How to Read Files Line by Line in Windows PowerShell

1. Read File Line by Line in PowerShell Using Get-Content with a ForEach Loop
2. Read File Line by Line in PowerShell Using Get-Content with Foreach-Object
3. Using the [System.IO.File] Class in PowerShell to Read File Line by Line
4. Using the System.IO.StreamReader Class in PowerShell to Read File Line by Line
5. Conclusion

In Windows PowerShell, we can use the Get-Content cmdlet to read files from a file. However, the cmdlet will load the entire file contents to memory at once, which will fail or freeze on large files.

To solve this problem, what we can do is we can read the files line by line, and this article will show you how.

Before we start, it is better if we create a sample text file with multiple lines. For example, I have created a file.txt containing the following lines.

file.txt:

one
two
three
four
five
six
seven
eight
nine
ten

According to Microsoft, a regular expression is a pattern used to match the text. It can be made up of literal characters, operators, and other constructs. This article will try different scripts involving a standard variable called Regular Expressions, denoted by $regex.

Read File Line by Line in PowerShell Using Get-Content with a ForEach Loop

We have initially discussed that the Get-Content cmdlet has its flaws when we are loading a large file as it will load the contents of the file into memory all at once. However, we can improve it by loading the lines one by one using the foreach loop.

Example Code:

foreach ($line in Get-Content .\file.txt) {
    if ($line -match $regex) {
        Write-Output $line
    }
}

Here’s a step-by-step explanation of the process:

1. Get the File Content:

   We start by using the Get-Content cmdlet to read the contents of the file.

   Get-Content .\file.txt
   

   Replace "file.txt" with the path to your file.

2. Iterate Over Each Line:

   Next, we use a ForEach loop to iterate over each line in the file. The loop assigns each line to a variable, $line, one at a time.

   Inside the loop, you can replace Write-Host $line with the code that processes each line as per your requirements. In this example, we are simply displaying each line using Write-Host, but you can perform any desired actions.

Read File Line by Line in PowerShell Using Get-Content with Foreach-Object

To read a file line by line in PowerShell using Get-Content with Foreach-Object, follow these steps:

1. Use the Get-Content Cmdlet:

   We start by using the Get-Content cmdlet to read the contents of the file. The Get-Content cmdlet reads each line of the file and outputs them as objects in a pipeline.

   powershellCopy codeGet-Content -Path "example.txt" | ForEach-Object {
       # Process each line here
       $_  # $_ represents the current line
   }
   

Replace "example.txt" with the path to your file.

2. Foreach-Object Loop:

   Next, we use the Foreach-Object cmdlet (% is an alias for Foreach-Object) to iterate over each line in the pipeline. Inside the script block (the code within {}), you can process each line using $_, which represents the current line being processed.

   powershellCopy codeGet-Content -Path "example.txt" | ForEach-Object {
       # Process each line here
       $_  # $_ represents the current line
   }
   

   Within this loop, you can perform any actions you need on each line.

Complete Working Code Example

Here’s a complete working code example that reads a file line by line and displays each line:

powershellCopy code# Specify the path to the file
$filePath = "example.txt"

# Check if the file exists
if (Test-Path $filePath) {
    # Read the file line by line and process each line
    Get-Content -Path $filePath | ForEach-Object {
        # Process each line here
        Write-Host $_
    }
} else {
    Write-Host "File not found: $filePath"
}

In this code:

• We specify the path to the file using the $filePath variable.
• We use the Test-Path cmdlet to check if the file exists.
• If the file exists, we use Get-Content to read its content and pass it to Foreach-Object for processing.
• Inside the Foreach-Object script block, we use Write-Host $_ to display each line.

Using the [System.IO.File] Class in PowerShell to Read File Line by Line

To read a file line by line in PowerShell using the [System.IO.File] class, follow these steps:

1. Specify the File Path:

   Start by specifying the path to the file you want to read. Replace "example.txt" with the actual path to your file.

   powershellCopy code
   $filePath = "example.txt"
   

2. Check If the File Exists:

   Before attempting to read the file, it’s a good practice to check if the file exists using the Test-Path cmdlet:

   powershellCopy codeif (Test-Path $filePath) {
       # File exists, proceed with reading
   } else {
       Write-Host "File not found: $filePath"
   }
   

3. Read the File Line by Line:

   Once you’ve verified that the file exists, you can use the [System.IO.File] class to read the file line by line. Here’s the code to do that:

   powershellCopy codetry {
       $lines = [System.IO.File]::ReadLines($filePath)
       foreach ($line in $lines) {
           # Process each line here
           Write-Host $line
       }
   } catch {
       Write-Host "Error reading the file: $_"
   }
   

   In this code:

   • We use the ReadLines method of the [System.IO.File] class to read the file line by line, and we store the lines in the $lines variable.
   • We then use a foreach loop to iterate over each line in $lines and process it. In this example, we are simply displaying each line using Write-Host, but you can perform any desired actions.

