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    #!/bin/bash
    n=0
    on=0
    fact=1
    echo -n "Enter a number to find factorial: "
    read n
    on=$n
    while [ $n -ge 1 ]
    do
    fact=`expr $fact \* $n`
    n=`expr $n - 1`
    done
    echo "Factorial for $on is $fact"
    XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

    Reverse a Number using Shell Script

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    #!/bin/bash
    if [ $# -ne 1 ]
    then
    echo "Usage: $0 number"
    echo " Reseverse a number"
    echo " For eg. 1234, It will print 4321"
    exit 1
    fi
    n=$1
    rev=0
    sd=0
    while [ $n -gt 0 ]
    do
    sd=`expr $n % 10`
    rev=`expr $rev \* 10 + $sd`
    n=`expr $n / 10`
    done
    echo "Reverse number is $rev"
    XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

    Program for Fibonacci Series

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    # Input for N
    read N
    # First Number of the Fibonacci Series
    a=0
    # Second Number of the Fibonacci Series
    b=1
    echo "The Fibonacci series is : "
    for (( i=0; i<N; i++ ))
    do
    echo -n "$a "
    fn=$((a + b))
    a=$b
    b=$fn
    done
    # End of for loop
    XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

    Armstrong number within a range

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    #!/bin/bash
    i=1
    read n
    while((i-le$n))
    do
    c=$i; d=$i; b=0; a=0
    #checking each number
    while ((c>0))
    do
    #separating each digit
    a=$((c%10))
    #finding cube of each digit and adding them
    b=$((b + a*a*a))
    c=$((c/10))
    done
    if ((b==d));
    then
    echo "$i"
    fi
    i=$((i+1))
    done
    XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

    Happy Exploring!

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