Automatic variables ($_, $PSVersionTable) in PowerShell

Introduction

Automatic variables in PowerShell hold special information that helps scripts run smoothly without extra setup.

When processing items in a list or pipeline and you want to refer to the current item easily.

When you need to check the version of PowerShell running your script.

When writing quick scripts that handle data flowing through commands without naming variables.

When debugging or logging to see environment details like PowerShell version.

Syntax

PowerShell

$_
$PSVersionTable

$_ represents the current object in the pipeline.

$PSVersionTable is a read-only table showing PowerShell version details.

Examples

This filters processes using more than 100 CPU units, using $_ to refer to each process.

PowerShell

Get-Process | Where-Object { $_.CPU -gt 100 }

This prints the PowerShell version number.

PowerShell

Write-Output $PSVersionTable.PSVersion

Prints the name of each service using $_ inside a loop.

PowerShell

Get-Service | ForEach-Object { "Service: $_.Name" }

Sample Program

This script first shows PowerShell version info, then lists all running services by using $PSVersionTable and $_.

PowerShell

Write-Output "PowerShell version info:"
Write-Output $PSVersionTable

"List of running services with status:" | Write-Output
Get-Service | Where-Object { $_.Status -eq 'Running' } | ForEach-Object { Write-Output "Service: $_.Name" }

Important Notes

$_ only works inside commands that process pipeline input like ForEach-Object or Where-Object.

$PSVersionTable is useful to check compatibility before running scripts.

Summary

$_ is the current item in a pipeline, making it easy to work with each piece of data.

$PSVersionTable shows details about your PowerShell version and environment.

These automatic variables help write shorter, clearer scripts without extra setup.

Practice

1. What does the automatic variable $_ represent in a PowerShell pipeline?

easy

A. The version of PowerShell installed

B. The current object being processed in the pipeline

C. The last error message generated

D. The total number of commands run

Solution

1. Step 1: Understand the role of $_ in pipelines

   $_ is a special variable that holds the current item being processed in a pipeline command.

2. Step 2: Differentiate from other automatic variables

   $PSVersionTable shows version info, errors use $Error, and command counts are not stored in $_.

3. Final Answer:

   The current object being processed in the pipeline -> Option B

4. Quick Check:

   $_ = current pipeline item [OK]

Hint: Remember: $_ always means current pipeline item [OK]

Common Mistakes:

• Confusing $_ with $PSVersionTable
• Thinking $_ holds error info
• Assuming $_ is a global variable

2. Which of the following is the correct way to display the PowerShell version using $PSVersionTable?

easy

A. Write-Output $PSVersionTable.VersionInfo

B. Write-Output $_.Version

C. Write-Output $PSVersion.VersionTable

D. Write-Output $PSVersionTable.Version

Solution

1. Step 1: Identify the correct property of $PSVersionTable

   The property Version holds the PowerShell version number.

2. Step 2: Confirm the syntax to output the version

   Write-Output $PSVersionTable.Version correctly prints the version; other options use wrong variable names or properties.

3. Final Answer:

   Write-Output $PSVersionTable.Version -> Option D

4. Quick Check:

   $PSVersionTable.Version outputs version [OK]

Hint: Use $PSVersionTable.Version to get PowerShell version [OK]

Common Mistakes:

• Using $_ instead of $PSVersionTable
• Swapping property names incorrectly
• Assuming $PSVersionTable is a method

3. What will be the output of this PowerShell command?

1..3 | ForEach-Object { "Number: $_" }

medium

A. Number: 1 Number: 2 Number: 3

B. Number: $_ Number: $_ Number: $_

C. 1 2 3

D. Error: $_ is undefined

Solution

1. Step 1: Understand the pipeline and $_ usage

   The pipeline sends numbers 1 to 3 one by one to ForEach-Object. Inside the script block, $_ holds the current number.

2. Step 2: Evaluate the output string for each number

   Each iteration outputs "Number: 1", then "Number: 2", then "Number: 3" on separate lines.

3. Final Answer:

   Number: 1 Number: 2 Number: 3 -> Option A

4. Quick Check:

   Pipeline item $_ prints each number [OK]

Hint: $_ holds current pipeline item inside ForEach-Object [OK]

Common Mistakes:

• Thinking $_ is a literal string
• Expecting all output on one line without spaces
• Confusing $_ with a variable outside pipeline

4. Identify the error in this script snippet:

Get-Process | Where-Object { $_.Name -eq 'powershell' } | ForEach-Object { Write-Output $PSVersionTable }

medium

A. The script outputs the entire $PSVersionTable multiple times unnecessarily

B. Using $PSVersionTable inside ForEach-Object is invalid

C. $_ is not defined inside Where-Object script block

D. Get-Process cannot be piped to Where-Object

Solution

1. Step 1: Analyze the pipeline and variable usage

   The pipeline filters processes named 'powershell' and then outputs $PSVersionTable for each matching process.

2. Step 2: Identify inefficiency or error

   $PSVersionTable is a static variable; outputting it inside the loop repeats the same info multiple times unnecessarily.

3. Final Answer:

   The script outputs the entire $PSVersionTable multiple times unnecessarily -> Option A

4. Quick Check:

   Repeated $PSVersionTable output inside loop [OK]

Hint: Avoid repeating static variables inside loops [OK]

Common Mistakes:

• Thinking $PSVersionTable is undefined in pipeline
• Believing $_ is invalid in Where-Object
• Assuming Get-Process can't be piped

5. You want to list all running processes and display their names along with your PowerShell version once at the end. Which script correctly uses $_ and $PSVersionTable to achieve this?

hard

A. Get-Process | ForEach-Object { Write-Output "Process: $PSVersionTable.Version" }; Write-Output "PowerShell Version: $_.Name"

B. Get-Process | ForEach-Object { Write-Output "Process: $_.Name; PowerShell Version: $PSVersionTable.Version" }

C. Get-Process | ForEach-Object { Write-Output "Process: $_.Name" }; Write-Output "PowerShell Version: $PSVersionTable.Version"

D. Write-Output "PowerShell Version: $PSVersionTable.Version"; Get-Process | ForEach-Object { Write-Output "Process: $_.Name" }

Solution

1. Step 1: Understand the requirement to show processes and version separately

   We want to list each process name using $_ inside the pipeline, then show PowerShell version once after.

2. Step 2: Evaluate each option's correctness

   Get-Process | ForEach-Object { Write-Output "Process: $_.Name" }; Write-Output "PowerShell Version: $PSVersionTable.Version" lists processes with $_ correctly, then outputs version once after the loop. Get-Process | ForEach-Object { Write-Output "Process: $_.Name; PowerShell Version: $PSVersionTable.Version" } repeats version for each process. Write-Output "PowerShell Version: $PSVersionTable.Version"; Get-Process | ForEach-Object { Write-Output "Process: $_.Name" } outputs version first, then processes (order reversed). Get-Process | ForEach-Object { Write-Output "Process: $PSVersionTable.Version" }; Write-Output "PowerShell Version: $_.Name" misuses variables inside the loop.

3. Final Answer:

   Get-Process | ForEach-Object { Write-Output "Process: $_.Name" }; Write-Output "PowerShell Version: $PSVersionTable.Version" -> Option C

4. Quick Check:

   List processes with $_, version once after [OK]

Hint: Output static info after pipeline, use $_ inside pipeline [OK]

Common Mistakes:

• Repeating version output inside loop
• Mixing order of outputs
• Using wrong variable names inside pipeline