Shell scripting input redirection oddities

Question

Can anyone explain this behavior? Running:

#!/bin/sh
echo \"hello world\" | read var1 var2
echo $var1
echo $var2

results in nothing being o

Answer 1

#!/bin/sh
echo "hello world" | read var1 var2
echo $var1
echo $var2

produces no output because pipelines run each of their components inside a subshell. Subshells inherit copies of the parent shell's variables, rather than sharing them. Try this:

#!/bin/sh
foo="contents of shell variable foo"
echo $foo
(
    echo $foo
    foo="foo contents modified"
    echo $foo
)
echo $foo

The parentheses define a region of code that gets run in a subshell, and $foo retains its original value after being modified inside them.

Now try this:

#!/bin/sh
foo="contents of shell variable foo"
echo $foo
{
    echo $foo
    foo="foo contents modified"
    echo $foo
}
echo $foo

The braces are purely for grouping, no subshell is created, and the $foo modified inside the braces is the same $foo modified outside them.

Now try this:

#!/bin/sh
echo "hello world" | {
    read var1 var2
    echo $var1
    echo $var2
}
echo $var1
echo $var2

Inside the braces, the read builtin creates $var1 and $var2 properly and you can see that they get echoed. Outside the braces, they don't exist any more. All the code within the braces has been run in a subshell because it's one component of a pipeline.

You can put arbitrary amounts of code between braces, so you can use this piping-into-a-block construction whenever you need to run a block of shell script that parses the output of something else.