01.$LASTEXITCODE和$?
类似于linux的shell脚本，powershell也可以用记录程序退出码的方式判断命令是否执行成功
其中
$?表示最后一个操作的执行状态。如果最后一个操作成功，则包含 TRUE，失败则包含 FALSE。$LASTEXITCODE则是返回上一次执行的退出码，因为linux程序通常用退出码0表示执行成功，所以我们判断$LASTEXITCODE是否为0就能判断上次程序执行是否成功。
总的来说$LASTEXITCODE，逻辑上还要多绕一圈，所以平时方便起见用$?就是了。
02.trap
trap关键词可以指定让程序终结的错误发生时，执行一系列语句。
- trap的默认行为是在执行完statement block里的语句 后显示错误，然后会继续执行脚本或者函数
- 在trap中使用break可以实现显示错误，然后退出执行
- 如果想要不显示错误继续执行，可以用continue
语法
trap [[<error type>]] {<statement list>}
statement list就是终结错误发生时，会执行的语句。
错误类型定义了trap的处理范围
trap处理所有终结错误
没有类型定义的trap会在所有错误发生的时候执行。
当一个让程序终结的错误没有找到其他处理的脚本或命令，就会执行trap里面的语句，
下面是一个例子
trap {"Error found."}
function TrapTest {
trap {"Error found."}
nonsenseString
}
TrapTest
会出现下面的错误
Error found.
nonsenseString:
Line |
3 | nonsenseString
| ~~~~~~~~~~~~~~
| The term 'nonsenseString' is not recognized as the name of a cmdlet,
function, script file, or operable program. Check the spelling of the name, or
if a path was included, verify that the path is correct and try again.
trap 中使用$_,也就是powershell自动生成的当前对象，就会被替换成输出的错误
function TrapTest {
trap {"Error found: $_"}
nonsenseString
}
TrapTest
Error found: The term 'nonsenseString' is not recognized as the name of a
cmdlet, function, script file, or operable program. Check the spelling of the
name, or if a path was included, verify that the path is correct and try again.
nonsenseString:
Line |
3 | nonsenseString
| ~~~~~~~~~~~~~~
| The term 'nonsenseString' is not recognized as the name of a cmdlet,
function, script file, or operable program. Check the spelling of the name, or
if a path was included, verify that the path is correct and try again.
注意: trap可以在一个作用域代码块（scope）的任何地方定义，即使定义在一个代码块的底部，也会捕获整个代码块的错误
trap处理特定的错误
下面是只处理没有找到命令的错误的trap，trap的使用的是.net的异常类型
trap [System.Management.Automation.CommandNotFoundException]
{"Command error trapped"}
下面是系统异常类型，System.Management.Automation.CommandNotFoundException是继承自System.Exception的
trap [System.Exception] {"An error trapped"}
当多个trap同时存在的时候powershell会匹配最精确特定的trap，如下
trap {"Other terminating error trapped" }
trap [System.Management.Automation.CommandNotFoundException] {
"Command error trapped"
}
nonsenseString
Command error trapped
nonsenseString:
Line |
5 | nonsenseString
| ~~~~~~~~~~~~~~
| The term 'nonsenseString' is not recognized as the name of a cmdlet,
function, script file, or operable program. Check the spelling of the name, or
if a path was included, verify that the path is correct and try again.
trap捕获错误的作用域
trap捕获错误以后，会执行trap内部的语句，但是错误处后续的代码还会继续执行
下面是在函数内部发生了错误的情况，错误被内部的trap捕获，并且函数的返回语句被正常执行
function function1 {
trap { "An error: " }
NonsenseString
"function1 was completed"
}
function1
An error:
NonsenseString:
Line |
3 | NonsenseString
| ~~~~~~~~~~~~~~
| The term 'NonsenseString' is not recognized as the name of a cmdlet,
function, script file, or operable program. Check the spelling of the name, or
if a path was included, verify that the path is correct and try again.
function1 was completed
下面的例子trap语句被放到函数外面，能够捕获到函数内部的错误，但是函数的返回语句没有被执行
function function2 {
NonsenseString
"function2 was completed"
}
trap { "An error: " }
function2
An error:
NonsenseString:
Line |
2 | NonsenseString
| ~~~~~~~~~~~~~~
| The term 'NonsenseString' is not recognized as the name of a cmdlet,
function, script file, or operable program. Check the spelling of the name, or
if a path was included, verify that the path is correct and try again.
