Basic usage of Shell's If-else

The IF statement determines which branch to execute by using relational operators to determine whether an expression is true or false. The Shell has three kinds of if ... else statements:

• If ... fi statement;
• If ... else ... fi statement;
• If ... else ... elif. Fi statement.

1) If ... else statement

Syntax for the IF ... else statement:

If [Expression]then   Statement (s) to was executed if expression is Truefi

If expression returns True,then, the statement is executed and no statement is executed if False is returned.

Finally must end with FI to close the if,fi is if the inverted spelling, the back will also meet.

Note: There must be a space between expression and square brackets ([]), or there will be a syntax error.

As an example:

2. #!/bin/sh
6. a = Ten
8. b = -
12. if [ $a = = $b ]
14. Then
16. Echo "A is equal to B"
18. Fi
22. if [ $a ! = $b ]
24. Then
26. Echo "A is not equal to B"
28. Fi

Operation Result:

A is not equal to B

2) If ... else ... fi statement

Syntax for the If ... else ... fi statement:

If [Expression]then   Statement (s) to being executed if expression is TrueElse   Statement (s) to be executed if express Ion is not Truefi

If expression returns true, then the statement behind the then will be executed, otherwise the statement behind the else is executed.

As an example:

1. #!/bin/sh
3. a = Ten
4. b = -
6. if [ $a = = $b ]
7. Then
8. Echo "A is equal to B"
9. Else
10. Echo "A is not equal to B"
11. Fi

Execution Result:

A is not equal to B

3) If ... elif ... fi statement

The If ... elif. FI statement can judge multiple conditions, with the syntax:

If [expression 1]then   Statement (s) to BES executed if expression 1 is trueelif [expression 2]then   Statement (s) To being executed if expression 2 is trueelif [expression 3]then   Statement (s) to was executed if Expression 3 is Trueels E   Statement (s) to being executed if no expression is Truefi

Which expression evaluates to true, executes the statement after which expression is executed, and if it is false, no statement is executed.

As an example:

2. #!/bin/sh
6. a = Ten
8. b = -
12. if [ $a = = $b ]
14. Then
16. Echo "A is equal to B"
18. elif [ $a -gt $b ]
20. Then
22. Echo "A is greater than B"
24. elif [ $a -lt $b ]
26. Then
28. Echo "A is less than B"
30. Else
32. echo "None of the condition met"
34. Fi

Operation Result:

A is less than B

The IF ... else statement can also be written as a line and run as a command, like this:

1. if Test $[2*3] -eq $[1+5]; then echo ' The numbers is equal! ' ; fi ;

The IF ... Else statement is also often used in conjunction with the Test command, as follows:

2. NUM1 = $[2*3]
4. num2 = $[1+5]
6. if Test $[num1] -eq $[num2]
8. Then
10. echo ' The numbers is equal! '
12. Else
14. echo ' The numbers is not equal! '
16. Fi

Output:

The numbers is equal!

The test command is used to check if a condition is true, similar to square brackets ([]).

Basic usage of the shell's If-else