powershell（powershell 开发）

Output Behavior

• Everything not captured goes to output — even without return or Write-Output
• return doesn't stop output — previous uncaptured expressions still output
• Write-Host bypasses pipeline — use for display only, not data
• Assign to $null to suppress — $null = SomeFunction
• [void] cast also suppresses — [void](SomeFunction)

Array Gotchas

• Single item result is scalar, not array — @(Get-Item .) forces array
• Empty result is $null, not empty array — check with if ($result) carefully
• Array unrolling in pipeline — @(1,2,3) | ForEach sends items one by one
• += on array creates new array — slow in loops, use [System.Collections.ArrayList]
• , is array operator — ,$item wraps single item in array

Comparison Operators

• -eq, -ne, -gt, -lt — not ==, !=, >, <
• -like with wildcards, -match with regex — both return bool
• -contains for array membership — $arr -contains $item, not $item -in $arr (though -in works too)
• Case-insensitive by default — -ceq, -cmatch for case-sensitive
• $null on left side — $null -eq $var prevents array comparison issues

String Handling

• Double quotes interpolate — "Hello $name" expands variable
• Single quotes literal — '$name' stays as literal text
• Subexpression for complex — "Count: $($arr.Count)" for properties/methods
• Here-strings for multiline — @" ... "@ or @' ... '@
• Backtick escapes — ` n ` for newline, t `` for tab

Pipeline

• $_ or $PSItem is current object — same thing, $_ more common
• ForEach-Object for pipeline — foreach statement doesn't take pipeline
• -PipelineVariable saves intermediate — Get-Service -PV svc | Where ...
• Pipeline processes one at a time — unless function doesn't support streaming

Error Handling

• $ErrorActionPreference sets default — Stop, Continue, SilentlyContinue
• -ErrorAction Stop per command — makes non-terminating errors terminating
• try/catch only catches terminating — set ErrorAction Stop first
• $? is last command success — $LASTEXITCODE for native commands

Common Mistakes

• No space before { in if — if($x){ works but if ($x) { preferred
• = is assignment in conditions — use -eq for comparison
• Function return array unrolls — return ,@($arr) to keep array
• Get-Content returns lines array — -Raw for single string
• Select-Object creates new object — properties are copies, not references

Cross-Platform

• pwsh is PowerShell 7+ — powershell is Windows PowerShell 5.1
• Paths use / or \ — Join-Path for portable
• Environment vars: $env:VAR — works on all platforms
• Aliases differ across platforms — ls, cat may not exist, use full cmdlet names