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about_If

  • Artikkeli

Short description

Describes a language command you can use to run statement lists based on the results of one or more conditional tests.

Long description

You can use the if statement to run code blocks if a specified conditional test evaluates to true. You can also specify one or more additional conditional tests to run if all the prior tests evaluate to false. Finally, you can specify an additional code block that's run if no other prior conditional test evaluates to true.

Syntax

The following example shows the if statement syntax:

if (<test1>)
    {<statement list 1>}
[elseif (<test2>)
    {<statement list 2>}]
[else
    {<statement list 3>}]

When you run an if statement, PowerShell evaluates the <test1> conditional expression as true or false. If <test1> is true, <statement list 1> runs, and PowerShell exits the if statement. If <test1> is false, PowerShell evaluates the condition specified by the <test2> conditional statement.

For more information about boolean evaluation, see about_Booleans.

If <test2> is true, <statement list 2> runs, and PowerShell exits the if statement. If both <test1> and <test2> evaluate to false, the <statement list 3> code block runs, and PowerShell exits the if statement.

You can use multiple elseif statements to chain a series of conditional tests. Each test is run only if all the previous tests are false. If you need to create an if statement that contains many elseif statements, consider using a Switch statement instead.

Examples:

The simplest if statement contains a single command and doesn't contain any elseif statements or any else statements. The following example shows the simplest form of the if statement:

if ($a -gt 2) {
    Write-Host "The value $a is greater than 2."
}

In this example, if the $a variable is greater than 2, the condition evaluates to true, and the statement list runs. However, if $a is less than or equal to 2 or isn't an existing variable, the if statement doesn't display a message.

By adding an Else statement, a message is displayed when $a is less than or equal to 2. As the next example shows:

if ($a -gt 2) {
    Write-Host "The value $a is greater than 2."
}
else {
    Write-Host ("The value $a is less than or equal to 2," +
        " is not created or is not initialized.")
}

To further refine this example, you can use the elseif statement to display a message when the value of $a is equal to 2. As the next example shows:

if ($a -gt 2) {
    Write-Host "The value $a is greater than 2."
}
elseif ($a -eq 2) {
    Write-Host "The value $a is equal to 2."
}
else {
    Write-Host ("The value $a is less than 2 or" +
        " was not created or initialized.")
}

Using the ternary operator syntax

PowerShell 7.0 introduced a new syntax using the ternary operator. It follows the C# ternary operator syntax:

<condition> ? <if-true> : <if-false>

The ternary operator behaves like the simplified if-else statement. The <condition> expression is evaluated and the result is converted to a boolean to determine which branch should be evaluated next:

  • The <if-true> expression is executed if the <condition> expression is true
  • The <if-false> expression is executed if the <condition> expression is false

For example:

$message = (Test-Path $path) ? "Path exists" : "Path not found"

In this example, the value of $message is Path exists when Test-Path returns $true. When Test-Path returns $false, the value of $message is Path not found.

$service = Get-Service BITS
$service.Status -eq 'Running' ? (Stop-Service $service) : (Start-Service $service)

In this example, if the service is running, it's stopped, and if its status is not Running, it's started.

If a <condition>, <if-true>, or <if-false> expression calls a command, you must wrap it in parentheses. If you don't, PowerShell raises an argument exception for the command in the <condition> expression and parsing exceptions for the <if-true> and <if-false> expressions.

For example, PowerShell raises exceptions for these ternaries:

Test-Path .vscode   ? Write-Host 'exists'   : Write-Host 'not found'
(Test-Path .vscode) ? Write-Host 'exists'   : Write-Host 'not found'
(Test-Path .vscode) ? (Write-Host 'exists') : Write-Host 'not found'

Test-Path: A positional parameter cannot be found that accepts argument '?'.
ParserError:
Line |
   1 |  (Test-Path .vscode) ? Write-Host 'exists'   : Write-Host 'not found'
     |                       ~
     | You must provide a value expression following the '?' operator.
ParserError:
Line |
   1 |  (Test-Path .vscode) ? (Write-Host 'exists') : Write-Host 'not found'
     |                                               ~
     | You must provide a value expression following the ':' operator.

And this example parses:

(Test-Path .vscode) ? (Write-Host 'exists') : (Write-Host 'not found')

exists

See also