[Richard Suchenwirth] 2003-07-09 - After having written the following code repeatedly, I just declare it to be an [idiom]. The ''[filter]'' function is frequently seen on Unix (and even DOS) tools, and I think was published in the Software Tools book(s):
   * read from stdin (or files specified on command line)
   * write to stdout
Such tools (grep, sed, awk, sort, ...) can easily be combined ("glued") in pipes to do powerful things. A little framework for Tcl scripts with the filter function:

 set about "usage: myFilter ?file...?

 Does something meaningful with the specified files, or stdin.
 "

 #-- This handler contains the real functionality for one stream (or file)
 proc myHandler channel {
   ...
 }
 #-- This prevents lengthy errors if a filter|more is terminated with 'q'
 proc puts! string {catch {puts stdout $string}}

 if {[lsearch $argv "--help"]>=0} {puts $about; exit}

 if {[llength $argv]==0} {
    puts! [myHandler stdin]
 } else {
        foreach file $argv {
            set fp [open $file]
            puts! [myHandler $fp]
            close $fp
        }
 }
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[Mike Tuxford] is a little confused here, although that in itself is not unusual. It appears to me that you provide a method of repeating a single set of functions upon a multiple set of files, whereas unix pipes provide multiple functions upon the returned data passed between the functions. That is, the 1st command passes its stdout to the 2nd command as its stdin, and so on...
Perhaps an example of usage might clarify things for me. - [RS]: Well, the above is the framework for one filter, which you can put into a pipe, but also can draw input from files specified on command line, like this (and similar to e.g. ''cat'' or ''more''):
 echo Data | myFilter | more
 cat data.file | myFilter | more
 myFilter *.file | more
 more data.File
You're right that real filters read from stdin only, but many have the added convenience of filenames on command line - that's what I meant, and implemented.

[Mike Tuxford] says: OK, I see, and that's still a useful thing. Not to mention that you got me thinking and learning. Thanks for the quick response.

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