PowerShell, like most other scripting languages, wouldn't be much use without an input and output system for retrieving or sending data. PowerShell has various output systems.
The Pipeline
Consider the example of a simple directory listing with the Get-ChildItem Cmdlet.
PS D:\Dev\PoshTweet> (Get-ChildItem).GetType()
IsPublic IsSerial Name BaseType
-------- -------- ---- --------
True True Object[] System.Array
PS D:\Dev\PoshTweet> (Get-ChildItem)[0].GetType()
IsPublic IsSerial Name BaseType
-------- -------- ---- --------
True True FileInfo System.IO.FileSystemInfo
PS D:\Dev\PoshTweet> Get-ChildItem
Directory: D:\Dev\iControl\PoshTweet
Mode LastWriteTime Length Name
---- ------------- ------ ----
-a--- 1/7/2009 12:44 PM 29167 PoshTweet.ps1
PS D:\Dev\PoshTweet> Get-Childitem | ForEach-Object -Process { Write-Host $_.CreationTime : $_.Name; }
1/9/2009 8:32:40 AM : PoshTweet.ps1
By having access to the object, I was able to extract the CreationTime which I wouldn't have been able to do with a traditional shell that just passed the text representation of the output of a directory listing.
The Cmdlets
Now that you've learned how to pass the output of a command to another command, you'll likely want to be able to make that output available to you. This can be done by writing the output to the console, a file, a variable, a printer, or some other form of persistent storage. The main output mechanisms can be found in the Out-* Cmdlets listed below:
Out-Default - Send the output to the default formatter and the default output cmdlet.
Out-File - Sends output to a file.
Out-GridView - Sends output to an interactive table in a separate window.
Out-Host - Sends the output to the command line.
Out-Null - Deletes output instead of sending it to the console.
Out-Printer - Sends output to a printer.
Out-String - Sends objects to the host as a series of strings.
PowerShell provides a mechanism in which the PowerShell runtime can be embedded inside other applications. The host you are probably most familiar with is the Interactive PowerShell console but the host could just as easily be your favorite GUI application that "hosts" the PowerShell runtime.
The Out-Host Cmdlet, sends the output to the default host but you may want more control with the format of your output. This can be accomplished with the the following Cmdlets.
Format-Custom - Uses a customized view to format the output.
Format-List - Formats the output as a list of properties in which each property appears on a new line.
Format-Table - Formats the output as a table.
Format-Wide - Formats objects as a wide table that displays only one property of each object.
Tee-Object - Saves command output in a file or variable and then displays it to the console.
Write-Debug - Writes a debug message to the console.
Write-Error - Writes a warning message.
Write-EventLog - Writes an event to the Windows Event Log.
Write-Host - Writes customized output to a host.
Write-Progress - Displays a progress bar within a Windows PowerShell command window.
Write-Verbose - Writes text to the verbose message stream.
Write-Warning - Writes a warning message.
Write-Output - Sends the specified objects to the next command in the pipeline.
Note that some of these Cmdlet's will not display anything to the console by default. The Write-Verbose and Write-Debug Cmdlet's require you to set the $VerbosePreference and $DebugPreference variables to see their output.
Redirection Operators
No shell language would be complete without allowing for easy input/output redirection. PowerShell has the Out-File Cmdlet that covers the outbound redirections, but the following traditional operators have been added that are aliases to the various formats of the Out-File Cmdlet.
| Operator | | | Example | | | Results | | | Description |
|---|---|---|---|---|---|---|
| > | dir > out.txt | Contents of out.txt | Redirect pipeline output to a file, | |||
| are replaced | overwriting the current contents. | |||||
| >> | dir >> out.txt | Contents of out.txt | Redirect pipeline output to a file, | |||
| are appended to. | appending to the existing content. | |||||
| 2> | dir nosuchfile.txt 2> err.txt | Contents of err.txt | Redirect error output to a file, | |||
| are replaced by the | overwriting the current contents. | |||||
| error messages | ||||||
| 2>> | dir nosuchfile.txt 2>> err.txt | Contents of err.txt | Redirect error output to a file, | |||
| are appended with | appending to the current contents. | |||||
| the error messages | ||||||
| 2>&1 | dir nosuchfile.txt 2>&1 | The error message is | The error messages are written | |||
| written to the output. | to the output pipe instead of the | |||||
| error pipe. | ||||||
| Not implemented in | The operator is reserved for input redirection | |||||
| PowerShell v1 or v2 | which is not in PowerShell v1 or v2. | |||||
| This will result in a syntax error. | ||||||
