Named pipe hanging?


 
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# 1  
Old 03-19-2014
Question Named pipe hanging?

Ok, I can't seem to figure this out or find anything on the web about this.

I'm on Sun Solaris, UNIX.

I have the following test script:

Code:
#!/bin/ksh

touch test.file

LOG=./tmp.log
rm -f ${LOG}

PIPE=./tmp.pipe
mkfifo ${PIPE}
trap "rm -f ${PIPE}" EXIT

tee -a ${LOG} < ${PIPE} &

echo "Hello world"  >${PIPE}  2>&1

ls -ltr test.file

echo "Goodbye world"  >${PIPE}  2>&1

which hangs on the final echo command when I run it.
Interestingly, if I comment out the ls, or either echo command, it works fine.
Smilie

So I'm trying to understand what's happening here. Hoping somebody could explain?

What I'm trying to do:
- capture stdout and stderr into a log file. [edit] correction: want to capture stdout and sterr to both display and log file[/edit]
- need to capture the return code of the command as well (ie $?) (hence why I'm mucking around with PIPE's so I can displace the tee command).

It seems to work sometimes, and then hang on other occasions, and I haven't found a pattern to it yet.
Hoping anyone can shed light on this - or provide a suitable work around.

Thanks!

[edit]
more specific version info:
Sun4u sparc SUNW
SPARC-Enterprise SunOS
5.10

uname -v
returns:
Generic_147440-02

[/edit]

[edit]
Seems as soon as I do an ls, cp, mv or other disk access, it hangs on the next command?
[/edit]

Last edited by Ditto; 03-19-2014 at 05:19 PM.. Reason: version
# 2  
Old 03-19-2014
What return code are you trying to capture?
What are you trying to capture in the log file? (What you have seems to want to capture the output from the echo commands, but not the output from the ls command.)
It would seem that:
Code:
#!/bin/ksh

touch test.file

LOG=./tmp.log

echo "Hello world"  >${LOG}  2>&1

ls -ltr test.file

echo "Goodbye world"  >>${LOG}  2>&1

would be much simpler than what you're doing and produce the same results.
# 3  
Old 03-19-2014
Are you trying to create a dual pipeline...if so you cant displace the tee command...
# 4  
Old 03-19-2014
Quote:
Originally Posted by Don Cragun
What return code are you trying to capture?
What are you trying to capture in the log file? (What you have seems to want to capture the output from the echo commands, but not the output from the ls command.)
It would seem that:
Code:
#!/bin/ksh

touch test.file

LOG=./tmp.log

echo "Hello world"  >${LOG}  2>&1

ls -ltr test.file

echo "Goodbye world"  >>${LOG}  2>&1

would be much simpler than what you're doing and produce the same results.
I want to display output and stderr to both display and the log file. [edit] sorry, original post forgot to mention display Smilie [/edit]
I need to use tee for that.
I also want to capture the return code of each command (ie echo, ls, cp, whatever). If I use:
<command> 2>&1 |tee -a <log>
then when I check $? I get the return code for tee.

So I found the logic for named pipes, and the above example seemed to work most of the time.
Then I hit this "side case?" Not really sure.

FYI: I did just find something on the web that helped.

Changed my code to this (added the exec), and now it works Smilie
(soo confused. )

Code:
#!/bin/ksh

touch test.file

LOG=./tmp.log
rm -f ${LOG}

PIPE=./tmp.pipe
mkfifo ${PIPE}
trap "rm -f ${PIPE}" EXIT

tee -a ${LOG} < ${PIPE} &

trap "exec 5>&-" EXIT
exec  5>${PIPE}

echo "Hello world"  >${PIPE}  2>&1

ls -ltr test.file

echo "Goodbye world"  >${PIPE}  2>&1

From what I understand, the named pipe thinks that after the ls (or cp, or mv), that I'm all done with it. (this occurs whether I send the output of the ls, cp, or mv to the PIPE or not). By throwing the exec 5>$PIPE in there, it seems to trick it into expecting more input ???
# 5  
Old 03-19-2014
Quote:
Originally Posted by Ditto
...which hangs on the final echo command when I run it.
The shell tries to open the pipe before it puts the command in background, and freezes waiting for it. You have to put it in a separate subshell to avoid this.

Code:
( command < pipe ) &

It still hangs, until the other end opens, but it's some other shell hanging, not yours.

What is your goal here, though?
# 6  
Old 03-19-2014
Quote:
Originally Posted by Corona688
The shell tries to open the pipe before it puts the command in background, and freezes waiting for it. You have to put it in a separate subshell to avoid this.

Code:
( command < pipe ) &

Yes, the tee command, I run using & to throw into background.
As I indicated, if you comment out the ls (which seems to have nothing to do with the PIPE - it works fine - no hang.
With the script as is - the first echo makes it into the pipe, the ls works fine - displays to screen only.
then the last echo just hangs. Yes - my script is hanging.

Quote:
What is your goal here, though?

What I'm trying to do:
- want to capture stdout and sterr to both display and log file
- need to capture the return code of the command as well (ie $?) (hence why I'm mucking around with PIPE's so I can displace the tee command).
# 7  
Old 03-19-2014
So, try:
Code:
#!/bin/ksh
touch test.file
LOG=./tmp.log
(
        echo "Hello world"
        ls -ltr test.file
        echo "Goodbye world"
) 2>&1 | tee ${LOG}

 
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