Powershell后台作业、异步操作实例

Powershell是单线程程序且一次只能做一件事情。后台作业能额外增加Powershell进程在后台处理作业。当需要程序同时运行且数据量不是很大时它能很好的解决问题。但从Powershell后台回传数据是一个非常麻烦的工作，它将浪费很多时间。将会导致脚本更慢。

这里有3个并发执行任务：
代码如下:
$start = Get-Date

# get all hotfixes
$task1 = { Get-Hotfix }

# get all scripts in your profile
$task2 = { Get-Service | Where-Object Status -eq Running }

# parse log file
$task3 = { Get-Content -Path $env:windir/windowsupdate.log | Where-Object { $_ -like '*successfully installed*' } }

# run 2 tasks in the background, and 1 in the foreground task
$job1 =  Start-Job -ScriptBlock $task1
$job2 =  Start-Job -ScriptBlock $task2
$result3 = Invoke-Command -ScriptBlock $task3

# wait for the remaining tasks to complete (if not done yet)
$null = Wait-Job -Job $job1, $job2

# now they are done, get the results
$result1 = Receive-Job -Job $job1
$result2 = Receive-Job -Job $job2

# discard the jobs
Remove-Job -Job $job1, $job2

$end = Get-Date
Write-Host -ForegroundColor Red ($end - $start).TotalSeconds

上面执行全部的任务消耗了5.9秒。三个任务的结果将分别存入$result1, $result2, 和 $result3.
让我们再继续查看相继在前台执行完命令需要多长时间：

代码如下:
$start = Get-Date

# get all hotfixes
$task1 = { Get-Hotfix }

# get all scripts in your profile
$task2 = { Get-Service | Where-Object Status -eq Running }

# parse log file
$task3 = { Get-Content -Path $env:windir/windowsupdate.log | Where-Object { $_ -like '*successfully installed*' } }

# run them all in the foreground:
$result1 = Invoke-Command -ScriptBlock $task1
$result2 = Invoke-Command -ScriptBlock $task2
$result3 = Invoke-Command -ScriptBlock $task3

$end = Get-Date
Write-Host -ForegroundColor Red ($end - $start).TotalSeconds

结果，这次只花费了5.05秒。与后台作业几乎同时完成，所以后台作业更适合解决长时间执行的任务。从三个任务返回的数据观察，好处是这种按顺数在前台获得数据能减少了执行过程的开销。