PowerShell Control Structures

Control structures are the "brain" of a program, determining under what conditions code executes, whether it repeats, and how to handle errors gracefully.

As a modern scripting language, PowerShell provides complete flow control syntax, including:

• Conditional statements:if、elseif、else、switch
• Loop structures:for、foreach、while、do-while
• Error handling mechanisms:try-catch-finally

1. Conditional Statements

if / elseif / else

Basic syntax:

if (条件1) {
    # 条件1为真执行
} elseif (条件2) {
    # 条件2为真执行
} else {
    # 其他情况
}

Example: Checking if Disk Space Is Insufficient

$disk = Get-PSDrive C
if ($disk.Free -lt 5GB) {
    Write-Output "磁盘空间不足！"
} elseif ($disk.Free -lt 10GB) {
    Write-Output "磁盘空间偏低。"
} else {
    Write-Output "磁盘空间充足。"
}

switch

When there are multiple possible values to evaluate,switchis more readable than multipleif[if] statements.

switch ($value) {
    "start" { Write-Output "开始任务" }
    "stop"  { Write-Output "停止任务" }
    "exit"  { Write-Output "退出程序" }
    default { Write-Output "未知命令" }
}

Supports matching patterns:

switch -Wildcard ($filename) {
    "*.txt" { "文本文件" }
    "*.jpg" { "图片文件" }
    default { "其他类型" }
}

2. Loop Structures

forfor Loop (Classic Counting Loop)

for ($i = 1; $i -le 5; $i++) {
    Write-Output "第 $i 次"
}

foreachforeach Loop (Iterating Collections)

$names = "张三", "李四", "王五"
foreach ($name in $names) {
    Write-Output "你好，$name"
}

You can also useForEach-Objectthe pipeline version:

$names | ForEach-Object { Write-Output "你好，$_" }

whilewhile Loop (Execute While Condition is True)

$count = 0
while ($count -lt 3) {
    Write-Output "计数：$count"
    $count++
}

do-whileanddo-until

doThe do-while loop willexecute at least once：

$count = 0
do {
    Write-Output "当前值：$count"
    $count++
} while ($count -lt 3)

do-until: it only stops when the condition becomes true

$count = 0
do {
    Write-Output "当前值：$count"
    $count++
} until ($count -ge 3)

3. Error Handling: try / catch / finally

PowerShell provides a structured exception handling mechanism for catching and responding to runtime errors.

3.1 Basic Structure

try {
    # 尝试运行可能出错的代码
}
catch {
    # 错误时执行
}
finally {
    # 无论是否出错，都会执行（可选）
}

Example: Handling Division by Zero Errors

try {
    $result = 10 / 0
}
catch {
    Write-Output "发生错误：$($_.Exception.Message)"
}
finally {
    Write-Output "运算结束"
}

Catching Specific Exception Types

try {
    Get-Content "不存在的文件.txt"
}
catch [System.IO.FileNotFoundException] {
    Write-Output "文件未找到！"
}
catch {
    Write-Output "其他错误：$($_.Exception.Message)"
}

Remove-Item "不存在的文件.txt" -ErrorAction Stop

$ErrorActionPreference = "Stop"

$number = 7
if ($number % 2 -eq 0) {
    "偶数"
} else {
    "奇数"
}

$items = "apple", "banana", "cherry"
foreach ($item in $items) {
    "$item 的长度是 $($item.Length)"
}

try {
    Get-Content "D:\not-exist.txt"
}
catch {
    "文件读取失败：" + $_.Exception.Message
}