about_Command_Precedence
Short description
Describes how PowerShell determines which command to run.
Long description
Command precedence describes how PowerShell determines which command to run when a session contains more than one command with the same name. Commands within a session can be hidden or replaced by commands with the same name. This article shows you how to run hidden commands and how to avoid command-name conflicts.
Command precedence
When a PowerShell session includes more than one command that has the same name, PowerShell determines which command to run using the following rules.
If you specify the path to a command, 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
You can run any executable command using its full path. As a security feature,
PowerShell doesn't run executable commands, including PowerShell scripts and
native commands, unless the command is located in a path listed in the
$env:Path environment variable.
To run an executable file that's in the current directory, specify the full
path or use the relative path .\ to represent the current directory.
For example, to run the FindDocs.ps1 file in the current directory, type:
.\FindDocs.ps1
If you don't specify a path, PowerShell uses the following precedence order when it runs commands.
- Alias
- Function
- Cmdlet (see Cmdlet name resolution)
- External executable files (including PowerShell script files)
Therefore, if you type help, 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 your session contains a cmdlet and a function, both named
Get-Map, when you type Get-Map, PowerShell runs the function.
Note
This only applies to loaded commands. If there is a build executable and an
Alias build for a function with the name of Invoke-Build inside a module
that is not loaded into the current session, PowerShell runs the build
executable instead. It doesn't auto-load modules if it finds the external
executable. It's only when no external executable is found that an alias,
function, or cmdlet with the given name is invoked.
Resolving items with the same names
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 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. You can run the cmdlet by specifying its module-qualified name.
When items are replaced or overwritten, you can no longer access the original item.
For example, if you import a variable that has the same name as a variable in the session, the original variable is replaced. You can't qualify a variable with a module name.
If you create a function at the command line and then import a function with the same name, the original function is replaced.
Finding hidden commands
The All parameter of the Get-Command cmdlet gets all commands with
the specified name, even if they're hidden or replaced. Beginning in 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. You can use Get-Command to determine which command is
chosen first.
Get-Command Get-Date
CommandType Name ModuleName
----------- ---- ----------
Function Get-Date
Uses the All parameter to list available Get-Date commands.
Get-Command Get-Date -All
CommandType Name Version Source
----------- ---- ------- ------
Function Get-Date
Cmdlet Get-Date 3.1.0.0 Microsoft.PowerShell.Utility
Get-Command where -All
CommandType Name Version Source
----------- ---- ------- ------
Alias where -> Where-Object
Application where.exe 10.0.22621.1 C:\Windows\system32\where.exe
You can run particular commands by including qualifying information that
distinguishes the command from other commands that might have the same name.
For cmdlets, you can use the module-qualified name. For executables, you can
include the file extension. For example, to run the executable version of
where use where.exe.
Use module-qualified names
Using the module-qualified name of a cmdlet allows you to run commands hidden
by an item with the same name. For example, you can run the Get-Date cmdlet
by qualifying it with its module name Microsoft.PowerShell.Utility or its
path. When you use module-qualified names, the module can be automatically
imported into the session depending on the value of
$PSModuleAutoLoadingPreference.
Note
You can't use module names to qualify variables or aliases.
Using module-qualified names ensures that you are running the command that you intend to run. This is the recommended method of calling cmdlets when writing scripts that you intend to distribute.
The following example illustrates how to qualify a command by including its module name.
Important
Module qualification uses the backslash character (\) to separate the
module name from the command name, regardless of the platform.
New-Alias -Name "Get-Date" -Value "Get-ChildItem"
Microsoft.PowerShell.Utility\Get-Date
Tuesday, May 16, 2023 1:32:51 PM
To run a New-Map command from the MapFunctions module, use its
module-qualified name:
MapFunctions\New-Map
To find the module from which a command was imported, use the ModuleName property of commands.
(Get-Command <command-name>).ModuleName
For example, to find the source of the Get-Date cmdlet, type:
(Get-Command Get-Date).ModuleName
Microsoft.PowerShell.Utility
If you want to qualify the name of the command using the path to the module,
you must use the forward slash (/) as the path separator and the backslash
character (\) before the command name. Use the following example to run the
Get-Date cmdlet:
//localhost/c$/Progra~1/PowerShell/7-preview/Modules/Microsoft.PowerShell.Utility\Get-Date
The path can be a full path or a path that is relative to the current location.
On Windows, you can't use a drive-qualified path. You must use a UNC path, as
shown in the previous example, or a path that's relative to the current drive.
The following example assumes that your current location is in the C: drive.
/Progra~1/PowerShell/7-preview/Modules/Microsoft.PowerShell.Utility\Get-Date
Use the call operator
You can also use the call operator (&) to run hidden commands by combining it
with a call to Get-ChildItem (the alias is dir), Get-Command or
Get-Module.
The call operator executes strings and script blocks in a child scope. For more information, see about_Operators.
For example, use the following command to run the function named Map that's
hidden by an alias named Map.
& (Get-Command -Name Map -CommandType 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.
$myMap = (Get-Command -Name map -CommandType function)
& ($myMap)
Replaced items
A replaced item is one that you can no longer access. You can replace items by importing items of the same name from a module.
For example, if you type a Get-Map function in your session, and you import a
function called Get-Map, it replaces the original function. You can't
retrieve it in the current session.
Variables and aliases can't be hidden because you can't use a call operator or a qualified name to run them. When you import variables and aliases from a module, they replace variables in the session with the same name.
Cmdlet name resolution
When you don't use the qualified name of a cmdlet, PowerShell checks to see if the cmdlet is loaded in the current session. If there are multiple modules loaded that contain the same cmdlet name, PowerShell uses the cmdlet from the first module found alphabetically.
If the cmdlet isn't loaded, PowerShell searches the installed modules and
autoloads the first module that contains the cmdlet and runs that cmdlet.
PowerShell searches for modules in each path defined in the $env:PSModulePath
environment variable. The paths are searched in the order that they're listed
in the variable. Within each path, the modules are searched in alphabetical
order. PowerShell uses the cmdlet from the first match it finds.
Avoiding name conflicts
The best way to manage command name conflicts is to prevent them. When you name your commands, use a unique name. For example, add your initials or company name acronym to the nouns in your commands.
When you import commands into your session from a PowerShell module or from
another session, you can 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 PowerShell when you import the
DateFunctions module.
Import-Module -Name DateFunctions -Prefix ZZ
Running external executables
On Windows. PowerShell treats the file extensions listed in the $env:PATHEXT
environment variable as executable files. Files that aren't Windows executables
are handed to Windows to process. Windows looks up the file association and
executes the default Windows Shell verb for the extension. For Windows to
support the execution by file extension, the association must be registered
with the system.
You can register the executable engine for a file extension using the ftype
and assoc commands of the CMD command shell. PowerShell has no direct method
to register the file handler. For more information, see the documentation for
the ftype command.
For PowerShell to see a file extension as executable in the current session,
you must add the extension to the $env:PATHEXT environment variable.
