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