ssh-agent-utils.ps1

# SSH Agent Functions
# Mark Embling (http://www.markembling.info/)
#
# How to use:
# - Place this file into %USERPROFILE%\Documents\WindowsPowershell (or location of choice)
# - Import into your profile.ps1:
#   e.g. ". (Resolve-Path ~/Documents/WindowsPowershell/ssh-agent-utils.ps1)" [without quotes]
# - Enjoy
#
# Note: ensure you have ssh and ssh-agent available on your path, from Git's Unix tools or Cygwin.

# Retrieve the current SSH agent PId (or zero). Can be used to determine if there
# is an agent already starting.
function Get-SshAgent() {
    $agentPid = [Environment]::GetEnvironmentVariable("SSH_AGENT_PID",  "User")
    if ([int]$agentPid -eq 0) {
        $agentPid = [Environment]::GetEnvironmentVariable("SSH_AGENT_PID", "Process")
    }
    
    if ([int]$agentPid -eq 0) {
        0
    } else {
        # Make sure the process is actually running
        $process = Get-Process -Id $agentPid -ErrorAction SilentlyContinue
		
        if(($process -eq $null) -or ($process.ProcessName -ne "ssh-agent")) {
            # It is not running (this is an error). Remove env vars and return 0 for no agent.
            [Environment]::SetEnvironmentVariable("SSH_AGENT_PID", $null, "Process")
            [Environment]::SetEnvironmentVariable("SSH_AGENT_PID", $null, "User")
            [Environment]::SetEnvironmentVariable("SSH_AUTH_SOCK", $null, "Process")
            [Environment]::SetEnvironmentVariable("SSH_AUTH_SOCK", $null, "User")
            0
        } else {
            # It is running. Return the PID.
            $agentPid
        }
    }
}

# Start the SSH agent.
function Start-SshAgent() {
    # Start the agent and gather its feedback info
    [string]$output = ssh-agent
    
    $lines = $output.Split(";")
    $agentPid = 0
    
    foreach ($line in $lines) {
        if (([string]$line).Trim() -match "(.+)=(.*)") {
            # Set environment variables for user and current process.            
            [Environment]::SetEnvironmentVariable($matches[1], $matches[2], "Process")
            [Environment]::SetEnvironmentVariable($matches[1], $matches[2], "User")
            
            if ($matches[1] -eq "SSH_AGENT_PID") {
                $agentPid = $matches[2]
            }
        }
	}
    
    # Show the agent's PID as expected.
    Write-Host "SSH agent PID:", $agentPid
}

# Stop a running SSH agent
function Stop-SshAgent() {
    [int]$agentPid = Get-SshAgent
    if ([int]$agentPid -gt 0) {
        # Stop agent process
        $proc = Get-Process -Id $agentPid
        if ($proc -ne $null) {
            Stop-Process $agentPid
        }
        
        # Remove all enviroment variables
        [Environment]::SetEnvironmentVariable("SSH_AGENT_PID", $null, "Process")
        [Environment]::SetEnvironmentVariable("SSH_AGENT_PID", $null, "User")
        [Environment]::SetEnvironmentVariable("SSH_AUTH_SOCK", $null, "Process")
        [Environment]::SetEnvironmentVariable("SSH_AUTH_SOCK", $null, "User")
    }
}

# Add a key to the SSH agent
function Add-SshKey() {
    if ($args.Count -eq 0) {
        # Add the default key (~/id_rsa)
        ssh-add
    } else {
        foreach ($value in $args) {
            ssh-add $value
        }
    }
}

# Start the agent if not already running; provide feedback
$agent = Get-SshAgent
if ($agent -eq 0) {
	Write-Host "Starting SSH agent..."
	Start-SshAgent		# Start agent
	Add-SshKey			# Add my default key
} else {
	Write-Host "SSH agent is running (PID $agent)"
}

In powershell 7.1 I was getting a

Powershell: You cannot call a method on a null-valued expression
on 45 | $lines = $output.Split(";")

So added to my solution: an If else and -eq on $null as $ouput / ssh-agent is null
[string]$output = ssh-agent

if ($output -eq $null) {

    $agentPid = 0

} else { 

This functionality is now available in posh-sshell: https://github.com/dahlbyk/posh-sshell

@mavaddat Yes it is 🙂 this is actually where that functionality first came from and you'll see I'm named in the readme there. I'd recommend using posh-sshell instead of this gist unless you have a good reason not to do so, but I'm keeping this gist up for reference and historical reasons.

@markembling I saw that! Thank you for this. I may submit some PR's myself when I take a look at the inner workings. Cheers.