为啥cmd powershell -command中的最后一行错误

Posted 2023-02-16

【中文标题】为啥cmd powershell -command中的最后一行错误

【英文标题】：Why does the last line in cmd powershell -command error为什么cmd powershell -command中的最后一行错误

【发布时间】：2021-11-29 03:18:52

【问题描述】：

'C:\Users\kevin>powershell -Command "$Url = 'http://shared4.info/psequotes/files/2021/stockQuotes_$CurrentDate.csv'"

C:\Users\kevin>powershell -Command "$Path = 'C:\Users\kevin\Desktop\stockQuotes_$CurrentDate.csv'"

C:\Users\kevin>powershell -Command "$WebClient = New-Object System.Net.WebClient"

C:\Users\kevin>powershell -Command "$WebClient.DownloadFile($url, $path)"
You cannot call a method on a null-valued expression.
At line:1 char:1
+ $WebClient.DownloadFile($url, $path)
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ CategoryInfo          : InvalidOperation: (:) [], RuntimeException
+ FullyQualifiedErrorId : InvokeMethodOnNull'

【问题讨论】：

设置 $Path 变量需要使用引号字符而不是撇号字符，以便允许对 $CurrentDate 进行变量插值。在引号字符内，必须使用转义。 "$Path = "C:\Users...otes_$CurrentDate.csv\"".

另外，使用$Path = Join-Path $Env:USERPROFILE -ChildPath "Desktop\stockQuotes_$CurrentDate.csv\"" 可能会更好。

【参考方案1】：

您正在使用每个命令启动一个新的 Powershell 会话。所以变量$WebClient在最后一个命令创建的powershell会话中不存在。

不要在每一行调用powershell -Command ，而是调用一次powershell ，然后在一个会话中运行所有这些语句。例如：

C:\Users\david>powershell
Windows PowerShell
Copyright (C) Microsoft Corporation. All rights reserved.

Install the latest PowerShell for new features and improvements! https://aka.ms/PSWindows

PS C:\Users\david> $Url = 'http://shared4.info/psequotes/files/2021/stockQuotes_$CurrentDate.csv'
PS C:\Users\david> $Path = 'C:\Users\kevin\Desktop\stockQuotes_$CurrentDate.csv'
PS C:\Users\david> $WebClient = New-Object System.Net.WebClient
PS C:\Users\david> $WebClient.DownloadFile($url, $path)

或者像这样的批处理文件：

powershell -Command ^
$Url = 'http://shared4.info/psequotes/files/2021/stockQuotes_$CurrentDate.csv'; ^
$Path = 'C:\Users\kevin\Desktop\stockQuotes_$CurrentDate.csv'; ^
$WebClient = New-Object System.Net.WebClient; ^
$WebClient.DownloadFile($url, $path);