Powershell Cheats

This is a compilation of powershell snippets to do various things. I can never remember the cryptic powershell syntax and API’s, so I use this cheatsheet just about every time I touch the language.

Define a Function

pretty straightforward, but I found parameterization to be really weird

function do_something{
  param ([string]$parameter1, [string]$parameter2)
  Write-Host 'did something'
  Write-Host "parameter1: ${parameter1}"
  Write-Host "parameter2: ${parameter2}"
}

Make Function Parameters Required

function do_something{
  param ([string]$parameter1=$(throw 'parameter1 is a required parameter'))
}

Call a Function

Don’t use commas!

do_something 'hello' 'world'

Execute Script

Pretty straightforward. From cmd.exe:

@powershell .\my-script.ps1

From powershell.exe:

.\my-script.ps1

In any case, the path must be absolute

Execute Script with Parameters

You need to put one of these funky param blocks at the top of your script:

param ( [string]$foo)

And then pass the parameter on the command line in a named fashion like so:

@powershell .\my-script -foo 'bar'

Treat Errors Like…. Errors

There’s no guarantee that errors will actually stop execution as you might expect, but having the following at the top makes powershell behavior much more inline with what you’d expect

$ErrorActionPreference = "Stop"

Constrain Powershell Version

Add the following to the top of your script and the version of powershell will be constrained at runtime accordingly

#Requires -Version 4.0

Suppress Command Output

Sometimes you want to execute something like a directory delete and you don’t want the detailed console output displayed. In *nix you’d do something like this: >dev/null. In powershell, you’d do this: | out-null. Such as:

New-Item -ItemType directory -Path $dir | out-null

Execute External Process

There’s a few ways to do this… the only way I found that waits properly and handles errors properly is:

&something.exe -arg1 'foo' -arg2 'bar'
if ($lastexitcode -ne 0) { throw 'something.exe failed' }

Read File and Display Content

Get-Content 'path\to\file.txt'

Recreate Directory If It Exists

$dir = 'path\to\dir'
If (Test-Path $dir) { Remove-Item $dir -Force -Recurse | out-null }

New-Item -ItemType directory -Path $dir | out-null

Get Installed Programs

function get_installed_apps
{
    if ([IntPtr]::Size -eq 4) {
        $regpath = 'HKLM:\Software\Microsoft\Windows\CurrentVersion\Uninstall\*'
    }
    else {
        $regpath = @(
            'HKLM:\Software\Microsoft\Windows\CurrentVersion\Uninstall\*'
            'HKLM:\Software\Wow6432Node\Microsoft\Windows\CurrentVersion\Uninstall\*'
        )
    }
    Get-ItemProperty $regpath | .{process{if($_.DisplayName -and $_.UninstallString) { $_ } }} | Select DisplayName, Publisher, InstallDate, DisplayVersion, UninstallString |Sort DisplayName
}

Check If App Is Installed

This uses the function above in the Get Installed Programs section

function inspect_for_app {
  param ([string] $name,
         [string] $targetVersion)

  Write-Host -NoNewLine " * Inspecting ${name}:  " -foregroundcolor Green
  $appToMatch = "$name*"
  $app = get_installed_apps | where {$_.DisplayName -like $appToMatch}

  if ($app -eq $null -or $app.DisplayVersion -eq $null -or $app.DisplayVersion -eq '') {
    Write-Host "No" -foregroundcolor Green
  } else {
    $version = $app.DisplayVersion
    Write-Host "$version, $targetVersion" -foregroundcolor Green
  }

  return $app
}

$vagrantApp = inspect_for_app 'Vagrant' $targetVagrant
$vagrantVersion = $vagrantApp.DisplayVersion

Check Is Administrator

If you script needs to run as elevated, this will do it:

if (-NOT ([Security.Principal.WindowsPrincipal][Security.Principal.WindowsIdentity]::GetCurrent()).IsInRole(`
  [Security.Principal.WindowsBuiltInRole] "Administrator")) {

}

Parse JSON

$versionsJson = Get-Content $versionTempPath | Out-String | ConvertFrom-Json

HTTP GET

performs an HTTP GET for an XML payload and interprets it

$url = "http://scm.csgicorp.com/buildhistory/api/modules/CSGCloudCMDB/versions/latest/builds/latest?links=fals"
$webClient = New-Object System.Net.WebClient
$webClient.Headers.add('content-type','text/xml')
$xml = $webClient.DownloadString($url)
$doc = [xml]($xml)
Write-Host $doc.Response.Build

Download File

function download_file{
  param ([string]$url, [string] $dest)
    $webClient = New-Object System.Net.WebClient
    $webClient.DownloadFile($url, $dest)
}