Creating Environment variables

To list all environment variables you dir the env: PSdrive

dir env:

-or-

>> [Environment]::GetEnvironmentVariables();

# truncated list

Name                 Value
----                     -----
Path                       C:\Program Files\Windows Resource Kits\Tools\;C:\Program F
SESSIONNAME         Console
PATHEXT                .COM;.EXE;.BAT;.CMD;.VBS;.VBE;.JS;.JSE;.WSF;.WSH;.PSC1;.VB
....
....
....
ALLUSERSPROFILE   C:\Documents and Settings\All Users
OS                          Windows_NT
HOMEDRIVE             C:

Creating Variables

Variables can be created for the current process, user or machine context.
Process variables typically will be lost once the process exists (volatile), while user or machine are permanent and stored in the registry.

You can use the .NET SetEnvironmentVariable Method to create , modify or delete an environment variable. The method syntax is as follows:

[Environment]::SetEnvironmentVariable(<name>,<value>,<EnvironmentVariableTarget>);

EnvironmentVariableTarget can hold one of the following constants:
Process = 0
User = 1
Machine = 2

All Italic lines were copied from the SetEnvironmentVariable method web page.

Process variable

"If the target parameter is not Process, the environment variable is not automatically copied to the current process."

Correct me if I'm wrong but I think the "is not" should be "is". Listing the current process environment items shows the new created "Shay" item. Anyway, here is the create process:

>> [Environment]::SetEnvironmentVariable("Shay","Levi","Process");


User variable

"If target is User, the environment variable is stored in the Windows operating system registry key reserved for the current user, and is copied to the current process when the user who stored the variable starts a new process."

>> [Environment]::SetEnvironmentVariable("Shay","Levi","User");


Screen shot taken from the System properties applet

Machine variable

"If target is Machine, the environment variable is stored in the Windows operating system registry key reserved for the local machine, and is copied to the current process when any user on the local machine starts a new process."

>> [Environment]::SetEnvironmentVariable("Shay","Levi","Machine");

Delete variables

To delete a variable assign the $null to the item value

[Environment]::SetEnvironmentVariable("Shay",$null,0); # Process
[Environment]::SetEnvironmentVariable("Shay",$null,1); # User
[Environment]::SetEnvironmentVariable("Shay",$null,2); # Machine

Getting System up time (locally)

Here is another way to get your servers up time (total time since last reboot).
This method works locally.

The .NET Environment class exposes the TickCount property. TickCount Gets the number of milliseconds elapsed since the system started. It's a 32-bit signed integer type and by its restrictions it will be "reset" back to zero every 49.7 days (see Environment.TickCount Property on MSDN).

>> [environment]::tickcount;
297580406

To format the result to something more meaningful we'll use the New-TimeSpan cmdlet.
New-TimeSpan doesn't expose a milliseconds parameter so we'll divide the tickcount result by 1000 and feed the New-TimeSpan -seconds parameter.

>> New-TimeSpan -seconds ([environment]::tickcount / 1000);

Days : 3
Hours : 10
Minutes : 42
Seconds : 42
Milliseconds : 0
Ticks : 2977620000000
TotalDays : 3.44631944444444
TotalHours : 82.7116666666667
TotalMinutes : 4962.7
TotalSeconds : 297762
TotalMilliseconds : 297762000

So, to get only the days since last reboot:

>> (New-TimeSpan -seconds ([environment]::tickcount / 1000)).days;
3

Another formatting variation:

>> $sup = New-TimeSpan -seconds ([environment]::tickcount / 1000);
>> [string]::format("D:{0:00},H:{1:00},M:{2:00}",$sup.days,$sup.hours,$sup.minutes);

D:03,H:10,M:53

But wait, There's more. Browsing to the TimeSpan class page at MSDN reveals that TimeSpan class has a TimeSpan.FromMilliseconds Method.

>> [timespan] gm -static fr*;

TypeName: System.TimeSpan
Name MemberType Definition
---- ---------- ----------
FromDays Method static System.TimeSpan FromDays(Double value)
FromHours Method static System.TimeSpan FromHours(Double value)
FromMilliseconds Method static System.TimeSpan FromMilliseconds(Double value)
FromMinutes Method static System.TimeSpan FromMinutes(Double value)
FromSeconds Method static System.TimeSpan FromSeconds(Double value)
FromTicks Method static System.TimeSpan FromTicks(Int64 value)

Now, we can provide milliseconds to TimeSpan in a "static notation form":

>> $sup = [timespan]::FromMilliseconds([environment]::tickcount);
>> [string]::format("{0:00}:{1:00}:{2:00}",$sup.days,$sup.hours,$sup.minutes);

03:11:04

Note: If the returned values are negative, we can use the [math]::abs method to return absolute values.

Shay