If I pipe the output of a command into `read variable', why doesn't the output show up in $variable when the read command finishes?

If I pipe the output of a command into `read variable', why doesn't the output show up in $variable when the read command finishes? 


This has to do with the parent-child relationship between Unix

processes.  It affects all commands run in pipelines, not just

simple calls to `read'.  For example, piping a command's output

into a `while' loop that repeatedly calls `read' will result in

the same behavior.



Each element of a pipeline runs in a separate process, a child of

the shell running the pipeline.  A subprocess cannot affect its

parent's environment.  When the `read' command sets the variable

to the input, that variable is set only in the subshell, not the

parent shell.  When the subshell exits, the value of the variable

is lost. 



Many pipelines that end with `read variable' can be converted

into command substitutions, which will capture the output of

a specified command.  The output can then be assigned to a

variable:



	grep ^gnu /usr/lib/news/active | wc -l | read ngroup



can be converted into



	ngroup=$(grep ^gnu /usr/lib/news/active | wc -l)



This does not, unfortunately, work to split the text among

multiple variables, as read does when given multiple variable

arguments.  If you need to do this, you can either use the

command substitution above to read the output into a variable

and chop up the variable using the bash pattern removal

expansion operators or use some variant of the following

approach.



Say /usr/local/bin/ipaddr is the following shell script:



#! /bin/sh

host `hostname` | awk '/address/ {print $NF}'



Instead of using



	/usr/local/bin/ipaddr | read A B C D



to break the local machine's IP address into separate octets, use



	OIFS="$IFS"

	IFS=.

	set -- $(/usr/local/bin/ipaddr)

	IFS="$OIFS"

	A="$1" B="$2" C="$3" D="$4"



Beware, however, that this will change the shell's positional

parameters.  If you need them, you should save them before doing

this.



This is the general approach -- in most cases you will not need to

set $IFS to a different value.



Some other user-supplied alternatives include:



read A B C D << HERE

    $(IFS=.; echo $(/usr/local/bin/ipaddr))

HERE



and, where process substitution is available,



read A B C D < <(IFS=.; echo $(/usr/local/bin/ipaddr))

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