Lesson 11• Decision Statements

• If then

• If then else

• if...elif...else...fi

• case

Decision Making

Unix Shell supports conditional statements Used to perform different actions based on different conditions

• if...else statements

• case...esac statement

If then

if...fi statement

if...fi statement is the fundamental control statement.Allows Shell to make decisions and execute statements conditionally.

Syntax: if [ expression ]then

Statement(s) to be executed if expression is true fi

• Shell expression is evaluated. If the resulting value is true, given statement(s) are executed. • If expression is false then no statement would be not executed. • Most of the times you will use comparison operators while making decisions.

Spaces between braces and expression is mandatory otherwise there’s syntax error.Most of the if statements check relations using relational operators

If expression is a shell command then it would be assumed true if it return 0 after its execution.If it is a boolean expression then it would be true if it returns true.

If then

Exemple:

Vi if.sh#!/bin/sha=10 b=20 if [ $a == $b ] thenecho "a is equal to b" fi if [ $a != $b ] thenecho "a is not equal to b" fi

Exemple Result:

$ a is not equal to b

If then

Exemple:

#!/bin/bashif [ “1” = “1” ]then

echo “ONE”fi

#!/bin/bashif [ “1” = “1” ]; then

echo “ONE”fi

Use of semicolon in which its purpose is to separate statements

if [ “1” = “1” ]; then echo “ONE”; fi # See?

all in one

If then else

if...else...fi statement

The if...else...fi statement is the next form of control statementAllows to execute statements in controlled way making decision between two choices

Syntax: if [ expression ]then

Statement(s) to be executed if expression is true else

Statement(s) to be executed if expression is not truefi

• Shell expression is evaluated. • If the resulting value is true, given statement(s) are executed.

• If expression is false then no statement would be not executed.

If then else

Exemple:

vi else.sh#!/bin/sha=10 b=20 if [ $a == $b ] thenecho "a is equal to b" elseecho "a is not equal to b" fi

Exemple Result:

$ a is not equal to b

If then else

Exemple:

#!/bin/bashif [ “1” = “1” ]; then

echo “ONE”else

echo “NONE”fi

else part will only be executed only if the expression fails

if...elif...else...fi

if...elif...else...fi statement

if...elif...fi One level advance form of control statement.Allows Shell to make correct decision out of several conditions. Syntax: if [ expression 1 ] then

Statement(s) to be executed if expression 1 is true elif [ expression 2 ] then

Statement(s) to be executed if expression 2 is true elif [ expression 3 ] then

Statement(s) to be executed if expression 3 is true else

Statement(s) to be executed if no expression is true fi

• Series of if statements, where each if is part of the else clause of the previous statement. • Here statement(s) are executed based on the true condition,• if non of the condition is true then else block is executed.

if...elif...else...fi

Exemple:

#!/bin/sha=10 b=20 if [ $a = $b ] thenecho "a is equal to b" elif [ $a -gt $b ] thenecho "a is greater than b" elif [ $a -lt $b ] thenecho "a is less than b" elseecho "None of the condition met" fi

Exemple Result:

$ a is less than b

if...elif...else...fi

Exemple:

#!/bin/bashif [ “1” = “1” ]; then

echo “ONE”elif [ “0” = “0” ]; then

echo “ZERO”else

echo “NONE”fi

Multiple related conditions use elif which is almost the same as if it comes only after if

case...esac

You can use multiple if...elif statements to perform a multiway branch. This is not always the best solution, when all branches depend on value of a single variable.

Shell support case...esac statement which handles exactly this situation.similar to case statement in programming languages C or C++ and PERL etc.

• Used to execute statements based on specific values. • Better than repeated if...elif statements if there are a large number of conditions

• Value used can be an expression

• Each set of statements must be ended by a pair of semicolons ; ;• a *) is used to accept any value not matched with list of values

case $var inval1)

statements;;val2)

statements;;*)

statements;;esac

case...esac

Syntax:

case word inpattern1)

Statement(s) to be executed if pattern1 matches ;;

pattern2) Statement(s) to be executed if pattern2 matches ;;

pattern3) Statement(s) to be executed if pattern3 matches ;;

esac

• String word is compared against every pattern until a match is found. Statement(s) following the matching pattern executes.If no matches are found, case statement exits without action.

• There is no maximum number of patterns, but the minimum is one.

• When statement(s) part executes, the command ;; indicates that program flow should jump to the

end of the entire case statement. This is similar to break in the C programming language.

case...esac

Exemple:

vi bananas.sh#!/bin/shFRUIT="kiwi" case "$FRUIT" in "apple") echo "Apple pie is quite tasty." ;; "banana") echo "I like banana nut bread." ;; "kiwi") echo "New Zealand is famous for kiwi." ;; esac

Exemple Result:

$ New Zealand is famous for kiwi.

case...esac

Exemple:

$ cat case.sh#!/bin/bash

echo -n “Enter a number 1 < x < 10: ”read x

case $x in1) echo “Value of x is 1.”;;2) echo “Value of x is 2.”;;3) echo “Value of x is 3.”;;4) echo “Value of x is 4.”;;5) echo “Value of x is 5.”;;6) echo “Value of x is 6.”;;7) echo “Value of x is 7.”;;8) echo “Value of x is 8.”;;9) echo “Value of x is 9.”;;0 | 10) echo “wrong number.”;;*) echo “Unrecognized value.”;;

esac

case...esac

Exemple of evaluation of command line arguments

#!/bin/shoption="${1}" case ${option} in

-f) FILE="${2}" echo "File name is $FILE" ;;

-d) DIR="${2}" echo "Dir name is $DIR" ;;

*) echo "`basename ${0}`:usage: [-f file] | [-d directory]" exit 1 # Command to come out of the program with status 1 ;;

esac

case...esac

Exemple output evaluation of command line arguments

$./test.sh test.sh: usage: [ -f filename ] | [ -d directory ]

$ ./test.sh -f index.htm $ vi test.sh

$ ./test.sh -f index.htm File name is index.htm

$ ./test.sh -d unixDir name is unix$