也就是说，trap如果在函数的外部，就可以实现捕获到错误就停止函数运行的功能。
注意:多个trap定义相同类型的错误条件的时候，只有最高处定义的trap会被使用
Remove-Item -ErrorAction Stop ThisFileDoesNotExist
trap { "whoops 1"; continue }
trap { "whoops 2"; continue }
注意：trap语句的定义域取决于它编译的位置，如果它在一个函数或者脚本中，那么当函数或脚本退出时，他们内部的trap语句就会被删除
看下面的例子，函数外的错误没有被函数内的trap捕获
function function1 {
trap { "An error: " }
"function1 was completed"
}
NonsenseString
function1 was completed
NonsenseString: C:\Projects\base\test\testexit.ps1:6:1
Line |
6 | NonsenseString
| ~~~~~~~~~~~~~~
| The term 'NonsenseString' is not recognized as a name of a cmdlet, function,
| script file, or executable program. Check the spelling of the name, or if a
| path was included, verify that the path is correct and try again.
使用break和continue关键字
在trap中使用break和continue可以决定终结错误发生时，继续运行还是停止
默认没有break关键字的情况下是会继续运行的，也会输出错误
function break_example {
trap {
"Error trapped"
break
}
1/$null
"Function completed."
}
break_example
Error trapped
ParentContainsErrorRecordException:
Line |
6 | 1/$null
| ~~~~~~~
| Attempted to divide by zero.
使用continue语句，powershell将从错误中恢复，并且不会输出错误流
function continue_example {
trap {
"Error trapped"
continue
}
1/$null
"Function completed."
}
continue_example
Error trapped
Function completed.
综上，trap是一种比较简单的广范围的错误处理方式，对于更细粒度的错误处理，建议使用try catch语句
03.try catch finally
try 捕获的错误，会被自动保存到$Error变量里面，powershell会寻找catch语句来处理错误。
这个语法就和c#的异常处理比较像
语法
try {<statement list>}`
catch [[<error type>][',' <error type>]*] {<statement list>}
finally {<statement list>}
捕获错误
try { NonsenseString }
catch { "An error occurred." }
An error occurred.
使用多个catch语句
try {
$wc = new-object System.Net.WebClient
$wc.DownloadFile("http://www.contoso.com/MyDoc.doc","c:\temp\MyDoc.doc")
}
catch [System.Net.WebException],[System.IO.IOException] {
"Unable to download MyDoc.doc from http://www.contoso.com."
}
catch {
"An error occurred that could not be resolved."
}
在try catch语句中使用trap
当try语句中包含trap，即使声明了catch语句，也会执行trap，如果trap的位置比try更高，并且没有catch语句，trap也会获得控制，即使父级作用域中包含匹配的catch语句。
访问错误信息 ACCESSING EXCEPTION INFORMATION
catch语句中的$_就包含了错误信息
try { NonsenseString }
catch {
Write-Host "An error occurred:"
Write-Host $_
}
An Error occurred:
The term 'NonsenseString' is not recognized as the name of a cmdlet, function,
script file, or operable program. Check the spelling of the name, or if a path
was included, verify that the path is correct and try again.
你还可以用$_打印堆栈信息
try { NonsenseString }
catch {
Write-Host "An error occurred:"
Write-Host $_.ScriptStackTrace
}
An Error occurred:
at <ScriptBlock>, <No file>: line 2
使用FINALLY来释放资源
不管try块是否遇到错误，finally语句都会执行。即使catch中使用exit退出脚本，或者使用ctrl+c中止脚本都会执行。
