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Re: What's the difference between a pipe and a filter?

2006-10-04 11:27:41
On Wednesday, Oct 4th 2006 at 18:38 +0100, quoth Steve A:

=>This is just for confirmation really...
=>
=>A pipe and a filter are invoked by the same command, '|'.  For a normal 
=>pipe, procmail hands off the message to the pipe, end of story.  For a 
=>pipe acting as a filter, procmail will take whatever is returned from 
=>the pipe (most likely changed), to be the message it previously handed 
=>off.  Is this true?
=>
=>If so, if you use the h flag in the recipe, procmail will only hand off 
=>the headers, and when using the fh flags, procmail will reattach the 
=>(changed?) header to the original body.  Is this true?
=>
=>Thanks,
=>Steve :)

I'll pretend you're asking the question generically so I don't look 
stoopid.

A pipe is the asynchronous mechanism used to transfer output from one 
program to being the input of another. It's that funny commandline with 
more than one comand stuck together with a | sign.

A filter is a program that reads from stdin and writes to stdout. Most 
unix programs do that. For example, you can tell grep what files to seach 
right on the commandline, but because grep is a filter, you can provide 
input to grep through a pipe.


grep foo file*

or

cat file* | grep

Yes, they're different and they *almost* do the same thing, but the poiont 
I was shooting for is made.

-- 
Time flies like the wind. Fruit flies like a banana. Stranger things have  .0.
happened but none stranger than this. Does your driver's license say Organ ..0
Donor?Black holes are where God divided by zero. Listen to me! We are all- 000
individuals! What if this weren't a hypothetical question?
steveo at syslang.net

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