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Top Forums Shell Programming and Scripting Doubt about pipes and subprocess Post 302388819 by Corona688 on Thursday 21st of January 2010 02:16:36 PM
Old 01-21-2010
We're not ignoring you if we haven't answered the instant you want an answer. You don't have to bump threads. In fact you shouldn't, it's against the rules you read and agreed to when you registered.

I don't think you've got that quite right. Without complex structures like brackets or loops, a command after a pipe needs no subshell. The shell just creates a new process, arranges the files how you wanted, and replaces the new process with the requested command.

A subshell does have to stick around to manage situations like you illustrated:
Code:
command1 | ( command2 ; command3 )

It has to stay resident in order to wait for command2 to finish before starting command3.

A subshell also exists in a situation like this:

Code:
command1 | while read LINE
do
        echo "${LINE}"
done

The subshell has to stay resident to process code after the pipe inside the loop.

These subshells have side-effects. Try this:

Code:
VARIABLE="hello"
echo asdf | ( cat ; VARIABLE="goodbye" )

echo "VARIABLE=${VARIABLE}"

It will output "hello", since variables changed inside the subshell aren't changed in its parent.
 

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echo(1B)					     SunOS/BSD Compatibility Package Commands						  echo(1B)

NAME
echo - echo arguments to standard output SYNOPSIS
/usr/ucb/echo [-n] [argument] DESCRIPTION
echo writes its arguments, separated by BLANKs and terminated by a NEWLINE, to the standard output. echo is useful for producing diagnostics in command files and for sending known data into a pipe, and for displaying the contents of envi- ronment variables. For example, you can use echo to determine how many subdirectories below the root directory (/) is your current directory, as follows: o echo your current-working-directory's full pathname o pipe the output through tr to translate the path's embedded slash-characters into space-characters o pipe that output through wc -w for a count of the names in your path. example% /usr/bin/echo "echo $PWD | tr '/' ' ' | wc -w" See tr(1) and wc(1) for their functionality. The shells csh(1), ksh(1), and sh(1), each have an echo built-in command, which, by default, will have precedence, and will be invoked if the user calls echo without a full pathname. /usr/ucb/echo and csh's echo() have an -n option, but do not understand back-slashed escape characters. sh's echo(), ksh's echo(), and /usr/bin/echo, on the other hand, understand the black-slashed escape characters, and ksh's echo() also understands a as the audible bell character; however, these commands do not have an -n option. OPTIONS
-n Do not add the NEWLINE to the output. ATTRIBUTES
See attributes(5) for descriptions of the following attributes: +-----------------------------+-----------------------------+ | ATTRIBUTE TYPE | ATTRIBUTE VALUE | +-----------------------------+-----------------------------+ |Availability |SUNWscpu | +-----------------------------+-----------------------------+ SEE ALSO
csh(1), echo(1), ksh(1), sh(1), tr(1), wc(1), attributes(5) NOTES
The -n option is a transition aid for BSD applications, and may not be supported in future releases. SunOS 5.10 3 Aug 1994 echo(1B)
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