Day 2 - Windows Powershell

Maybe you're a windows sysadmin. Maybe you're not. Either way, you might find the features in Powershell pretty cool.

Powershell is Windows-only and free to use. Some syntactic differences asside, it looks and feels like a unix shell language. It has standard features you might expect such as functions, recursion, variables, variable scope, objects, and a handful of built-in functionality to help you get work done, but it does many things better.

In addition to these baseline expectations, functions in powershell trivially take flag arguments by simply declaring a function argument (function foo($bar) { ... } can be invoked as foo -bar "somevalue". You can create arbitrary objects with properties and methods defined on the fly. Exception handling, logging, trace debugging, and other goodies are packed in by default.

It supports pipes like your favorite unix shell, except instead of piping text, you pipe objects. The key word is object. When you run 'dir' (or ls, which is an alias), it outputs file objects. When you run 'ps' (which is an alias of get-process), you get process objects. When you run 'get-content' a file, you get an array of strings.

Why is this significant? As a unix sysadmin, you quickly become intimate with piping one command to another, smoothly sandwiching filter invocations between others tools. Filter tools like awk, sed, grep, xargs, etc, all helping you convert one output text into another input text for another command. What if you didn't have to do that, or had to do it less? No more parsing the output of ls(1), stat(1), or du(1) to ask for file attributes when powershell's file object has them. What about getting process attributes?

# Yes, this is a comment in Powershell
# Show the top 3 consumers of virtual memory:
PS > get-process | sort {$_.VirtualMemorySize} | select -last 3

Handles  NPM(K)    PM(K)      WS(K) VM(M)   CPU(s)     Id ProcessName
-------  ------    -----      ----- -----   ------     -- -----------
    745      58    66648       4316   632    21.03   3564 CCC
   1058     107   230788      28384   680   600.23   5048 Steam
    446      78  1328988    1267960  1616 6,223.72   3692 firefox

# Kill firefox
PS > get-process firefox | stop-process
# Alternately
PS > get-process firefox | foreach { $_.kill() }

'select' is an alias for 'select-object' which lets you (among other things) trim an object to only selected properties. Inspection is done with 'get-member' (or 'gm' for short) and you can inspect objects output by 'ls' by doing: ls | gm, or processes with get-process | gm. You can ask an object what type it is with obj.gettype(); such as (get-item .).gettype()

But what if you want to manipulate the registry easily? The registry, filesystem, aliases, variables, environment, functions, and more are all considered "providers" in Powershell. Each provider gives you access to a certain data store using standard built-in commands. A provider can be invoked by prefixing a path with the provider name. For example, to access a registry key, you could use dir Registry::HKEY_CURRENT_USER to list keys in that part of the registry.

In addition to other neat features, you've got nice access to both COM and .NET. Want to create a tempfile?

PS > $tmp = [System.IO.Path]::GetTempFileName()
PS > ls $tmp
    Directory: Microsoft.PowerShell.Core\FileSystem::C:\Users\Jordan\AppData\Local\Temp

Mode                LastWriteTime     Length Name
----                -------------     ------ ----
-a---         12/2/2008   1:25 AM          0 tmp55FC.tmp

PS > rm $tmp

Help is also conveniently available from the powershell prompt. Help, which can be accessed with a muscle-memory-friendly 'man,' comes in different details levels. Try help select-object and then help select-object -detailed. There's also other useful builtins like foreach-object (like 'for' in bourne), select-object (like cut, tail, head, and uniq, but cooler) , sort-object (like sort, but cooler), where-object (like grep, but cooler), measure-object (like wc, but cooler), and format-list and format-table for sanely printing object properties.

Are you still scripting in DOS batch or VBScript? Do you use Cygwin as a means of escaping to a scripting language on windows that is less frustrating or awkward? Are you suddenly facing windows administration when your background is unix? Check out Powershell.

Further reading:

Download Powershell

Powershell homepage

Hey, Scripting Guy!

A pretty good resources for practical powershell examples