If you don’t have UnxUtils and you don’t have Cygwin and you don’t want to use PowerShell and you just want to use the standard Windows command prompt…

A basic starter command:

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findstr /SPIN /C:"foo" *

…where:

/S - search subdirectories
/P - skip binary files
/I - case-insensitive searching
/N - show line numbers of matches
C:"foo" - search for the literal string foo (no regex)
* - search all files

From there you can use findstr /? for more help - such as /r for regex support.

Another useful one, where /M only prints the file name:

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findstr /SPIM /C:"foo" *