PowerShell: Pipe Output

You can send one command's output to another command's input. This is called piping. you write it as: command1 | command2 | command3.

Here are some examples:

# list current dir, sort it
dir | sort

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

# list, sort, select, then format for display
dir | sort | select -first 5 | format-table name, length

# move jpg files to another dir
Get-Item $home/Documents/*jpg | Move-Item -Destination $home/Pictures/

# filter out bot access from web log, save to new file
get-content weblog.txt |
select-string -notmatch "Googlebot" |
out-file -Encoding utf8 -width 999000 weblog2.txt

How Pipe Works

• The output of PowerShell commands are .NET objects. They display on screen as formatted text representation. Unlike unix shells, it is not passing text streams.
• Each .NET object has a type, and members. “Member” means property or method. You can find the object type of command output by pipe it to Get-Member. [see PowerShell: Object Type, Properties, Methods]
• Output of a command may be more than one object. For example, output of dir.
• Objects in output may be different types. For example, output of dir may be files (type System.IO.FileInfo) and directories (type System.IO.DirectoryInfo).
• Pipeline output is passed as arguments into one of the paramater of the receiving command. The command's doc (help cmd) specifies which paramaters (in any) can accept argument from pipeline. When more than one paramater 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.)
• In a pipeline, when there's more than one object in output, it'll be fed into receiving command one at a time, in order. For example, Get-ChildItem | Select-Object -First 5. (Note: it is not passing one single object of array) However, if a command receives collection argument from its paramater (not via pipeline), it is received as one single array object. For example, Select-Object -InputObject (Get-ChildItem) -First 5 doesn't work.
• Technically, only some commands work well together in a pipeline and in particular order. But in general, this is intuitive and never a question. In general, you can tell by the cmdlet name. Get-nounX usually works with somethig-nounX. Also, certain commands are almost always used with pipeline, such as Select-Object (select items by known info, such as length, property type, values.) and Where-Object (select items by arbitrary tests), Format-Table, Format-List, Sort-Object.

See also: PowerShell: Object Type, Properties, Methods