This is a blog post I’ve intended to write for some time now. I intended to come up with a great example, but I’ve decided to go ahead and publish it and let you come up with your own examples. Please share your examples in the comments.
One of the great strengths of Unix is the shell pipeline. Unix has thousands of little utilities that can be strung together via a pipeline. The output of one program can be the input to another. But in practice, things don’t go quite so smoothly. Suppose the conceptual pattern is
A | B | C
meaning the output of
A goes to
B, and the output of
B goes to
C. This is actually implemented as
A | <grubby text munging> | B | <grubby text munging> | C
B doesn’t really take the output of
A. There’s some manipulation going on to prepare the output of
A as the input of
B. Strip these characters from these columns, replace this pattern with this other pattern, etc. The key point is the Unix commands spit out text. Maybe at a high level you care about programs
C, but in between are calls to utilities like
awk to bridge the gaps between output formats and input formats.
The PowerShell pipeline is different because PowerShell commands spit out objects. For example, if the output of a PowerShell command is a date, then the command returns a .NET object representing a date, not a text string. The command may display a string on the command line, but that string is just a human-readable representation. But the string representation of an object is not the object. If the output is piped to another command, the latter command receives a .NET date object, not a string. This is the big idea behind PowerShell. Commands pass around objects, not strings. The grubby, error-prone text munging between commands goes away.
Not all problems go away just because commands pass around objects. For example, maybe one command outputs a COM object and another takes in a .NET object. This is where more PowerShell magic comes in. PowerShell does a lot of work behind the scenes to implicitly convert output types to input types when possible. This sort of magic makes me nervous when I’m programming. I like to know exactly what’s going on, especially when debugging. But when using a shell, magic can be awfully convenient.
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