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Full Discussion: while read loop; scope of variables (shell)
Top Forums Shell Programming and Scripting while read loop; scope of variables (shell) Post 302293581 by mjd_tech on Tuesday 3rd of March 2009 11:11:18 AM
Old 03-03-2009
while read loop; scope of variables (shell)
If I set a variable within a while-read loop, sometimes it's local to the loop, sometimes it's global, depending on how the loop is set up. I'm testing this on a Debian Lenny system using both bash and dash with the same results.

For example:
Code:
# Pipe command into while-read loop

count=
ls -1 | while read line; do
    count=$((count + 1))
done
echo "Count = $count"

In this case, $count is null outside the loop.
The loop set up a separate variable named count and ignored the global variable named count.

Note: Yes, there are better ways to count files in a directory.

So let's try this:
Code:
# Redirect file into read-while loop

ls -1 > /tmp/junk
while read line; do
    count=$((count + 1))
done < /tmp/junk
rm /tmp/junk
echo "Count = $count"

That works as expected.
We didn't even need to declare count as a global.

Let's try to avoid the temp file by using a Here Doc.
Code:
# Redirect Here Document into read-while loop

while read line; do
    count=$((count + 1))
done << EOF
$(ls -1)
EOF
echo "Count = $count"

This also works as expected.

The example using the pipe did not work.
My guess is that the pipe somehow launches a child process or subshell or something like that, and variables created within a child are not visible to the parent.
Ok, I can live with that. I think it even says so in the man pages.

But, another thing I noticed is the variable "line" used in all three examples is never visible outside the loop. This would imply a child process as well. Or maybe not?
This is where I am confused.
Why is "line" always invisible outside the loop, but "count" is visible, provided we don't use a pipe?

-MD
 

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LEARN ABOUT X11R4

break

break(1)                                                           User Commands                                                          break(1)

NAME

       break, continue - shell built-in functions to escape from or advance within a controlling while, for, foreach, or until  loop

SYNOPSIS

   sh
       break [n]

       continue [n]

   csh
       break

       continue

   ksh
       *break [n]

       *continue [n]

DESCRIPTION

   sh
       The break utility exits from the enclosing for or while loop, if any. If n is specified, break n levels.

       The continue utility resumes the next iteration of the enclosing for or while loop. If n is specified, resume at the n-th enclosing loop.

   csh
       The  break  utility  resumes  execution after the end of the nearest enclosing foreach or while loop. The remaining commands on the current
       line are executed. This allows multilevel breaks to be written as a list of break commands, all on one line.

       The continue utility continues execution of the next iteration of the nearest enclosing while or foreach loop.

   ksh
       The break utility exits from the enclosed for, while, until, or select loop, if any. If n is specified,  then  break  n  levels.  If  n  is
       greater than the number of enclosing loops, the outermost enclosing loop shall be exited.

       The  continue utility resumes the next iteration of the enclosed for, while, until, or select loop. If n is specified then resume at the n-
       th enclosed loop. If n is greater than the number of enclosing loops, the outermost enclosing loop shall be used.

       On this man page, ksh(1) commands that are preceded by one or two * (asterisks) are treated specially in the following ways:

       1.  Variable assignment lists preceding the command remain in effect when the command completes.

       2.  I/O redirections are processed after variable assignments.

       3.  Errors cause a script that contains them to abort.

       4.  Words that follow a command preceded by ** that are in the format of a variable assignment are expanded with the same rules as a  vari-
           able  assignment.  This means that tilde substitution is performed after the = sign, and also that word splitting and file name genera-
           tion are not performed.

ATTRIBUTES

       See attributes(5) for descriptions of the following attributes:

       +-----------------------------+-----------------------------+
       |      ATTRIBUTE TYPE         |      ATTRIBUTE VALUE        |
       +-----------------------------+-----------------------------+
       |Availability                 |SUNWcsu                      |
       +-----------------------------+-----------------------------+

SEE ALSO

       csh(1), exit(1), ksh(1), sh( 1), attributes(5)

SunOS 5.10                                                          17 Jul 2002                                                           break(1)
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