About Command Precedence
Describes how Windows PowerShell determines which command to run.
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.
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:
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:
-- If you do not specify a path, Windows PowerShell uses the following precedence order when it runs commands:
- 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, if you have a Get-Map function in the session and you import a cmdlet named Get-Map. By default, when you type "Get-Map", Windows PowerShell runs the Get-Map function.
-- 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, if you have a cmdlet named Get-Date and 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.
FINDING HIDDEN COMMANDS
The All parameter of the Get-Command cmdlet gets all commands with the specified name, even if they are hidden or replaced. Beginning in Windows PowerShell 3.0, by default, Get-Command gets only the commands that run when you type the command name.
In the following examples, the session includes a Get-Date function and a Get-Date cmdlet.
The following command gets the Get-Date command that runs when you type "Get-Date".
PS C:> Get-Command Get-Date
CommandType Name ModuleName
The following command uses the All parameter to get all Get-Date commands.
PS C:> Get-Command Get-Date -All
CommandType Name ModuleName
Function get-date Cmdlet Get-Date Microsoft.PowerShell.Utility
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.
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:
To run a New-Map command that was added by the MapFunctions module, use its module-qualified name:
To find the snap-in or module from which a command was imported, use the ModuleName property of commands.(Get-Command
For example, to find the source of the Get-Date cmdlet, type:
PS C:> (Get-Command Get-Date).ModuleName Microsoft.PowerShell.Utility
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.
- 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 -
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)
If a command originated in a module, you can use the following format to run it.&
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
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.