bash coproc and leftover coproc output

Question

I need to read some configuration data into environment variables in a bash script.

The \"obvious\" (but incorrect) pattern is:

egrep \"pattern\" con         


        

Answer 1

What happens is, as soon as the subshell finishes, the parent shell cleans up and closes the FDs. You're lucky you even got to read the first line!

Try this in an interactive shell:

$ coproc ECHO { echo foo; echo bar; }
[2] 16472
[2]+  Done                    coproc ECHO { echo foo; echo bar; }
$ read -u ${ECHO[0]}; echo $REPLY
bash: read: -u: option requires an argument
read: usage: read [-ers] [-a array] [-d delim] [-i text] [-n nchars] [-N nchars] [-p prompt] [-t timeout] [-u fd] [name ...]

It even mops up the environment variable.

Now try this:

$ coproc ECHO { echo foo; echo bar; sleep 30; }
[2] 16485
$ read -u ${ECHO[0]}; echo $REPLY
foo
$ read -u ${ECHO[0]}; echo $REPLY
bar
$ read -u ${ECHO[0]}; echo $REPLY # blocks until the 30 seconds are up

[2]+  Done                    coproc ECHO { echo foo; echo bar; sleep 30; }

As for solving the problem behind the question: Yes, redirection and process substitution is the better choice for the particular example given.