Linux shell condition to judge if else, if Elif else....__linux

In the shell of Linux

The IF statement determines which branch to execute through the relational operator to determine the true or false of an expression. The Shell has three kinds of if ... else statements:

    If ... fi statement;
    if ... else ... fi statement, if ... elif ... other ...
    fi statement.

1 If ... else statement

Syntax for if ... else statement:

If [expression]
then
   Statement (s) to was executed if expression is true
fi

Note: If expression returns the statement after True,then, it will be executed, and if false, no statements are executed.
Finally, you have to end up with fi. If,fi is if the spelling is reversed and will be met later.
Note: There must be a space between the expression and the square brackets ([]), otherwise there will be a syntax error.

For example:

    #!/bin/sh
    a=10
    b=20
    if [$a = = $b]
    then
       echo "A is equal to B"
    fi
    If [$a!= $b]
    t Hen
       echo "A isn't equal to B"
    fi

Run Result:

A is isn't equal to B

2 If ... else ... fi statementSyntax for if ... else ... fi statement:

If [expression]
then
   Statement (s) to be executed if expression am true
else
   Statement (s) to be executed If expression is not true
fi

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

As an example:

    #!/bin/sh
    a=10
    b=20
    if [$a = = $b]
    then
       echo "A is equal to B"
    else
       echo "a" isn't equal to B "
    fi

Execution results:

A is isn't equal to B

3 If ... elif ... fi statementIf ... the elif-fi statement can be used to determine multiple conditions, the syntax is:

If [expression 1]
then
   Statement (s) to am executed if expression 1 is true
elif [expression 2]
Then
  statement (s) to was executed if Expression 2 is true
elif [expression 3]
then
   Statement (s) to be executed I F expression 3 is true
else
   Statement (s) to am executed if no expression is true
fi

Which expression evaluates to true, executes the statement after which expression, and if all is false, does not execute any statements.

As an example:

    #!/bin/sh
    a=10
    b=20
    if [$a = = $b]
    then
       echo "A is equal to B"
    elif [$a-gt $b]
    Then
       echo "A is greater than B"
    elif [$a-lt $b]
    then
       echo "A is less than B"
    else
       Echo None Of the condition met
    fi

Run Result:

A is less than B

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

if test $[2*3]-eq $[1+5]; Then echo ' The two numbers are equal! '; Fi
The IF ... else statement is often used in conjunction with the Test command, as follows:

    NUM1=$[2*3]
    num2=$[1+5]
    if test $[num1]-eq $[num2]
    then
        Echo '
    two numbers are ' else
        Echo ' The two numbers are not equal! '
    Fi

Output:

The two numbers are equal!

The test command checks to see if a condition is set up, similar to the square brackets ([]).