script to find given string and replace in all files in given directory

Maybe something like this

    $files = Get-ChildItem "DirectoryContainingFiles"
    foreach ($file in $files) 
    {
    $content = Get-Content -path $file.fullname
    $content | foreach {$_ -replace "toreplace", "replacewith"} | 
Set-Content $file.fullname
    }

If the string to replace spans multiple lines then using Get-Content isn't going to cut it unless you stitch together the output of Get-Content into a single string. It's easier to use [io.file]::ReadAllText() in this case e.g.:

Get-ChildItem | Where {!$_.PSIsContainer} | 
    Foreach { $txt = [IO.File]::ReadAllText($_.fullname); 
              $txt -replace $old,$new; $txt | Out-File $_}

Note with with $old, you may need to use a regex directive like '(?s)' at the beginning to indicate that . matches newline characters also.

I believe that you can get the list of all files in a directory (simple?). Now comes the replacement part. Here is how you can do it with power shell:

type somefile.txt | %{$_ -replace "string_to_be_replaces","new_strings"}

Modify it as per your need. You can also redirect the output to a new file the same way you do other redirection (using: >).

To get the list of files, use:

Get-ChildItem <DIR_PATH> -name