Question

I was trying to understand how “while read” works in bash, and i came accross a behaviour that i can’t explain:

root@antec:/# var=old_value
root@antec:/# echo "new_value" | read var && echo $var
old_value
root@antec:/# 

It works fine with “while read”:

root@antec:/# var=old_value
root@antec:/# echo "new_value" | while read var; do echo $var; done
new_value
root@antec:/# 

Can someone explain why it does not work when read is used without while ? Moreover, i don’t understand how the value of “var” in the main shell can be seen from the allegedly subshell after the pipe ..

Thank you

Answer 1

I believe this is precedence problem, with | having higher precedence than &&. First is grouped as:

(echo "new_value" | read var) && echo $var

Second is grouped as:

echo "new_value" | (while read var; do echo $var; done)