如何将 Powershell 命令的输出用作另一个 Powershell 命令的参数？

Question

我想折叠这些 Powershell 命令：

$colour = ""
$colour1 = "Red"
$colour2 = "Green"
if ($var -eq 0) {
    # failure
    $colour = $colour1
} Else {
    # success
    $colour = $colour2
}
Write-Host ("Answer = " + $var) -ForegroundColor $colour

成一行，使用if上面表达式的输出作为 的输入参数Write-Host，例如：

Write-Host ("Answer = " + $var) -ForegroundColor &{if ($var -eq 0) {'Red'} else {'Green'}

我尝试过但失败的事情：

Write-Host ("Answer = " + $var) -ForegroundColor &{if ($var -eq 0) {'Red'} else {'Green'}
Write-Host ("Answer = " + $var) -ForegroundColor `&{if ($var -eq 0) {'Red'} else {'Green'}`
Write-Host ("Answer = " + $var) -ForegroundColor `{if ($var -eq 0) {'Red'} else {'Green'}`
Write-Host ("Answer = " + $var) -ForegroundColor &`{if ($var -eq 0) {'Red'} else {'Green'}`
Write-Host ("Answer = " + $var) -ForegroundColor (&`{if ($var -eq 0) {'Red'} else {'Green'}`)
Write-Host ("Answer = " + $var) -ForegroundColor `{if ($var -eq 0) {return 'Red'} else {return 'Green'}`
Write-Host ("Answer = " + $var) -ForegroundColor `&{if ($var -eq 0) {return 'Red'} else {return 'Green'}`
Write-Host ("Answer = " + $var) -ForegroundColor `&{if ($var -eq 0) {return "Red"} else {return "Green"}`
Write-Host ("Answer = " + $var) -ForegroundColor &{if ($var -eq 0) {return "Red"} else {return "Green"}
Write-Host ("Answer = " + $var) -ForegroundColor &{if ($var -eq 0) {return "Red"} else {return "Green"}}
Write-Host ("Answer = " + $var) -ForegroundColor `&{if ($var -eq 0) {return "Red"} else {return "Green"}}`
Write-Host ("Answer = " + $var) -ForegroundColor &`{if ($var -eq 0) {return "Red"} else {return "Green"}}`
Write-Host ("Answer = " + $var) -ForegroundColor `{if ($var -eq 0) {return "Red"} else {return "Green"}}`
Write-Host ("Answer = " + $var) -ForegroundColor `if ($var -eq 0) {return "Red"} else {return "Green"}`

Answer 1

要将整个语句（例如if）的输出作为命令参数传递，请将它们括在$(...)、子表达式运算符 中。

一个简化的例子：

# Note the use of $(...) around the `if` statement.
PS> $var = 0; Write-Host $(if ($var -eq 0) { 'zero' } else { 'nonzero' })
zero

请注意，如果只需要表达式或命令的输出，则使用(...),分组运算符就足够了（并且没有副作用）。

例如，PowerShell (Core) 7+ 替代if 语句是三元运算符，其用途是表达式：

# PowerShell (Core) 7+ only.
# Since an expression is used, (...) is sufficient.
PS> $var = 0; Write-Host ($var -eq 0 ? 'zero' : 'nonzero')
zero

对于命令（管道），(...)也足够了。

# Since a command is used, (...) is sufficient.
Write-Output (Get-Item *.txt | Where-Object Length -ge 1kb)

有关$(...)vs. 的更多信息(...)，请参阅此答案。