Export (0) Print
Expand All

about_Command_Precedence

Updated: January 26, 2011

Applies To: Windows PowerShell 2.0

TOPIC
    about_Command_Precedence

SHORT DESCRIPTION
    Describes how Windows PowerShell determines which command to run.

LONG DESCRIPTION
    This topic explains how Windows PowerShell determines which command to 
    run, especially when a session contains more than one command with the
    same name. It also explains how to run commands that do not run by 
    default, and it explains how to avoid command-name conflicts in your
    session.


  COMMAND PRECEDENCE
      When a session includes commands that have the same name, Windows 
      PowerShell uses the following rules to decide which command to run.

      These rules become very important when you add commands to your session
      from modules, snap-ins, and other sessions.

    
      -- If you specify the path to a command, Windows PowerShell runs the 
         command at the location specified by the path. 

         For example, the following command runs the FindDocs.ps1
         script in the C:\TechDocs directory:

             C:\TechDocs\FindDocs.ps1

         As a security feature, Windows PowerShell does not run executable
         (native) commands, including Windows PowerShell scripts, unless the 
         command is located in a path that is listed in the Path environment 
         variable ($env:path) or unless you specify the path to the script 
         file. 

         To run a script that is in the current directory, specify the full 
         path, or type a dot (.) to represent the current directory.

         For example, to run the FindDocs.ps1 file in the current directory,
         type:

             .\FindDocs.ps1

        
      -- If you do not specify a path, Windows PowerShell uses the following
         precedence order when it runs commands:

              1. Alias
              2. Function
              3. Cmdlet 
              4. Native Windows commands

          Therefore, if you type "help", Windows PowerShell first looks for an
          alias named "help", then a function named "Help", and finally a 
          cmdlet named "Help". It runs the first "help" item that it finds.

          For example, assume you have a function named Get-Map. Then, you add
          or import a cmdlet named Get-Map. By default, Windows PowerShell
          runs the function when you type "Get-Map".        


      -- When the session contains items of the same type that have the same
         name, such as two cmdlets with the same name, Windows PowerShell 
         runs the item that was added to the session most recently.

         For example, assume you have a cmdlet named Get-Date. Then, you import
         another cmdlet named Get-Date. By default, Windows PowerShell runs 
         the most-recently imported cmdlet when you type "Get-Date".


  HIDDEN and REPLACED ITEMS
      As a result of these rules, items can be replaced or hidden by items with
      the same name.  

      --  Items are "hidden" or "shadowed" if you can still access the original
          item, such as by qualifying the item name with a module or snap-in
          name. 

          For example, if you import a function that has the same name as a
          cmdlet in the session, the cmdlet is hidden (but not replaced) 
          because it was imported from a snap-in or module.
            

      --  Items are "replaced" or "overwritten" if you can no longer access
          the original item.

          For example, if you import a variable that has the same name as a 
          a variable in the session, the original variable is replaced and is
          no longer accessible. You cannot qualify a variable with a module 
          name.
           
          Also, if you type a function at the command line and then import 
          a function with the same name, the original function is replaced and
          is no longer accessible.


  RUNNING HIDDEN COMMANDS
      You can run particular commands by specifying item properties that 
      distinguish the command from other commands that might have the same 
      name. 

      You can use this method to run any command, but it is especially useful
      for running hidden commands. 

      Use this method as a best practice when writing scripts that you intend 
      to distribute because you cannot predict which commands might be present
      in the session in which the script runs.


      QUALIFIED NAMES
      You can run commands that have been imported from a Windows PowerShell
      snap-in or module or from another session by qualifying the command 
      name with the name of the module or snap-in in which it originated.  

      You can qualify commands, but you cannot qualify variables or aliases.      

      For example, if the Get-Date cmdlet from the Microsoft.PowerShell.Utility
      snap-in is hidden by an alias, function, or cmdlet with the same name, you
      can run it by using the snap-in-qualified name of the cmdlet:

          Microsoft.PowerShell.Utility\Get-Date


      To run a New-Map command that was added by the MapFunctions module, use 
      its module-qualified name:

          MapFunctions\New-Map


      To find the snap-in or module from which a command was imported, use the
      following Get-Command command format:

          get-command <command-name> | format-list -property Name, PSSnapin, Module


      For example, to find the source of the Get-Date cmdlet, type:


          get-command get-date | format-list -property Name, PSSnapin, Module

          Name     : Get-Date
          PSSnapIn : Microsoft.PowerShell.Utility
          Module   :
       

      CALL OPERATOR
      You can also use the Call operator (&) to run any command that you
      can get by using a Get-ChildItem (the alias is "dir"), Get-Command, or 
      Get-Module command. 

      To run a command, enclose the Get-Command command in parentheses,
      and use the Call operator (&) to run the command.

          &(get-command ...)  

      - or -

          &(dir ... )


      For example, if you have a function named Map that is hidden by an
      alias named Map, use the following command to run the function.

          &(get-command -name map -type function)  

      - or -

          &(dir function:\map)


      You can also save your hidden command in a variable to make it
      easier to run.

      For example, the following command saves the Map function in the
      $myMap variable and then uses the Call operator to run it.

          $myMap = (get-command -name map -type function)

          &($myMap)


      If a command originated in a module, you can use the following
      format to run it.

         & <PSModuleInfo-object> <command>

      For example, to run the Add-File cmdlet in the FileCommands
      module, use the following command sequence.
           
         $FileCommands = get-module -name FileCommands
           
         & $FileCommands Add-File
        
   

  REPLACED ITEMS
      Items that have not been imported from a module or snap-in, such as 
      functions, variables, and aliases that you create in your session or
      that you add by using a profile can be replaced by commands that have
      the same name. If they are replaced, you cannot access them.

      Variables and aliases are always replaced even if they have been 
      imported from a module or snap-in because you cannot use a call operator
      or a qualified name to run them.

      For example, if you type a Get-Map function in your session, and you 
      import a function called Get-Map, the original function is replaced. 
      You cannot retrieve it in the current session.

   
  AVOIDING NAME CONFLICTS
      The best way to manage command name conflicts is to prevent them. When 
      you name your commands, use a name that is very specific or is likely to
      be unique. For example, add your initials or company name acronym to the 
      nouns in your commands.

      Also, when you import commands into your session from a Windows PowerShell
      module or from another session, use the Prefix parameter of the 
      Import-Module or Import-PSSession cmdlet to add a prefix to the nouns 
      in the names of commands.

      For example, the following command avoids any conflict with the Get-Date 
      and Set-Date cmdlets that come with Windows PowerShell when you import
      the DateFunctions module.

          import-module -name DateFunctions -prefix ZZ

      For more information, see Import-Module and Import-PSSession.  


SEE ALSO
    about_Path_Syntax
    about_Aliases
    about_Functions
    Alias (provider)
    Function (provider)
    Get-Command
    Import-Module
    Import-PSSession
Was this page helpful?
(1500 characters remaining)
Thank you for your feedback

Community Additions

Show:
© 2014 Microsoft