Powershell Safe Invocation

PowerShell Safe Invocation

Shell

Use PowerShell 7 through pwsh.exe unless Windows PowerShell 5.1 is explicitly required.

When the active shell is uncertain, verify:

$PSVersionTable.PSVersion
$PSNativeCommandArgumentPassing

Do not assume installing PowerShell 7 makes powershell.exe use PowerShell 7:

• pwsh.exe = PowerShell 7
• powershell.exe = Windows PowerShell 5.1

Native Programs

Never construct one large command string when arguments can be passed separately.

Use:

$exe = 'C:\Path With Spaces\tool.exe'
$argList = @(
    '--input'
    'C:\Data Folder\input.json'
    '--flag'
)

& $exe @argList

$exitCode = $LASTEXITCODE
if ($exitCode -ne 0) {
    throw "$exe failed with exit code $exitCode"
}

Rules:

• Treat every native argument as one array item.
• Invoke executable paths stored in variables with &.
• Capture $LASTEXITCODE immediately.
• Avoid using $args as your own argument array name; it is a PowerShell automatic variable. Use names like $argList or $nativeArgs.
• Do not use Invoke-Expression.
• Do not add a cmd.exe /c layer merely to launch an executable.
• Do not use Bash-style \" escaping in PowerShell.

Cmdlets

Use hashtable splatting for PowerShell cmdlets:

$params = @{
    LiteralPath = 'C:\Data[1]\input.txt'
    Destination = 'C:\Output'
    Force       = $true
    ErrorAction = 'Stop'
}

Copy-Item @params

Use -LiteralPath for concrete paths when the cmdlet supports it. For cmdlets that expose only -Path, such as New-Item, use -Path; when unsure, check Get-Command <cmdlet> -Syntax.

Do not use $LASTEXITCODE to test a PowerShell cmdlet. Use terminating errors:

$ErrorActionPreference = 'Stop'

Wrap expressions passed as parameter values in parentheses or assign them first: use Select-Object -Index (100..120), not -Index 100..120.

Do not pipe directly from statement syntax such as foreach (...) { ... } | ...; assign the statement output first or use the pipeline cmdlet ForEach-Object.

Complex Commands

Avoid deeply quoted commands such as:

cmd.exe /c pwsh.exe -Command "..."

For multiline code, nested quotes, JSON, XML, regular expressions, pipelines, redirection, or non-ASCII paths:

1. Write a temporary .ps1 file.
2. Execute it with:

pwsh.exe -NoLogo -NoProfile -NonInteractive -File script.ps1

Prefer -File over -Command for anything beyond a short, simple expression.

Do not add -ExecutionPolicy Bypass unless execution policy is actually blocking a trusted script.

When you must pass a script through -Command from an outer PowerShell process, remember that the outer shell expands $variables first. Use an outer single-quoted script string when the inner script contains $p, $env:..., $PSVersionTable, or similar:

pwsh.exe -NoLogo -NoProfile -Command '$p = "C:\Data Folder\input.txt"; Test-Path -LiteralPath $p'

If the command has to cross multiple interpreters or wrappers, stop and write a .ps1 file instead of stacking more quoting.

Strings And Multiline Code

• Use single quotes for literal strings and paths.
• Use double quotes only when PowerShell expansion is needed.
• Avoid backtick line continuation; use arrays, hashtables, splatting, parentheses, or script blocks.
• Do not use Bash heredocs such as python - <<'PY'; PowerShell parses < differently. Use a temporary script file or a PowerShell here-string piped to the program.
• For JSON, create objects and use ConvertTo-Json; do not hand-escape JSON.
• Use single-quoted here-strings for literal multiline text: put no characters after opening @', and close with '@ alone at the start of a line.

Encoding

PowerShell 7 defaults to UTF-8 without BOM for text output; Windows PowerShell 5.1 defaults vary by cmdlet.

• For a new text file consumed by another tool, specify its required encoding explicitly (usually utf8).
• Before changing an existing text file, identify and preserve its existing encoding. Do not silently convert GBK, UTF-16, or BOM-sensitive files; if the encoding is unclear, inspect or ask.
• Do not set Console.InputEncoding or Console.OutputEncoding by default. Set them only for a confirmed terminal or native-program encoding mismatch; $OutputEncoding instead controls PowerShell text sent to native programs.
• Do not apply text encoding options to binary files. Use byte APIs for binary data.