Loops

• Loops in Shell Scripts
  • Loop defined as: Computer can repeat particular instruction again and again, until particular condition satisfies. A group of instruction that is executed repeatedly is called a loop.
  • Bash supports:
    1. for loop
    2. while loop
  • Note that in each and every loop,
    • First, the variable used in loop condition must be initialized, then execution of the loop begins.
    • A test (condition) is made at the beginning of each iteration.
    • The body of loop ends with a statement that modifies the value of the test (condition) variable.
• 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
  

  Try the following script: http://siber.cankaya.edu.tr/SystemsProgramming/cfiles/testfortestfor

  for i in 1 2 3 4 5
  do
          echo "Welcome $i times"
  done
  

  Also try the following script: http://siber.cankaya.edu.tr/SystemsProgramming/cfiles/mtablemtable

  #!/bin/sh
  #
  #Script to test for loop
  #
  #
  if [ $# -eq 0 ]
  then
          echo "Error - Number missing form command line argument"
          echo "Syntax : $0 number"
          echo "Use to print multiplication table for given number"
          exit 1
  fi
  n=$1
  for i in 1 2 3 4 5 6 7 8 9 10
  do
          echo "$n * $i = `expr $i \* $n`"
  done
  

  Syntax:

  for (( expr1; expr2; expr3 ))
  do
          repeat all statements between do and
          done until expr2 is TRUE
  Done
  

  http://siber.cankaya.edu.tr/SystemsProgramming/cfiles/testfor2testfor2

  #!/bin/sh
  #
  #Script to test for loop 2
  #
  #
  for (( i = 0 ; i <= 5; i++ ))
  do
          echo "Welcome $i times"
  done
  

• Nesting of for Loop
  • As you see the if statement can nested, similarly loop statement can be nested.
  • To understand the nesting of for loop see the following shell script. http://siber.cankaya.edu.tr/SystemsProgramming/cfiles/nestedfornestedfor

    #!/bin/sh
    #
    # Nesting of for loop
    #
    #
    for (( i = 1; i <= 5; i++ )) ### Outer for loop ###
    do
            for (( j = 1 ; j <= 5; j++ )) ### Inner for loop ###
            do
                    echo -n "$i "
            done
            echo "" #### print the new line ###
    done
    

    Here, for each value of i the inner loop is cycled through 5 times, with the variable j taking values from 1 to 5. The inner for loop terminates when the value of j exceeds 5, and the outer loop terminates when the value of i exceeds 5.
  • Following script is quite interesting, it prints the chess board on screen. http://siber.cankaya.edu.tr/SystemsProgramming/cfiles/chessboardchessboard

    #!/bin/sh
    #
    # Chessboard 
    #
    #
    for (( i = 1; i <= 9; i++ )) ### Outer for loop ###
    do
            for (( j = 1 ; j <= 9; j++ )) ### Inner for loop ###
            do
                    tot=`expr $i + $j`
                    tmp=`expr $tot % 2`
                    if [ $tmp -eq 0 ]; 
                    then
                            echo -e -n "\033[47m "
                    else
                            echo -e -n "\033[40m "
                    fi
            done
    echo -e -n "\033[40m " #### set back background color to black
    echo "" #### print the new line ###
    done
    echo -e -n "\033[47m " #### set back background color to white
    echo "" #### print the new line ###
    echo "" #### print the new line ###
    echo "" #### print the new line ###
    echo "" #### print the new line ###
    

• while loop
  • Syntax:

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

  • Loop is executed as long as given condition is true. Above for loop program (shown in last section of for loop) can be written using while loop as; http://siber.cankaya.edu.tr/SystemsProgramming/cfiles/nt1nt1

    #!/bin/sh
    #
    #Script to test while statement
    #
    #
    if [ $# -eq 0 ]
    then
            echo "Error - Number missing form command line argument"
            echo "Syntax : $0 number"
            echo " Use to print multiplication table for given number"
            exit 1
    fi
    n=$1
    i=1
    while [ $i -le 10 ]
    do
            echo "$n * $i = `expr $i \* $n`"
            i=`expr $i + 1`
    done
    

• 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
    

  • The $variable-name is compared against the patterns until a match is found.
  • The shell then executes all the statements up to the two semicolons that are next to each other.
  • The default is *) and its executed if no match is found. Example; http://siber.cankaya.edu.tr/SystemsProgramming/cfiles/carcar

    #!/bin/sh
    #
    # if no vehicle name is given
    # i.e. -z $1 is defined and it is NULL
    #
    # if no command line arg
    if [ -z $1 ]
    then
            rental="*** Unknown vehicle ***"
    elif [ -n $1 ]
    then
            # otherwise make first arg as rental
            rental=$1
    fi
    case $rental in
            "car") echo "For $rental Rs.20 per k/m";;
            "van") echo "For $rental Rs.10 per k/m";;
            "jeep") echo "For $rental Rs.5 per k/m";;
            "bicycle") echo "For $rental 20 paisa per k/m";;
            *) echo "Sorry, I can not get a $rental for you";;
    esac