PowerShell: how to grep command output?

Tag: powershell Author: songj6688 Date: 2009-09-11

In PowerShell I have tried:

alias | select-string Alias

This fails even though Alias is clearly in the output. I know this is because select-string is operating on some object and not the actual output string.

What can be done about it?

Other Answer1

If you truly want to "grep" the formatted output (display strings) then go with Mike's approach. There are definitely times where this comes in handy. However if you want to try embracing PowerShell's object pipeline nature, then try this. First, check out the properties on the objects flowing down the pipeline:

PS> alias | Get-Member


   TypeName: System.Management.Automation.AliasInfo

Name                MemberType     Definition
----                ----------     ----------
Equals              Method         bool Equals(System.Object obj)
GetHashCode         Method         int GetHashCode()
GetType             Method         type GetType()
ToString            Method         string ToString()
<snip>
*Definition*        Property       System.String Definition {get;}
<snip>

Note the Definition property which is a header you see when you display the output of Get-Alias (alias) e.g.:

PS> alias

CommandType     Name           *Definition*
-----------     ----           ----------
Alias           %              ForEach-Object
<snip>

Usually the header title matches the property name but not always. That is where using Get-Member comes in handy. It shows you what you need to "script" against. Now if what you want to "grep" is the Definition property contents then consider this. Rather than just grepping that one property's value, you can instead filter each AliasInfo object in the pipepline by the contents of this property and you can use a regex to do it e.g.:

PS> alias | Where-Object {$_.Definition -match 'alias'}

CommandType     Name                   Definition
-----------     ----                   ----------
Alias           epal                   Export-Alias
Alias           gal                    Get-Alias
Alias           ipal                   Import-Alias
Alias           nal                    New-Alias
Alias           sal                    Set-Alias

In this example I use the Where-Object cmdlet to filter objects based on some arbitrary script. In this case, I filter by the Defintion property matched against the regex 'alias'. Only those objects that return true for that filter are allowed to propagate down the pipeline and get formatted for display on the host.

BTW if you're typing this, then you can use one of two aliases for Where-Object - 'Where' or '?'. For example:

PS> gal | ?{$_.Definition -match '-Item*'}

comments:

You can also use findstr where PowerShell will handle conversion to text for you, since it's not a cmdlet but a program (just out of completeness; in general it's always better to filter according to properties, imho :-))
FYI, as per Powershell 3, the following is more concise: gal | Where Definition -match 'alias'

Other Answer2

There are two problems. As in the question, select-string needs to operate on the output string, which can be had from "out-string". Also, select-string doesn't operate linewise on strings that are piped to it. Here is a generic solution

(alias|out-string) -split "`n" | select-string Write

comments:

In Powershell 1.0 there is no -split. You can do this instead: (alias|out-string -stream) | select-string Write
Aren't you all completely missing the point here? You don't need to grep powershell's output.
@x0n Well, there are cases in which when I'm quicly searching for something through PS console and I do not know and/or care if it's in particular property. I find it too tedious to use Get-Member and then use the correct property and expression in the where command. And grep is particularly good in this one.
@OnesimusUnbound Ok, I get you. Good point.
The use of split is unnecessary. Just use the -Stream parameter on Out-String and then you can pipe directly to Select-String.

Other Answer3

The proposed solution is just to much work for something that can be done like this:

Get-Alias -Definition Write*

Other Answer4

I think this solution is easier and better, use directly the function findstr:

alias | findstr -i Write

You can also make an alias to use grep word:

new-alias grep findstr

Other Answer5

Your problem is that alias emits a stream of AliasInfo objects, rather than a stream of strings.

This does what I think you want.

alias | out-string -stream| select-string Alias

When you don't handle things that are in the pipeline (like when you just ran 'alias'), the shell knows to use the ToString() method on each object (or use the output formats specified in the ETS info).

comments:

I think you want to use the -Stream parameter of Out-String so that each item output by alias is sent down the pipe as a string. As it stands now, the entire alias output is sent as a single string to Select-String resulting in Select-String outputting the entire string on a match.