Re: Redirect a backgrounded process' stdout toward COPROC's stdin
On 6/3/12 8:20 AM, Davide Baldini wrote:
Machine: i486
OS: linux-gnu
Compiler: gcc
Compilation CFLAGS: -O2 -march='i486' (plus the flags added by Makefile)
uname output: Linux debianBunker 2.6.26-2-686 #1 SMP Wed Sep 21 04:35:47
UTC 2011 i686 GNU/Linux
Machine Type: i486-pc-linux-gnu
Bash Version: 4.2.0(5)-release
Patch Level: 5
Release Status: release
Description:
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
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.
Chet
--
``The lyf so short, the craft so long to lerne.'' - Chaucer
``Ars longa, vita brevis'' - Hippocrates
Chet Ramey, ITS, CWRUc...@case.eduhttp://cnswww.cns.cwru.edu/~chet/
Redirect a backgrounded process' stdout toward COPROC's stdin
Machine: i486
OS: linux-gnu
Compiler: gcc
Compilation CFLAGS: -O2 -march='i486' (plus the flags added by Makefile)
uname output: Linux debianBunker 2.6.26-2-686 #1 SMP Wed Sep 21 04:35:47
UTC 2011 i686 GNU/Linux
Machine Type: i486-pc-linux-gnu
Bash Version: 4.2.0(5)-release
Patch Level: 5
Release Status: release
Description:
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.
What's wrong with the coproc? And what's the correct position for the
ampersand, before redirection, or after?
Re: Redirect a backgrounded process' stdout toward COPROC's stdin
2012/6/3 Davide Baldini baldiniebald...@gmail.com:
Description:
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
The coproc fds are only available in the same shell. The subshell
created with cannot use them.
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 is equivalent to
echo
${COPROC[1]}
ends the command, so the redirection is not applied to the echo.
See http://wiki.bash-hackers.org/syntax/keywords/coproc
--
Geir Hauge
Re: Redirect a backgrounded process' stdout toward COPROC's stdin
On 06/03/12 17:01, Geir Hauge wrote:
2012/6/3 Davide Baldini baldiniebald...@gmail.com:
Description:
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
The coproc fds are only available in the same shell. The subshell
created with cannot use them.
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 is equivalent to
echo
${COPROC[1]}
ends the command, so the redirection is not applied to the echo.
See http://wiki.bash-hackers.org/syntax/keywords/coproc
Thank you Geir, I really appreciated you help and the useful
bash-hackers.org link. Definitely, the GNU Bash manual would need some
deeper details on these niche features.
