Linux read command - simple question

Question

Hi,

why is output empty?

echo "a b c d" | read X Y Z V; echo $X

Thanks.

Answer 1

I believe it is because echo "a b c d" | read X Y Z V and echo $X are separate statements (I'm not sure of the exact term)? So one doesn't know about the other.

EDIT: Give echo "a b c d" | ( read X Y Z V; echo $X ) a try...

Answer 2

The issue is that, in order to run the read command with its input redirected from the echo, a new subshell process is spawned. This process reads the values, assigns them to the variables - and then exits; then the second echo command is run. To demonstrate this you can do the second echo and the read both from a subshell:

$ echo "a b c d" | ( read X Y Z V; echo $X )
a

Answer 3

In Bash, you can do a couple of different things to accomplish that:

A here string:

read X Y Z V <<< $(echo "a b c d"); echo $X

Process substitution:

read X Y Z V < <(echo "a b c d"); echo $X

A here document with command substitution:

read X Y Z V <<EOF
$(echo "a b c d")
EOF
echo $X

The here document method will also work with POSIX shells in addition to Bash.

If you're reading from a file instead of from the output of another command, it's a little simpler.