Bash structured language constructs

You can use the if command to test a condition. For example, shell script may need to execute tar command only if a certain condition exists (such as backup only on Friday night).

If today is Friday
   execute tar command
otherwise
   print an error message on screen.

.

More About Logic

• So far, the script you've used followed sequential flow:

#!/bin/bash
echo "Today is $(date)"
echo "Current directory : $PWD"
echo "What Users Are Doing:"
w

• Each command and/or statement is executed once, in order in above script.
• With sequential flow scripts, you cannot write complex applications (intelligent Linux scripts).
• However, with if command you will be able to selectively run certain commands (or part) of your script.
• You can create a warning message and run script more interactively using if command to execute code based on a condition.

But What Is A Condition?

• A condition is nothing but an expression that evaluates to a boolean value (true or false).
• In other words condition can be either true or false.
• A condition is used in shell script loops and if statements.

So, How Do I Make One?

A condition is mainly a comparison between two values. Open a shell prompt (console) and type the following command:

echo $(( 5 + 2 ))

Sample Output:

7

Addition is 7. But,

echo $(( 5 < 2 ))

Sample Output:

0

Answer is zero (0). Shell simple compared two number and returned result as true or false. Is 5 is less than 2? No. So 0 is returned. The Boolean (logical data) type is a primitive data type having one of two values

• True
• False

In shell:

• 0 value indicates false.
• 1 or non-zero value indicate true.

Examples

Operator	Example	Description	True / False	Evaluates To
5 > 12	echo $(( 5 > 12 ))	Is 5 greater than 12?	No (false)	0
5 == 10	echo $(( 5 == 10 ))	Is 5 equal to 10?	No (false)	0
5 != 2	echo $(( 5 != 2 ))	5 is not equal to 2?	Yes (true)	1
1 < 2	echo $(( 1 < 2 ))	Is 1 less than 2?	Yes (true)	1
5 == 5	echo $(( 5 == 5 ))	Is 5 equal to 5?	Yes (true)	1

Now, it makes no sense to use echo command for comparisons. But, when you compare it with some value it becomes very useful. For example:

if [ file exists /etc/resolv.conf ]
then
    make a copy
else
    print an error on screen
fi