Bash structured language constructs
Yo 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 bakcup 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.
- Each command and/or statement is executed once, in order.
- With sequential flow scripts you cannot write complex applications or intelligent Linux scripts.
- However, with if command you will be able to selectively run certain commands of your script.
- You can create 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.
- 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:
<syntax lang="bash">echo $(( 5 + 2 ))
Addition is 7. But,
<syntax lang="bash">echo $(( 5 < 2 ))
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. In shell:
- 0 indicates false.
- 1 or (non-zero) value indicate true.