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Top Forums Shell Programming and Scripting ret val of a command in a pipe which is NOT the last one Post 31029 by Perderabo on Thursday 31st of October 2002 08:24:19 AM
Old 10-31-2002
Try your code fragment. The tee process will never write to the logfile. It *might* recreate the logfile if it doesn't exist. Other than that it clearly will do nothing.

A co-process certainly does not use the same file descriptors as the main process. That would render it useless.

These days pipes pretty much are bidirectional. And with some use of the ksh <> construct it might be possible to exploit this. But I never do. Each fd that I use in a script is for reading only or writing only. I always assume pipes to be unidirectional.

The first exec saves the current stdout and stderr so that the last exec can restore them. The stuff in between connects the output from the subshell to the input of the co-process and the output of the co-process to the original output.
 

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FORK(2) 							System Calls Manual							   FORK(2)

NAME
fork - spawn new process SYNOPSIS
fork( ) DESCRIPTION
Fork is the only way new processes are created. The new process's core image is a copy of that of the caller of fork. The only distinc- tion is the fact that the value returned in the old (parent) process contains the process ID of the new (child) process, while the value returned in the child is 0. Process ID's range from 1 to 30,000. This process ID is used by wait(2). Files open before the fork are shared, and have a common read-write pointer. In particular, this is the way that standard input and output files are passed and also how pipes are set up. SEE ALSO
wait(2), exec(2) DIAGNOSTICS
Returns -1 and fails to create a process if: there is inadequate swap space, the user is not super-user and has too many processes, or the system's process table is full. Only the super-user can take the last process-table slot. ASSEMBLER
(fork = 2.) sys fork (new process return) (old process return, new process ID in r0) The return locations in the old and new process differ by one word. The C-bit is set in the old process if a new process could not be cre- ated. FORK(2)
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