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

It's because the pipe version is creating a subshell, which reads the variable into its local space which then is destroyed when the subshell exits.

Execute this command

$ echo $$;cat | read a
10637

and use pstree -p to look at the running processes, you will see an extra shell hanging off of your main shell.

    |                       |-bash(10637)-+-bash(10786)
    |                       |             `-cat(10785)