1. The Linux Command Line Chapter 27
Flow Control: Branching with if
Prepared by Dr. Reyes, New York City College of Technology

2. Branching with if
if – a statement that allows you to evaluate a condition and branch based on the results
Syntax Example

3. Exit Status
Exit Status – integer value returned by a command when it terminates execution
Range from
Value of 0 indicates success
Values indicates error or failure
Man pages explain what each exit status means for a command
$? – parameter used to examined the exit status of the last command executed

4. Test and Expressions An if statement will test an expression
You may use the keyword test or enclose the expression within [ ]
test expression
[ expression ]
When using [ expression ] you must leave a space after opening the first bracket and another space before closing the last bracket
[expression] may give unexpected results (notice the lack of spacing)
A test will return zero if the expression is true, non-zero otherwise.
Strange, but to be consistent with the exit status explained before

5. String Expressions

6. String Script Example

7. Integer Expressions

8. Integer Script Example

9. Other Versions of test
[[ expression ]] – allows the use of regular expressions and patterns.
(( arithmetic expression )) – makes arithmetic tests easier

10. Combining Expressions
Recommendation