Powershell to manipulate host file

If anyone is looking for a more advanced example, I've always been particularly fond of this gist: https://gist.github.com/markembling/173887

#
# Powershell script for adding/removing/showing entries to the hosts file.
#
# Known limitations:
# - does not handle entries with comments afterwards ("<ip>    <host>    # comment")
#

$file = "C:\Windows\System32\drivers\etc\hosts"

function add-host([string]$filename, [string]$ip, [string]$hostname) {
    remove-host $filename $hostname
    $ip + "`t`t" + $hostname | Out-File -encoding ASCII -append $filename
}

function remove-host([string]$filename, [string]$hostname) {
    $c = Get-Content $filename
    $newLines = @()

    foreach ($line in $c) {
        $bits = [regex]::Split($line, "\t+")
        if ($bits.count -eq 2) {
            if ($bits[1] -ne $hostname) {
                $newLines += $line
            }
        } else {
            $newLines += $line
        }
    }

    # Write file
    Clear-Content $filename
    foreach ($line in $newLines) {
        $line | Out-File -encoding ASCII -append $filename
    }
}

function print-hosts([string]$filename) {
    $c = Get-Content $filename

    foreach ($line in $c) {
        $bits = [regex]::Split($line, "\t+")
        if ($bits.count -eq 2) {
            Write-Host $bits[0] `t`t $bits[1]
        }
    }
}

try {
    if ($args[0] -eq "add") {

        if ($args.count -lt 3) {
            throw "Not enough arguments for add."
        } else {
            add-host $file $args[1] $args[2]
        }

    } elseif ($args[0] -eq "remove") {

        if ($args.count -lt 2) {
            throw "Not enough arguments for remove."
        } else {
            remove-host $file $args[1]
        }

    } elseif ($args[0] -eq "show") {
        print-hosts $file
    } else {
        throw "Invalid operation '" + $args[0] + "' - must be one of 'add', 'remove', 'show'."
    }
} catch  {
    Write-Host $error[0]
    Write-Host "`nUsage: hosts add <ip> <hostname>`n       hosts remove <hostname>`n       hosts show"
}

The Carbon module has a Set-HostsEntry function for setting a hosts entry:

Set-HostsEntry -IPAddress 10.2.3.4 -HostName 'myserver' -Description "myserver's IP address"

All of these answers are pretty elaborate. This is all you need to add a hosts file entry:

Add-Content -Path $env:windir\System32\drivers\etc\hosts -Value "`n127.0.0.1`tlocalhost" -Force

IP address and hostname are separated by `t which is the PowerShell notation for a tab character.

`n is the PowerShell notation for a newline.

First up, if you're on Vista or Windows 7 make sure you run these commands from an elevated prompt:

# Uncomment lines with localhost on them:
$hostsPath = "$env:windir\System32\drivers\etc\hosts"
$hosts = get-content $hostsPath
$hosts = $hosts | Foreach {if ($_ -match '^\s*#\s*(.*?\d{1,3}.*?localhost.*)')
                           {$matches[1]} else {$_}}
$hosts | Out-File $hostsPath -enc ascii

# Comment lines with localhost on them:
$hosts = get-content $hostsPath
$hosts | Foreach {if ($_ -match '^\s*([^#].*?\d{1,3}.*?localhost.*)') 
                  {"# " + $matches[1]} else {$_}} |
         Out-File $hostsPath -enc ascii

Given this I think you can see how to use a regex to manipulate entries as necessary.