Escape Powershell Variables Starting with - (dash) in a Cmdlet or program call?

user_768028 0
Viewed 1348

So, I've just been pulling my hair out and yelling for the past 30 minutes, because I've learned that (apparently?) PowerShell will silently strip out the first - in any variable, regardless of if its escaped, if that variable starts with a - and is being used in a path. Here's my script, which is a simple wrapper around FFMPEG to merge two audio tracks into one and clip a video:

function IsNull($objectToCheck) {
    if ($objectToCheck -eq $null) {
        return $true
    }

    if ($objectToCheck -is [String] -and $objectToCheck -eq [String]::Empty) {
        return $true
    }

    if ($objectToCheck -is [DBNull] -or $objectToCheck -is [System.Management.Automation.Language.NullString]) {
        return $true
    }

    return $false
}

$_input = [Management.Automation.WildcardPattern]::Escape($args[0])
$time = $args[1]
$duration = $args[2]

if ($time -match ":") {
    $_matches = ($time | Select-String ":" -AllMatches).Matches
    $seconds = $time.Substring($_matches[$_matches.Count - 1].Index + 1, 2) -as [int]
    $finalSeconds = $seconds - 1
    $timeFlag2 = "-ss 1"
    $time = $time.Substring(0, $_matches[$_matches.Count - 1].Index) + (":{0:d2}" -f $finalSeconds)
    # there's totally a bug in here where we don't handle :00 seconds, plz ignore
}
else {
    $timeFlag2 = ""
}

if (-not (IsNull($duration))) {
    $d = "-t " + $duration
}
else {
    $d = ""
}

Write-Verbose "C:\ffmpeg.exe -ss $time -i $_input $timeFlag2 -filter_complex '[0:a:0]volume=0.6[l];[0:a:1]volume=1.2[k];[k][l]amerge=inputs=2[a]' -map '0:v:0' -map '[a]' -c:v copy -c:a libmp3lame -q:a 3 -ac 2 $d 'output-muxxed.mp4'" -Verbose

C:\ffmpeg.exe -ss $time -i $_input $timeFlag2 -filter_complex '[0:a:0]volume=0.6[l];[0:a:1]volume=1.2[k];[k][l]amerge=inputs=2[a]' -map '0:v:0' -map '[a]' -c:v copy -c:a libmp3lame -q:a 3 -ac 2 $d 'output-muxxed.mp4'

All I want to do is: take in a file, take in a timestamp to start clipping from, and then a duration. I do a small bit of math so I can pass -ss before and after the -i call to fix a weird green frame bug when working with ShadowPlay.

This one is currently broken and will give the error: Unrecognized option ss 1. K, so I figured the variable wasn't getting escaped properly. So I updated the last command to add quotes around it, and still no dice. So then I decided to do: $timeFlag2 = "`-ss 1", still absolutely no luck. During all of this, the command Write-Verbose is producing is:

VERBOSE: C:\ffmpeg.exe -ss 00:02:01 -i .\input.mp4 -ss 1 -filter_complex '[0:a:0]volume=0.6[l];[0:a:1]volume=1.2[k];[k][l]amerge=inputs=2[a]' -map '0:v:0' -map '[a]' -c:v copy -c:a libmp3lame -q:a 3 -ac 2  -t 40 'output-muxxed.mp4'

Which is 100% correct and works when copy-pasted to the terminal. After doing a shit ton of searching, I decided I'd try one last thing and I updated the $timeFlag2 and $d assignment calls to be:

$timeFlag2 = " -ss 1"
$d = " -t " + $duration

And the world worked... All errors were gone. Removing the spaces reintroduces them.

So, my question is what in the world is going on here? Why doesn't `, the escape operator, escape -'s? Why does the space escape it? Why does this appear to be properly escaped when passed to a string for Write-Verbose, but not when passed to a Path cmdlet or whatever the hell that's called?

I'm a loooonnnnnggg-time bash user, but have been using PS for like 6 weeks, so I'm sure there's tons wrong here and that I'm missing, so all advice/help/critiques are highly appreciated!

