About Command Precedence

Article
12/01/2022

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 by 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

As a security feature, PowerShell does not run executable (native) commands, including 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

Using wildcards in execution

You may use wildcards in command execution. Using wildcard characters is also known as globbing.

PowerShell executes a file that has a wildcard match, before a literal match.

For example, consider a directory with the following files:

Get-ChildItem C:\temp\test


    Directory: C:\temp\test


Mode                LastWriteTime         Length Name
----                -------------         ------ ----
-a----        5/20/2019   2:29 PM             28 a.ps1
-a----        5/20/2019   2:29 PM             28 [a1].ps1

Both script files have the same content: $MyInvocation.MyCommand.Path. This command displays the name of the script that is invoked.

When you run [a1].ps1, the file a.ps1 is executed even though the file [a1].ps1 is a literal match.

C:\temp\test\[a1].ps1

C:\temp\test\a.ps1

Now let's delete the a.ps1 file and attempt to run it again.

Remove-Item C:\temp\test\a.ps1
C:\temp\test\[a1].ps1

C:\temp\test\[a1].ps1

You can see from the output that [a1].ps1 runs this time because the literal match is the only file match for that wildcard pattern.

For more information about how PowerShell uses wildcards, see about_Wildcards.

Note

To limit the search to a relative path, you must prefix the script name with the .\ path. This limits the search for commands to files in that relative path. Without this prefix, other PowerShell syntax may conflict and there are few guarantees that the file will be found.

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

Alias
Function
Cmdlet
Native Windows commands

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.

When the session contains items of the same type that have the same name, PowerShell runs the newer item.

For example, if you import another Get-Date cmdlet from a module, when you type Get-Date, PowerShell runs the imported version over the native one.

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 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 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.

Get-Command Get-Date

CommandType     Name                      ModuleName
-----------     ----                      ----------
Function        Get-Date

The following command uses the All parameter to get all Get-Date commands.

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.

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.

Use this preferred method when writing scripts that you intend to distribute. You cannot predict which commands might be present in the session in which the script runs.

New-Alias -Name "Get-Date" -Value "Get-ChildItem"
Microsoft.PowerShell.Utility\Get-Date

Tuesday, September 4, 2018 8:17:25 AM

To run a New-Map command that was added by 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

Note

You cannot qualify variables or aliases.

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, 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 -CommandType Function)

&(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 or snap-in.

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 cannot retrieve it in the current session.

Variables and aliases cannot be hidden because you cannot use a call operator or a qualified name to run them. When you import variables and aliases from a module or snap-in, they replace variables in the session with the same name.

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.

Also, when you import commands into your session from a 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 PowerShell when you import the DateFunctions module.

Import-Module -Name DateFunctions -Prefix ZZ

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

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