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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 OPENDARWIN

script

SCRIPT(1)						    BSD General Commands Manual 						 SCRIPT(1)

NAME

     script -- make typescript of terminal session

SYNOPSIS

     script [-a] [-k] [-q] [-t time] [file [command ...]]

DESCRIPTION

     The script utility makes a typescript of everything printed on your terminal.  It is useful for students who need a hardcopy record of an
     interactive session as proof of an assignment, as the typescript file can be printed out later with lpr(1).

     If the argument file is given, script saves all dialogue in file.	If no file name is given, the typescript is saved in the file typescript.

     If the argument command ... is given, script will run the specified command with an optional argument vector instead of an interactive shell.

     Options:

     -a       Append the output to file or typescript, retaining the prior contents.

     -k       Log keys sent to program as well as output.

     -q       Run in quiet mode, omit the start and stop status messages.

     -t time  Specify time interval between flushing script output file.  A value of 0 causes script to flush for every character I/O event.  The
	      default interval is 30 seconds.

     The script ends when the forked shell (or command) exits (a control-D to exit the Bourne shell (sh(1)), and exit, logout or control-d (if
     ignoreeof is not set) for the C-shell, csh(1)).

     Certain interactive commands, such as vi(1), create garbage in the typescript file.  The script utility works best with commands that do not
     manipulate the screen.  The results are meant to emulate a hardcopy terminal, not an addressable one.

ENVIRONMENT

     The following environment variable is utilized by script:

     SHELL  If the variable SHELL exists, the shell forked by script will be that shell.  If SHELL is not set, the Bourne shell is assumed.  (Most
	    shells set this variable automatically).

SEE ALSO

     csh(1) (for the history mechanism).

HISTORY

     The script command appeared in 3.0BSD.

BUGS

     The script utility places everything in the log file, including linefeeds and backspaces.	This is not what the naive user expects.

     It is not possible to specify a command without also naming the script file because of argument parsing compatibility issues.

     When running in -k mode, echo cancelling is far from ideal.  The slave terminal mode is checked for ECHO mode to check when to avoid manual
     echo logging.  This does not work when in a raw mode where the program being run is doing manual echo.

BSD
								   June 6, 1993 							       BSD
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