Linux, Conditional Statements, testing conditions, performing specific actions

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Conditional Statements

An important aspect of programming is being able to determine if a condition is true or false and then perform specific actions depending upon the outcome of the condition test. Shell scripting supplies several ways of testing conditions and then performing specific actions. Some examples of condition statements are the if condition, the test or expression statement, the while loop, the until loop, and the case statement.

The If condition

The simple form of the if statement consists of the if condition, a then statement followed by commands to be executed when the if condition is true, and the fi statement (if spelled backward) which ends the if statement. The condition usually consists of a simple comparison between two values, such as A=B or number=5.

A simple if conditional statement

$ x=3

$ y=9

$ if [ $x -lt $y ]

> then

> echo " "

> echo " x=${x} which is less than y=${y}"

> echo " "

> fi

x=3 which is less than y=9

The simple if statement also provides an else clause which allows the execution of commands when the condition is false.

A simple if-then-else statement

$ x=3

$ y=2

$ if [ $x -lt $y ]

> then

> echo " "

> echo " ${x} is less than ${y}"

> echo " "

> else

> echo " "

> echo " ${x} is not less than ${y}"

> echo " "

> fi

3 is not less than 2

$

It is also possible to create compound conditional statements by using one or more elif (else if) clauses. If the first condition is false then subsequent elif statements are checked. When an elif condition is found to be true then the statements following the associated then statement are executed.

The above book excerpt is from:

Easy Linux Commands
Working Examples of Linux Command Syntax

ISBN: 0-9759135-0-6

Terry Clark

http://www.rampant-books.com/book_2005_1_linux_commands.htm

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