Some helpful parameters in Select-String are -Pattern and -Path.
Both -Pattern and -Path accept a list of string. Each item is separated by comma, for example, *.txt, *.log
-Pattern is the pattern of text you want to search
-Path is the list of files you want to search.
However, Select-String doesn't search files under a directory. You have to pass file paths to it via -Path. We can use Get-ChildItem to get a list of files, and get all files under a directory recursively by using -Recurse
The basic pattern to use both cmdlet for greping is as follow:
Get-ChildItem -Recurse -Path C:\Path\To\Folder -Include *.txt | Select-String "SearchText"
I create a script in my GitHub (https://github.com/kceiw/PowerScript/blob/master/scripts/GrepShell.ps1) so that I can reuse it.
This script is not signed though. If you want to use it, you need to change your execution policy to allow it to run. See Set-Execution.