Practical Use: Find & Replace in a Text File

A number of applications rely on simple text-based configuration files versus some proprietary format.  This makes editing files – ones that can’t simply be replaced via GPO/GPP or Login Script – really easy to update.

Back in my VBScript days I would likely

  1. Ingest the file via ReadAll()
  2. Check if it contains the value via InStr
  3. Replace(old_value,new_value)
  4. Write out the file with the updated content

I figured Get-Content was going to behave similarly but I discovered each line is its own separate object which meant iterating through the array for the content.  No big deal but something new and good to know.

The method for locating your text is important depending on what you’re searching for.

Locating X File

If it’s simple text, you can get away with either operator: -match or -like.

  • Match will simply return true or false and is geared towards regular expression based searches.
  • Like will return the actual objects that match.

So if you just want to know whether or not the file contains X, you could go either way.

[string]$File = 'C:\windows\Temp\ASPNETSetup_00000.log'
$OriginalContent = Get-Content -Path $File
[string]$Find = 'Vista'

# Search via Match
($OriginalContent | % { $_ -match $Find }) -contains $true

# Search via Like (similar concept)
($OriginalContent | % { $_ -like "*$Find*" }) - contains $true

But if you want to locate X and do something with it, -like is your friend.

#Like
[string]$File = 'C:\windows\Temp\ASPNETSetup_00000.log'
$OriginalContent = Get-Content -Path $File
[string]$Find = 'Vista'
$Results = $OriginalContent | % { $_ -like "*$Find*" }

 

I don’t want to get too deep into this, because it’s well documented elsewhere, but I just want to mention that if you’re searching for something that contains special characters some additional care is necessary.

If you’re using -like, you should be fine:

# Like
[string]$File = "C:\WINDOWS\temp\ASPNETSetup_00000.log"
$OrigContent = Get-Content -Path $File
[string]$Find = '\/\/I/\/D0WZ'

# Return true/false
($OrigContent | % { $_ -like $Find }) -contains $true

# Get the lines that match
$Results = $OriginalContent | % { $_ -like "*$Find*" }

 

But if you’re using -match, you’ll need to either escape those characters manually:

# Match
[string]$File = "C:\WINDOWS\temp\ASPNETSetup_00000.log"
$OriginalContent = Get-Content -Path $File
[string]$Find = '\\\/\\\/I\/\\\/D0WZ'
($OriginalContent | % { $_ -match $Find }) -contains $true

Or rely on the Escape() method of the Regex class:

# Match
[string]$File = "C:\WINDOWS\temp\ASPNETSetup_00000.log"
$OriginalContent = Get-Content -Path $File
[string]$Find = '\/\/I/\/D0WZ'

($OriginalContent | % { $_ -match [regex]::Escape($Find) }) -contains $true

Replacing the Content

Now that you’ve confirmed file X contains Y, its time to replace it.  Since humans are prone to making mistakes, I always like to have a way of backing out of programmatic changes like, so the steps below include a backup process.

# Replace X with Y and store it in a new variable
$NewContent = $OriginalContent | % { $_ -replace $Find,$Replace }

# Create a new file that will ultimately replace the existing file.
#     If you want a UTF-8 file with BOM use this
#$NewContent | Out-File -FilePath "$File.NEW" -Encoding utf8 -Force

#     If you just want a UTF-8 without BOM, this does the trick.
$NewContent | Out-File -FilePath "$File.NEW" -Encoding default -Force

# Backup the existing file
Copy-Item -Path $File -Destination "$File.ORIG.$(Get-date -Format 'yyyymmdd_hhmmss')" -Force

# Move the new file that we staged to overwrite the orignal
Move-Item -Path "$File.NEW" -Destination $File -Force

To the experts, this is really simple and basic stuff.  To those less seasoned, this is practical. 🙂

Good Providence!

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