Question

In the following test script I run an elementary coprocess to which the echo built-in, run in background, attaches its standard-output:

#!/bin/bash
# TEST 1
coproc /bin/sleep 100
echo >&${COPROC[1]} &

The script always fails, for no apparent reason, giving the output:

./test.sh: line 4: ${COPROC[1]}: Bad file descriptor

I wonder if the correct syntax should be rather this one (ampersand moved before redirection):

#!/bin/bash
# TEST 2
coproc /bin/sleep 100
echo & >&${COPROC[1]}

This second example seems to work since it reports no errors during execution, but with this syntax, the redirection is not performed in practice; in fact, consider this other test:

#!/bin/bash
# TEST 3
/bin/echo abc & >xfile

Test 3 creates the file xfile, but does not write anything into it. Curiously, trying again to position the ampersand after the redirection make the echo work fine:

#!/bin/bash
# TEST 4
/bin/echo abc >xfile &

Test 4 creates the file xfile with inside the string abc.

Have some idea on what is causing the coproc redirection error or what the correct syntax is?

Answer 1

You’ve got the answer elsewhere http://lists.gnu.org/archive/html/bug-bash/2012-06/msg00027.html:

Coproc file descriptors are not available to subshells. They’re
implemented using pipes, and leaving pipe file descriptors open in
subshells causes processes to hang and not terminate properly, which
results in very hard-to-track-down-and-reproduce bugs.