How to prepend to a file in PowerShell?

Question

I'm generating two files, userscript.meta.js and userscript.user.js. I need the output of userscript.meta.js to be placed at the very beginning of userscript.user.js.

Add-Content doesn't seem to accept a parameter to prepend and Get-Content | Set-Content will fail because userscript.user.js is being used by Get-Content.

I'd rather not create an intermediate file if it's physically possible to have a clean solution.

How to achieve this?

Answer 1

The Subexpression operator $( ) can evaluate both Get-Content statements which are then enumerated and passed through the pipeline to Set-Content:

$(
    Get-Content userscript.meta.js -Raw
    Get-Content userscript.user.js -Raw
) | Set-Content userscript.user.js

Consider using the Absolute Path of the files if your current directory is not where those files are.

---

An even more simplified approach than the above would be to put the paths in the desired order since both, the -Path and -LiteralPath parameters can take multiple values:

(Get-Content userscript.meta.js, userscript.user.js -Raw) |
    Set-Content userscript.user.js

And in case you want to get rid of excess leading or trailing white-space, you can include the String.Trim Method:

(Get-Content userscript.meta.js, userscript.user.js -Raw).Trim() |
    Set-Content userscript.user.js

Note that in above examples the grouping operator ( ) is mandatory as we need to consume all output from Get-Content before being passed through the pipeline to Set-Content. See Piping grouped expressions for more details.

Answer 2

For future folks, here's a snippet if you need to prepend the same thing to multiple files:

example: prepending an #include directive to a bunch of auto-generated C++ files so it works with my Windows environment.

Get-ChildItem -Path . -Filter *.cpp | ForEach-Object { 
    $file = $_.FullName

    # the -Raw param was important for me as it didn't read the entire
    # file properly without it. I even tried [System.IO.File]::ReadAllText
    # and got the same thing, so there must have been some characater that
    # caused the file read to return prematurely
    $content = Get-Content $file -Raw

    $prepend = '#include "stdafx.h"' + "`r`n"
    #this could also be from a file: aka
    # $prepend = Get-Content 'path_to_my_file_used_for_prepending'
    
    $content =  $prepend + $content
    Set-Content $file $content
}