Thanks in advance!

powershell escaping windows
1 Answers

Alright, got this figured out, especially from an ex-BASHer's PoV. So, BASH is REAL loose about argument escaping and how spaces work, a common idiom in bash is to construct commands as: mv "$src" "$dest", just in case $src or $dest have spaces in them. zsh and modern ilk handle this, but BASH doesn't. This is how I was used to writing shell scripts.

This is not how Windows PowerShell works (obviously). PS does a really good job of escaping its variables and command parameters entirely behind the scenes. What was happening to me here was that when I set the var: $timeFlag2 = "-ss 1" and passed that, PS was literally passing: "-ss 1" to the underlying command, regardless of how I escaped it or what I did. This isn't immediately obvious with the errors you get back, unless you sort of understand what errors command escaping can result in (Invalid flag with name: "$THIS_IS_CLEARLY_A_VALID_FLAG"), and what not.

The right way to do this is to split the flags up into two vars: $timePiece1 = "-ss" and $timePiece2 = "1" and then call the command using both. Even better is splatting, which is similar to BASH's space expansion, but MUCH smarter. Here's what the main fix ended up being:

$FFMPEG_PARAMS = @(
  "-ss",
  $time,
  "-i",
  $_input,
  $time2Flag,
  $time2Val,
  "-filter_complex",
  "[0:a:0]volume=0.6[l];[0:a:1]volume=1.2[k];[k][l]amerge=inputs=2[a]",
  "-map",
  "0:v:0",
  "-map",
  "[a]",
  "-c:v",
  "copy",
  "-c:a",
  "libmp3lame",
  "-q:a",
  "3",
  "-ac",
  "2",
  $d,
  $duration,
  "output-muxxed.mp4"
)

C:\ffmpeg.exe @FFMPEG_PARAMS

Breaking the params up and using splatting solved it! Here's the entire full script, for reference:

function IsNull($objectToCheck) {
    if ($objectToCheck -eq $null) {
        return $true
    }

    if ($objectToCheck -is [String] -and $objectToCheck -eq [String]::Empty) {
        return $true
    }

    if ($objectToCheck -is [DBNull] -or $objectToCheck -is [System.Management.Automation.Language.NullString]) {
        return $true
    }

    return $false
}

$_input = [Management.Automation.WildcardPattern]::Escape($args[0])
$time = $args[1]
$duration = $args[2]
if ($time -match ":") {
    $_matches = ($time | Select-String ":" -AllMatches).Matches
    $seconds = $time.Substring($_matches[$_matches.Count - 1].Index + 1, 2) -as [int]
    $finalSeconds = $seconds - 1
    $time2Flag = "-ss"
    $time2Val = "1"
    $time = $time.Substring(0, $_matches[$_matches.Count - 1].Index) + (":{0:d2}" -f $finalSeconds)
}
else {
    $time2Flag = ""
    $time2Val = ""
}

if (-not (IsNull($duration))) {
    $d = "-t"
}
else {
    $d = ""
}

$FFMPEG_PARAMS = @(
  "-ss",
  $time,
  "-i",
  $_input,
  $time2Flag,
  $time2Val,
  "-filter_complex",
  "[0:a:0]volume=0.6[l];[0:a:1]volume=1.2[k];[k][l]amerge=inputs=2[a]",
  "-map",
  "0:v:0",
  "-map",
  "[a]",
  "-c:v",
  "copy",
  "-c:a",
  "libmp3lame",
  "-q:a",
  "3",
  "-ac",
  "2",
  $d,
  $duration,
  "output-muxxed.mp4"
)
Write-Verbose "C:\ffmpeg.exe -ss $time -i $_input $time2Flag $time2Val -filter_complex '[0:a:0]volume=0.6[l];[0:a:1]volume=1.2[k];[k][l]amerge=inputs=2[a]' -map '0:v:0' -map '[a]' -c:v copy -c:a libmp3lame -q:a 3 -ac 2 $d $duration 'output-muxxed.mp4'" -Verbose

C:\ffmpeg.exe @FFMPEG_PARAMS

I've learned a valuable PS lesson today.

user_7680280
Related