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Date: Sun, 14 Jan 2001 02:50:03 -0800 (PST)
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From: Peter Pentchev <roam@orbitel.bg>
Subject: Re: bin/24228: /bin/sh problem : variable after pipeline won't be set
Reply-To: Peter Pentchev <roam@orbitel.bg>
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The following reply was made to PR bin/24228; it has been noted by GNATS.

From: Peter Pentchev <roam@orbitel.bg>
To: imura@af.airnet.ne.jp
Cc: FreeBSD-gnats-submit@freebsd.org
Subject: Re: bin/24228: /bin/sh problem : variable after pipeline won't be set
Date: Sun, 14 Jan 2001 12:47:51 +0200

 On Thu, Jan 11, 2001 at 01:32:48AM +0900, imura@af.airnet.ne.jp wrote:
 > 
 > >Number:         24228
 > >Category:       bin
 > >Synopsis:       when using /bin/sh, setting variable after pipeline won't work
 > >Originator:     Ryuichiro Imura
 > >Release:        FreeBSD 4.2-STABLE i386
 > >Environment:
 > 
 > FreeBSD 4.2-STABLE
 > 
 > >Description:
 > 
 > When using /bin/sh, setting variables after "|"
 > (in other words, I mean setting variables in a pipeline sequence),
 > will be ignored out of the pipeline sequence.
 > 
 > If it is a definition of /bin/sh, it's ok, I'm sorry,
 > otherwise I think it should be fixed.
 
 AFAIK, yes, this is expected sh(1) behavior.  From the manpage:
 
      Note that unlike some other shells, sh executes each process in the
      pipeline as a child of the sh process.  Shell builtin commands are the
      exception to this rule.  They are executed in the current shell, although
      they do not affect its environment when used in pipelines.
 
 That is, sh(1) forks a subshell for each subsequent command/construct
 in the pipe, and variable assignments within a construct are only valid
 for the subshell that construct is executing in, and not back-propagated
 to the main shell.
 
 Thus, in your example, 'variable' would only be set to 'bar' for
 the subshell executing the while loop.  See below for a suggestion
 for a workaround.
 
 > >How-To-Repeat:
 > 
 > write a simple shell script like this:
 > -----------starts here----------
 > #/bin/sh
 > 
 > cat FILE | while read line ; do
 > 	if [ $line = foo ]; then
 > 		variable=bar
 > 	fi
 > 	echo $variable   <---- this will be printed
 > done
 > 
 > echo $variable           <---- this will NOT be printed
 > -----------ends here------------
 
 You may do better with something like the following:
 
 #!/bin/sh
 # (make sure that's #!, not just # ;)
 
 exec < FILE
 
 while read line; do
 	if [ "$line" = "foo" ]; then
 		variable="bar"
 	fi
 	echo "line is $line, variable is $variable"
 done
 
 echo "after the loop, variable is $variable"
 
 Hope that makes things clearer :)
 
 G'luck,
 Peter
 
 -- 
 Hey, out there - is it *you* reading me, or is it someone else?
 


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