Shell Programming and Scripting

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

With something like:
A=""
echo test | read A
echo A

You have a problem because the "read" statement is executed in a subprocess with almost every shell. The only exception I know is the real Korn shell. The Korn shell will execute the last command of a pipeline in the current shell if it is a korn shell builtin. This does not apply to pdksh, only the real Korn shell. Your "while" statement is just a compound command.

But a loop like:
while read line ; do some-command ; done
runs until the "read line" get no input. This changes the value of line to nothing. So even if the loop is running in the current shell, line will be empty when the loop exits.

Ah! (light bulb turns on inside head)
I get it now.
I was thinking the "line" variable would hold the last line read.

so with:
while read line ; do some-command ; done < some-file

"read" tries to read the next line after the end of file, which is null,
and sets "line" to null, and returns "false" so the loop will end.

The "line" variable is not "invisible" outside the loop,
it just looks like that because it ends up null.

Just to make sure,

Code:

while [ "$line" != "Fred" ]; do
    read line
done << EOF
Wilma
Barney
Fred
Betty
EOF

echo "Line = $line"

The result is "Fred" in both bash and dash.

I've used this method before, but never really understood how it worked.

Thanks.

-MD