UNIX - Class3 - Programming Constructs


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It will provide brief knowledge regarding different types of constructs such as conditional statement execution and looping etc, which is crucial for shell Scripting.

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UNIX - Class3 - Programming Constructs

  1. 1. UNIX Programming Construct Presentation By Nihar R Paital
  2. 2. Nested ifs You can write the entire if-else construct within either the body of the if statement of the body of an else statement. This is called the nesting of ifs. Ex : chos=0 echo "1. Unix (Sun Os)" echo "2. Linux (Red Hat)" echo -n "Select your os choice [1 or 2]? " read chos if [ $chos -eq 1 ] ; then echo "You Pick up Unix (Sun Os)" else if [ $chos -eq 2 ] ; then echo "You Pick up Linux (Red Hat)" else echo "What you dont like Unix/Linux OS." fi fi Nihar R Paital
  3. 3. Multilevel if-then-else #!/bin/ksh echo Enter a number : read n if [ $n -gt 0 ]; then echo "$n is positive" elif [ $n -lt 0 ] then echo "$n is negative" elif [ $n -eq 0 ] then echo "$n is zero" else echo "Opps! $n is not number, give number" fi Nihar R Paital
  4. 4. Loops in Shell ScriptsBash supports: – for loop – while loopIn each and every loop,(e) First, the variable used in loop condition must be initialized, then execution of the loop begins.(b) A test (condition) is made at the beginning of each iteration.(c) The body of loop ends with a statement that modifies the value of the test (condition) variable. Nihar R Paital
  5. 5. for Loop 1) for x in 10 20 30 40 50 do echo $x done 2) Ex: Arrays with for loop #!/bin/ksh y="shell scripting Training" for x in ${y[*]} do echo $x done Nihar R Paital
  6. 6. Ex: for statement 3) for x in `ls` do echo $x done 4) for x in `ls` `cat forloop` do echo $x done Nihar R Paital
  7. 7. while loop Syntax: while [ condition ] do command1 command2 command3 .. .... done Nihar R Paital
  8. 8. Ex: while statement. x=1 while [ $x -lt 10 ] do echo $x x=`expr $x + 1` done Nihar R Paital
  9. 9. until statementSyntax: until control command do <commands> DoneEx: x=1 until [ $x -gt 10 ] do echo $x x=`expr $x + 1` done Nihar R Paital
  10. 10. 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 choice1) commands ;; choice2) commands ;; .... ....EsacThe $variable-name is compared against the cases 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. For e.g. write script as follows: Nihar R Paital
  11. 11. Ex: case statement echo enter value for x read x case $x in 1)ls;; 2)cal;; 3)date;; *)echo invalid esac Nihar R Paital
  12. 12. Useful Shell Scripting commands. break – To come out of a loop. continue – To jump to the start of loop. exit – To prematurely terminate a program. # – To interpret the rest of line as comments. Nihar R Paital
  13. 13. Nihar R Paital