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Full Discussion: Problems understanding pipes
Top Forums Programming Problems understanding pipes Post 302562733 by Corona688 on Saturday 8th of October 2011 12:23:41 AM
Old 10-08-2011
Quote:
Originally Posted by ab_tall
Why should'nt builtins work in the middle of the pipe?

In my home directory,
I tried ls | set | grep path
First, there's generally no point to doing so -- set doesn't read from standard input.

There's constructs that can:
Code:
grep filename | while read LINE
do
        echo "$LINE"
done

...but there's two problems:

1) csh can't do this. Just one of its many design flaws.
2) The Bourne shell can, but it must launch a subshell to do so. It's run in a shell, but not your shell. (As a side-effect, LINE doesn't get set in the rest of the program, just inside the while-loop.)

I already hinted at why it needs to do this...

Imagine a shell where all of those are builtins, running inside the same shell process. How does it decide which gets to go when? If you tried read when the pipe was empty it'd freeze, you'd never have a chance to run grep.

You could set everything nonblocking and just poll, running each thing a bit at a time. It'd never freeze, but your program would consume 100% CPU even sitting there waiting for sleep.

You could cheat and just run them in order, saving to temp file, waiting for one to finish before sending the file into the next one. I think this is how DOS pretended to have rudimentary pipes even though it didn't have processes at all.

Or you could use buffers and logic and mutual exclusion between things to decide which gets to run when and prevent one part of the program from waiting forever for the other... Except there's already a system on your computer which does that -- the operating system.

Instead of reinventing the wheel, they fork() off another process to run a specific piece of script-within-a-script independently of the rest. When it waits for input, it's not freezing the entire program.
Quote:
so what I'm basically asking is, if a builtin command occurs in the middle of a pipe sequence, is it executed in the parent shell process or as a subprocess?
Basically -- the shell fork()'s to run it. But since it's pure shell code, it doesn't exec() anything, the child process is used to run a subsection of shell code.

Again, compounded by the problem that csh is bad at this.
Last edited by Corona688; 10-08-2011 at 01:32 AM..
 

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LEARN ABOUT OSX

platform::shell

platform::shell(n)					       Tcl Bundled Packages						platform::shell(n)

__________________________________________________________________________________________________________________________________________________

NAME

       platform::shell - System identification support code and utilities

SYNOPSIS

       package require platform::shell ?1.1.4?

       platform::shell::generic shell
       platform::shell::identify shell
       platform::shell::platform shell
_________________________________________________________________

DESCRIPTION

       The platform::shell package provides several utility commands useful for the identification of the architecture of a specific Tcl shell.

       This  package  allows the identification of the architecture of a specific Tcl shell different from the shell running the package. The only
       requirement is that the other shell (identified by its path), is actually executable on the current machine.

       While for most platform this means that the architecture of the interrogated shell is identical to the architecture of  the  running  shell
       this  is  not generally true. A counter example are all platforms which have 32 and 64 bit variants and where a 64bit system is able to run
       32bit code. For these running and interrogated shell may have different 32/64 bit settings and thus different identifiers.

       For applications like a code repository it is important to identify the architecture of the shell which will  actually  run  the  installed
       packages, versus the architecture of the shell running the repository software.

COMMANDS

       platform::shell::identify shell
	      This command does the same identification as platform::identify, for the specified Tcl shell, in contrast to the running shell.

       platform::shell::generic shell
	      This command does the same identification as platform::generic, for the specified Tcl shell, in contrast to the running shell.

       platform::shell::platform shell
	      This command returns the contents of tcl_platform(platform) for the specified Tcl shell.

KEYWORDS

       operating system, cpu architecture, platform, architecture

platform::shell 						       1.1.4							platform::shell(n)
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