How does Select-String work with Pipelines of objects?

PowerShell Team

<Edited to convert Match-String to Select-String 7/9/2006>

In our newsgroup (Microsoft.Public.Windows.Server.Scripting) , Vasu asked about how Select-string works in pipelines:

Here is what I observe:
1. MSH C:\> get-alias
Alias           ri                                   remove-item
Alias           rni                                  rename-item

2. MSH C:\> get-alias | select-string ri
MSH C:\>

The question is why did the string “ri” get emitted instead of the alias record.

So what is going on here?

Select-String works on Streams of STRINGS.  When you pass it a string of OBJECTS, the MONAD engine converts those objects to strings before passing them to Select-String.  Thus the following 2 commands are functionaly equivalent:

get-alias | select-string “ri”
get-alias | foreach {“$_”} |Select-string “ri”

Converting an object to a string is accomplished by call that object’s ToString() method (vs using MONAD’s formating subsystem).  Notice the difference – the first uses Monad’s formating and the second is the object’s ToString() 

MSH> (gal ri)

CommandType     Name                          Definition
———–     —-                          ———-
Alias           ri                            remove-item

MSH> (gal ri).ToString()

If you wanted Select-String to work on the Monad formatted output, you’ll need to get that as a string.  Here is the thing to grok about our outputing.  When your command sequence emits a stream of strings, we emit it without processing.  If  instead, your command sequence emits a stream of objects, then we redirect those objects to the command Out-Default.  Out-Default looks at the type of the object and the registered formating metadata to see if there is a default view for that object type.  A view defines a FORMATTER and the metadata for that command.  Most objects get vectored to either Format-Table or Format-List (though they could go to Format-Wide or Format-Custom).  THESE FORMATTERS DO NOT EMIT STRINGS!  You can see this for yourself by the following:

MSH> gps |ft |group {$_.GetType().name} |ft Count,Name -auto

Count Name
—– —-
    1 FormatStartData
    1 GroupStartData
   53 FormatEntryData
    1 GroupEndData
    1 FormatEndData

These formating records are then vectored to an OUT-xxx command to be rendered into the appropriate data for a particular output device.  By default, they go to Out-Host but you can pipe this to Out-File, Out-Printer or Out-String.  (NOTE: these OUT-xxx commands are pretty clever, if you pipe formating objects to them, they’ll render them.  If you pipe raw object to them, they’ll first call the appropriate formatter and then render them.)

So to make Select-string work against the string version of Monad’s output, you’d do this:

MSH> get-alias | out-string -stream | Select-string “ri”
Alias           clv                       clear-variable
Alias           gdr                       get-drive
Alias           gv                        get-variable
Alias           ndr                       new-drive
Alias           nv                        new-variable
Alias           rdr                       remove-drive
Alias           ri                        remove-item
Alias           rv                        remove-variable

Notice a couple things 1) you had to say “Out-String -STREAM” (if you didn’t say -STREAM you would have gotten a single string) 2) now Select-string matches any line that has an “ri” in it not just the ones where the NAME has an “ri” in it.

Think about the last statement ‘just the ones where the NAME has an “ri” in it’.  One of the things we designed Monad to do was to give you an experience where you think it, you type it, you get it.  So let’s say it, “I want to GET the ALIASes WHERE the NAME matches the string RI”.  Now lets type it:

MSH> GET-ALIAS |WHERE {$_.NAME -match “ri”}

CommandType     Name                       Definition
———–     —-                       ———-
Alias           ri                         remove-item
Alias           write                      write-object

It take a while to stop thinking in terms of streams of text and start thinking in terms of streams of objects.  The benefit is that once you do this, you begin to ask questions in a way that makes it really easy to answer.


Jeffrey Snover
Monad Architect

[Edit: Monad has now been renamed to Windows PowerShell. This script or discussion may require slight adjustments before it applies directly to newer builds.]

PSMDTAG:FAQ: How does Select-String work with objects?

PSMDTAG:FAQ: How does outputing (i.e. out-default) work?

PSMDTAG:PHILOSOPHY: Stop thinking in terms of streams of text and start thinking of streams of objects

PSMDTAG:CMDLET: Get-Alias, Select-String

PSMDTAG:CMDLET:FORMAT: out-default, out-string -stream

PSMDTAG:INTERNAL: Formatters do NOT emit strings, they emit format records


1 comment

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  • Deqi Dai 0

    Hi Jeffrey,

    I just finished watching your 2013 Powershell class with Jason Helmick and bought “Learn Windows Powershell in a Month of Lunches” based on the recommendation from the class. Both your class and the book are great! The class certainly is standing the test of time!

    I come from a *nix background and was saying gal | sls ‘string’ and couldn’t understand what was going on. Googled my problen and found your most excellent explanation! Thank you!

    I still don’t understand why gal | sls ‘sls’ produces sls as output but gal | sls ‘string’ produces nothing. I’ll reread your post above and see what I can figure out. Again, thanks for a great class and for the book recommendation.

    Very Respectfully!

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