Why does Read-Host require to hit enter 2 times enter before sending result?

Question

I have a simeple loop to read for text and also detect the escape ESC [0x1b = chr(27)] key, to quit.

The Expected Behaviour

<Enter loop> 

<Enter any string, such as "AT" and hit Return>
# Some Output
<Repeat above OR>, 
...
<Hit the ESC (Escape) key to exit loop>

<Exit Loop> 

Actual Behaviour

I have to:

• Hit [Enter] button 2 times, after entering a string and before getting any output.
• ESC key to exit loop does not seem to work.

---

The Code:

        do {
            $key = if ($host.UI.RawUI.KeyAvailable) { $host.UI.RawUI.ReadKey('NoEcho, IncludeKeyDown') }
            if ($port.IsOpen) {
                $at = Read-Host
                $port.Write("${at}`r")
            } else {
                Write-Host -Fo Yellow "[INFO] Port was Closed!"
                break
            }
    
        } until ($key.VirtualKeyCode -eq 27)    # Repeat until a 'ESC'
    }

---

Q: How Can I fix the above to get the intended functionality?

(Why do I need to hit enter 2 times before the input string is processed?)

---

Experimenting, this one-liner is behaving very weird...

while (1) { if($host.UI.RawUI.ReadKey('IncludeKeyDown').VirtualKeyCode -eq 81) { break };$s=''; $s=Read-Host; if ($s -ne "w") { Write-Host ": $s" -Non | Out-Host } else { "Hit W!"}  }

If you want to list the names and codes of all available keys, you can use:

1..255| ForEach-Object { '{0} = {1}' -f $_, [System.Windows.Forms.Keys]$_ }

# <output>
...
16 = ShiftKey
17 = ControlKey
18 = Menu
19 = Pause
...

Answer 1

The following solution preserves the usual Read-Host command-line editing experience, so that editing the string being typed by the user is possible:

$wshell = (New-Object -ComObject WScript.Shell) # For sending keystrokes
while ($true) {
  # Solicit an initial keypress without echoing it.
  $c = $host.UI.RawUI.ReadKey('NoEcho,IncludeKeyDown').Character
  if ($c -eq [char] 27) { # ESC pressed? Exit the loop.
    'You pressed ESC - Quitting'; break
  } 
  elseif ($c -in '', "`r") {
    # Ignore keypresses that don't translate into printable characters
    # as well as an initial Enter keypress with nothing to submit.
    continue
  }
  # Send the character as a keypress to Read-Host to prefill its edit buffer...
  $wshell.SendKeys("{$c}")
  # ... and solicit a line of input, as usual.
  $userInput = Read-Host
  # Process the input.
  Write-Host -Fo DarkGreen "[\r] Sending: " -NoNewline
  Write-Host -Fo DarkYellow $userInput
}

• The above waits for a single, initial keypress.

  • If it turns out to be ESC, the loop is exited.
  • If it turns out to be a keypress that doesn't produce a printable character or produces Enter, it is ignored, given that there's nothing to submit.

• Otherwise:

  • The initially typed character is sent as a keystroke to the keyboard buffer, where Read-Host will pick it up; that is, the edit buffer will be prefilled with the typed character, and it will appear as if the user had just typed that character into the prompt.

  • Caveat: Due to use of New-Object -ComObject WScript.Shell, this solution is Windows-only and isn't fully robust: Keeping Shift held down in order to type a sequence of uppercase letters doesn't work reliably - for certain characters, such as o, everything from the 2nd character unexpectedly remains lowercase^[1] - see the next section for a potential workaround IF you have WSL installed.

    • On Linux, you could replace the keystroke-sending + Read-Host call more elegantly with (syntax assumes PowerShell 7.3+):

       $userInput = bash -c ('read -e -i ''{0}''; printf %s "$REPLY"' -f ($c -eq "'" ? "'\''" : $c)) || $(break)
      

    • On macOS, this doesn't work, unfortunately, and there's no simple workaround.

Note:

• Pressing Ctrl-C exits too, but not just the loop, but invariably the entire script (call stack); while you cannot prevent that, you can perform cleanup operations and/or write to any of PowerShell's output streams other than the success output stream in the finally block of an enclosing try / catch / finally statement.

• Pressing Esc to exit the loop only works before entering the Read-Host prompt; that is, once you've typed a printable character, Ctrl-C is your only option to directly abort the prompt - along with your script and all its callers.

  • As of PowerShell 7.3.4, there is no mechanism that would allow you to respond to keypresses in a Read-Host prompt in realtime, and pressing ESC invariably just clears what has been typed so far, without leaving the prompt.

---

Workaround for the .SendKeys() problem on Windows, assuming you have WSL installed:

# Switch the console encoding (temporarily) to UTF-8, so that
# PowerShell interprets WSL output correctly.
$prevEnc = [Console]::OutputEncoding; [Console]::OutputEncoding = [Text.Utf8Encoding]::new()

while ($true) {
  # Solicit an initial keypress without echoing it.
  $c = ($host.UI.RawUI.ReadKey('NoEcho,IncludeKeyDown')).Character
  if ($c -eq [char] 27) { # ESC pressed? Exit the loop.
    'You pressed ESC - Quitting'; break
  } 
  elseif ($c -in '', "`r") {
    # Ignore keypresses that don't translate into printable characters
    # as well as an initial Enter keypress with nothing to submit.
    continue
  }
  # Use Bash, via WSL, to prompt for the input, taking advantage of its ability
  # to pre-fill the edit buffer
  $userInput = wsl -e bash -c ('read -e -i ''{0}''; printf %s "$REPLY"' -f $(if ($c -eq "'") { "'\''" } else { $c }))
  if ($LASTEXITCODE) { 
    # Implies that Ctrl-C was pressed.
    # Exit the loop. Note that this is NOT the same as pressing Ctrl-C
    # with Read-Host, which terminates the entire script and its call stack.
    break 
  } 
  # Process the input.
  Write-Host -Fo DarkGreen "[\r] Sending: " -NoNewline
  Write-Host -Fo DarkYellow $userInput
}

# Restore the previous console endoding, if needed.
[Console]::OutputEncoding = $prevEnc

---

^{[1] Seemingly, the simulation of keyboard input, which involves programmatically pressing and releasing the Shift key, can interfere with properly detecting the physical keyboard state.}

Answer 2

After having wasted way too much time trying to find a good solution, I got to an acceptable one, based on the hint from mklement0's post. Next time I'm just gonna ask one of the AI's.

while(1) {
    while (($c=($host.UI.RawUI.ReadKey('NoEcho,IncludeKeyDown')).Character) -eq 0) {};
    switch -CaseSensitive ($c) {
        "`r"    { Write-Host -Fo DarkGreen "[\r] Sending: " -Non | Out-Host; Write-Host -Fo DarkYellow "$a"; $a=''}
        "`e"    { 'You pressed ESC - Quitting'; Break; }
        default { Write-Host -Fo DarkYellow "$c" -Non; $a += $c; }
    }
    if ($c -ceq "`e") { Break; } # Can also us a different character here, that can be used as a secret escape code.
}

Also, remember to use the case insensitive -ceq, and not -eq.
...and I'm also grateful for the simpleness of pwsh 7+ that also supports ESC [`e].

---

UPDATE: Although this loop is better (easier), I still have to hit enter 2 times in my use case, as implemented in another SO question.

Other Useful References:

• https://powershell.one/tricks/input-devices/detect-key-press
• https://ss64.com/vb/sendkeys.html