PowerShell: Pipe

Example of Pipe

You can send one command's output to another command's input. This is called piping.

you write it as:

command1 | command2 | command3

# list current dir, sort it
dir | sort

# list, sort, show first 5 elements
dir | sort | select -first 5

# move jpg files to another dir
dir -file *jpg | move ~/Pictures/

# filter out bot access from web log, save to new file
Get-Content weblog | Select-String -notmatch "Googlebot" | Out-File path -width 111222333 -NoNewLine weblog2 

How Pipe Works

• The output of PowerShell commands are dotnet objects. They display on screen as formatted text representation.
• Each dotnet object has a type, and members. Member means property or method.
• PowerShell: Object Type, Properties, Methods

• Output of a command may be a collection type. (collection means any array, list, hashtable etc.) e.g. output of dir.
• Objects in collection may be different types. e.g. output of dir may be a single System.IO.FileInfo or a single System.IO.DirectoryInfo, or a collection of either or both.

• Pipeline output is passed as arguments into one of the paramater of the receiving command.
• The command's doc (help cmd) specifies which paramaters (if any) can accept argument from pipeline.
• When more than one paramater can accept pipeline input, PowerShell makes a choice automatically based on the object type.
• User cannot chose which parameter to use for pipeline input. (unlike unix, the pipeline does not go thru stdout and stdin.)

• When output is a collection, it is piped into receiving command one element in the collection at a time, in order.
• If a command receives collection type argument from its paramater directly, it is received as one single collection object. e.g. select -InputObject (dir) -First 5 does not work.