1.5.9: Trouble with setting variables using 'read' in a script
Andy Rushton
ajr1@ecs.soton.ac.uk
Thu May 6 14:20:00 GMT 2004
Kevan Gelling wrote:
>I'm having trouble setting variables using the 'read' command in bash.
>
>All of the following lines fail to set $var and return a blank line.
> - echo "text" | read var ; echo $var
> - cat file | read var ; echo $var
> - read var < file | echo $var
>
>
The problem is that the pipeline is executed by spawning a sub-process
and running a sub-shell in it. The variable that is being read into is
actually a copy that exists within that sub-shell, not the original that
you declared in the script. Variables set in sub-shells cannot be passed
back to their parent shell (the script itself).
>I can get it work by explicitly declaring the file descriptor with the file
>redirection, but I'd prefer to use a pipe.
> - read -u 0 var <file ; echo $var
>
>
This doesn't create a sub-shell, so the variable being read is the one
in the script.
>Another related quirk, is that variables set within 'while read' loops lose
>their values once the loop ends. The following example displays "text text"
>within the loop and blank line outside.
> - echo "text" |\
> while read
> do
> foo=$REPLY ; bar="text"
> echo $foo $bar
> done
> echo $foo $bar
>
>
The while loop also becomes a sub-shell. Same problem as the pipeline.
This is a 'feature' of bash and indeed all Bourne-shell derivatives. It
makes a lot of shell features much harder to use than you would expect.
Incidentally, this is why you cannot write shell scripts to change your
environment. The script runs in a sub-shell, changes the local copy of
the environment and then discards the local copy on exit leaving the
parent shell in the same state as it was in before.
Andy
--
Andy Rushton, Southampton, UK
We may eventually come to realize that chastity is no more a virtue
than malnutrition.
-- Alex Comfort
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