Read file line by line in PowerShell

Get-Content has bad performance; it tries to read the file into memory all at once.

C# (.NET) file reader reads each line one by one

Best Performace

foreach($line in [System.IO.File]::ReadLines("C:\path\to\file.txt"))
{
       $line
}

Or slightly less performant

[System.IO.File]::ReadLines("C:\path\to\file.txt") | ForEach-Object {
       $_
}

The foreach statement will likely be slightly faster than ForEach-Object (see comments below for more information).

Not much documentation on PowerShell loops.

Documentation on loops in PowerShell is plentiful, and you might want to check out the following help topics: about_For, about_ForEach, about_Do, about_While.

foreach($line in Get-Content .\file.txt) {
    if($line -match $regex){
        # Work here
    }
}

Another idiomatic PowerShell solution to your problem is to pipe the lines of the text file to the ForEach-Object cmdlet:

Get-Content .\file.txt | ForEach-Object {
    if($_ -match $regex){
        # Work here
    }
}

---

Instead of regex matching inside the loop, you could pipe the lines through Where-Object to filter just those you're interested in:

Get-Content .\file.txt | Where-Object {$_ -match $regex} | ForEach-Object {
    # Work here
}

The almighty switch works well here:

'one
two
three' > file

$regex = '^t'

switch -regex -file file { 
  $regex { "line is $_" } 
}

Output:

line is two
line is three

Reading Large Files Line by Line

Original Comment (1/2021) I was able to read a 4GB log file in about 50 seconds with the following. You may be able to make it faster by loading it as a C# assembly dynamically using PowerShell.

[System.IO.StreamReader]$sr = [System.IO.File]::Open($file, [System.IO.FileMode]::Open)
while (-not $sr.EndOfStream){
    $line = $sr.ReadLine()
}
$sr.Close() 

Addendum (3/2022) Processing the large file using C# embedded in PowerShell is even faster and has less "gotchas".

$code = @"
using System;
using System.IO;

namespace ProcessLargeFile
{
    public class Program
    {
        static void ProcessLine(string line)
        {
            return;
        }

        public static void ProcessLogFile(string path) {
            var start_time = DateTime.Now;
            StreamReader sr = new StreamReader(File.Open(path, FileMode.Open));
            try {
                while (!sr.EndOfStream){
                    string line = sr.ReadLine();
                    ProcessLine(line);
                }
            } finally {
                sr.Close();
            }
            var end_time = DateTime.Now;
            var run_time = end_time - start_time;
            string msg = "Completed in " + run_time.Minutes + ":" + run_time.Seconds + "." + run_time.Milliseconds;
            Console.WriteLine(msg);
        }

        static void Main(string[] args)
        {
            ProcessLogFile("c:\\users\\tasaif\\fake.log");
            Console.ReadLine();
        }
    }
}
"@
 
Add-Type -TypeDefinition $code -Language CSharp

PS C:\Users\tasaif> [ProcessLargeFile.Program]::ProcessLogFile("c:\\users\\tasaif\\fake.log")
Completed in 0:17.109