PowerShell does not replace string although you can see it in cmd - string

Question

I normally find the answer to my problem by going through the site, but this time I have read every question yet still I am in despair and really need an experienced eye.
What I have is basically a structural health monitoring system. I measure strains and receive raw data. This raw data is processed by a MATLAB executable that I wrote myself and then uploaded to an ftp-server. We had a student that automated this with a PowerShell script which was working perfectly until I changed literally one small line in MATLAB and recompiled the code.
I do not understand much about PowerShell, so please be patient with me. The error I receive is you cannot call a method on a null-valued expression. This occurs when I try to replace a set of strings (just called xxx_xxx) with a date that exists as a variable in PowerShell. I can see xxx_xxx in the command window (see attached image), I can print out the date that I want to use as replacement, but somehow it does not work.
I cannot provide a working code snippet because you would need the DAQ to generate data, and as I said, I don't understand the language much. But below is the code. For easier reading, the line that I am receiving the error is the following:
$outData = $cmdOutput.Replace("xxx_xxx",$snaps[$i].Substring(6,4)+"-"+$snaps[$i].Substring(3,2)+"-"+$snaps[$i].Substring(0,2)+" "+$snaps[$i].Substring(11,8)+";")
If anyone could help me with this, I would be eternally grateful!
$retry=3
while(1){
#$dir = "C:\Users\Petar\Documents\Zoo\PetarData\INPUT DATA\New folder\"
$dir = "C:\Users\Yunus\Documents\Micron Optics\ENLIGHT\Data\" + $(get-date -f yyyy) + "\" + $(get-date -f MM) + "\"
#$outdir = "C:\Users\Petar\Documents\Zoo\PetarData\OUTPUT DATA\New folder\"
$archivedirin = "C:\Users\Yunus\Documents\Elefantenhaus\Archive\IN\"
$archivedirout = "C:\Users\Yunus\Documents\Elefantenhaus\Archive\OUT\"
$tempdir = "C:\Users\Yunus\Documents\Elefantenhaus\Archive\TEMP\"
$prefix = "EHZZ";
$filecount=(Get-ChildItem $dir).Count
$latest = Get-ChildItem -Path $dir | Sort-Object LastAccessTime -Descending | Select-Object -First 1
if($filecount -gt 1){
$exclude = $latest.name
$Files = GCI -path $dir | Where-object {$_.name -ne $exclude}
$dest = $archivedirin + "batch_"+$(get-date -f MM-dd-yyyy_HH_mm_ss)+"\"
new-item -type directory $dest
foreach ($file in $Files){move-item -path ($dir+$file) -destination $dest}
$latest = Get-ChildItem -Path $dest | Sort-Object LastAccessTime -Descending | Select-Object -First 1
$filename = $dest + $latest.name
$s=Get-Content $filename
while($s -eq $null){
if($retry -lt 0){break}
write-host "could not read file"
$retry = $retry -1
$s=Get-Content $filename
}
#read content of input file
$snaps = $s
#loop through the lines in the file until the first occurence of a timestamp, that is our desired line
for ($i = 0; $i -lt $snaps.length; $i++)
{
$ismatch =[regex]::Matches($snaps[$i], '^(\d\d.\d\d.\d\d\d\d\s\d\d+)')
if ( $ismatch -ne $null -and $ismatch[0].Groups[1].Value)
{
$temp=Get-Content $filename | select -skip $i
$filenametemp = $tempdir+"\temp.txt" #temp file path, don't change the filename "temp.txt"
#$filename3 = $tempdir+"\test.txt"
Add-Content $filenametemp $temp
$filename = $archivedirout+$prefix+"_"+$snaps[$i].Substring(8,2)+$snaps[$i].Substring(3,2)+$snaps[$i].Substring(0,2)+"_"+$snaps[$i].Substring(11,2)+$snaps[$i].Substring(14,2)+$snaps[$i].Substring(17,2)+".txt"
$cmdOutput = (cmd /c new_modified.exe $tempdir) | Out-String
write-output $cmdOutput #"$cmdOutput is:"
#IF ([string]::IsNullOrWhitespace($cmdOutput)){
# break
#}
$outData = $cmdOutput.Replace("xxx_xxx",$snaps[$i].Substring(6,4)+"-"+$snaps[$i].Substring(3,2)+"-"+$snaps[$i].Substring(0,2)+" "+$snaps[$i].Substring(11,8)+";")
Add-Content $filename $outData
remove-item -path $filenametemp
break
}
}
#break
}
else
{
write-host "waiting for file"
}
Start-Sleep -s 30
}

Answer 1

I think what is happening is that the output of the external program isn't being piped into a variable correctly. I haven't had a chance to test this but Tee-Object looks like the appropriate method for you.
I would suggest you try replacing...
$cmdOutput = (cmd /c new_modified.exe $tempdir) | Out-String
with...
cmd /c new_modified.exe $tempdir | Tee-Object -variable $cmdOutput