Complete Working Code Example

Here’s a complete working code example that reads a file line by line and displays each line:

powershellCopy code# Specify the path to the file
$filePath = "example.txt"

# Check if the file exists
if (Test-Path $filePath) {
    try {
        # Read the file line by line
        $lines = [System.IO.File]::ReadLines($filePath)
        foreach ($line in $lines) {
            # Process each line here
            Write-Host $line
        }
    } catch {
        Write-Host "Error reading the file: $_"
    }
} else {
    Write-Host "File not found: $filePath"
}

In this code:

• We specify the path to the file using the $filePath variable.
• We check if the file exists using Test-Path.
• If the file exists, we use the [System.IO.File]::ReadLines() method to read the file line by line and process each line using a foreach loop.
• We include error handling to catch any exceptions that might occur during file reading.

Using the System.IO.StreamReader Class in PowerShell to Read File Line by Line

To read a file line by line in PowerShell using the System.IO.StreamReader class, follow these steps:

1. Specify the File Path:

   Begin by specifying the path to the file you want to read. Replace "example.txt" with the actual path to your file.

   powershellCopy code
   $filePath = "example.txt"
   

2. Check If the File Exists:

   Before attempting to read the file, it’s a good practice to check if the file exists using the Test-Path cmdlet:

   powershellCopy codeif (Test-Path $filePath) {
       # File exists, proceed with reading
   } else {
       Write-Host "File not found: $filePath"
   }
   

3. Read the File Line by Line:

   Once you’ve verified that the file exists, you can use the System.IO.StreamReader class to read the file line by line. Here’s the code to do that:

powershellCopy codetry {
    $reader = [System.IO.StreamReader]::new($filePath)
    while ($reader.Peek() -ge 0) {
        $line = $reader.ReadLine()
        # Process each line here
        Write-Host $line
    }
    $reader.Close()
} catch {
    Write-Host "Error reading the file: $_"
}

In this code:

• We create a new instance of System.IO.StreamReader using [System.IO.StreamReader]::new($filePath), where $filePath is the path to the file.
• We use a while loop to read the file line by line. The $reader.Peek() -ge 0 condition ensures that we continue reading until the end of the file is reached.
• Inside the loop, we use $reader.ReadLine() to read each line and process it as needed. In this example, we are simply displaying each line using Write-Host, but you can perform any desired actions.
• Finally, we close the StreamReader using $reader.Close() to release the file resources.

Complete Working Code Example

Here’s a complete working code example that reads a file line by line and displays each line:

powershellCopy code# Specify the path to the file
$filePath = "example.txt"

# Check if the file exists
if (Test-Path $filePath) {
    try {
        # Create a StreamReader to read the file
        $reader = [System.IO.StreamReader]::new($filePath)

        # Read the file line by line
        while ($reader.Peek() -ge 0) {
            $line = $reader.ReadLine()
            # Process each line here
            Write-Host $line
        }
        
        # Close the StreamReader
        $reader.Close()
    } catch {
        Write-Host "Error reading the file: $_"
    }
} else {
    Write-Host "File not found: $filePath"
}

In this code:

• We specify the path to the file using the $filePath variable.
• We check if the file exists using Test-Path.
• If the file exists, we create a System.IO.StreamReader instance to read the file line by line and process each line using a while loop.
• We include error handling to catch any exceptions that might occur during file reading.

Conclusion

In summary, PowerShell provides several versatile methods for reading files line by line, catering to diverse scripting and automation needs. Get-Content with a ForEach Loop is a user-friendly choice for straightforward tasks, offering simplicity and readability. Get-Content with Foreach-Object enhances flexibility, making it suitable for more intricate operations and filtering tasks within the pipeline.

For scenarios requiring greater control and comprehensive error handling, the [System.IO.File] class offers a powerful solution. This method allows you to fine-tune file operations and gracefully handle exceptions, making it well-suited for robust and error-resistant scripts.

In situations where you’re dealing with large files and memory efficiency is crucial, the System.IO.StreamReader class shines. It allows you to read files efficiently while managing memory resources more effectively, ensuring smooth processing of substantial datasets.

Ultimately, the choice of method depends on your specific script requirements, file size, and complexity of processing. By leveraging these methods, you can confidently read files line by line in PowerShell, effectively addressing a wide range of file-related tasks in your automation workflows.