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Named pipe hanging?

 
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# 8  
Old 03-19-2014
You'd use a named pipe only to send the output of the preceding pipeline to another command as the below lines of shell code show...
Code:
mkfifo somepipe   # create a named pipe
wc -l < somepipe &   # start a reader process
ls -ltr | tee somepipe | grep "some_file_name"   # pipe output of "ls -ltr" to "grep" via tee and to "wc -l" via somepipe

but the stuff you're trying to do doesn't need a fifo and I'd go with what Don Cragun suggests...
# 9  
Old 03-19-2014
Quote:
Originally Posted by Ditto
- want to capture stdout and sterr to both display and log file
- need to capture the return code of the command as well (ie $?)
Is it okay if the streams are merged before they're printed, or does stderr still need to be redirected outside the script?
# 10  
Old 03-19-2014
Quote:
Originally Posted by shamrock
You'd use a named pipe only to send the output of the preceding pipeline to another command as the below lines of shell code show...
Code:
mkfifo somepipe   # create a named pipe
wc -l < somepipe &   # start a reader process
ls -ltr | tee somepipe | grep "some_file_name"   # pipe output of "ls -ltr" to "grep" via tee and to "wc -l" via somepipe

but the stuff you're trying to do doesn't need a fifo and I'd go with what Don Cragun suggests...
wc -l < somepipe & will hang in a shell, while wc -l somepipe & will not... The difference is subtle but important: One tells wc -l to open the pipe, which happens after it's run and backgrounded. The other tells the shell to open the pipe, which happens before it's run and backgrounded.
# 11  
Old 03-19-2014
Quote:
Originally Posted by Corona688
wc -l < somepipe & will hang in a shell, while wc -l somepipe & will not... The difference is subtle but important: One tells wc -l to open the pipe, which happens after it's run and backgrounded. The other tells the shell to open the pipe, which happens before it's run and backgrounded.
Nope...the only difference is that wc -l somepipe & will give the no. of line plus the filename whereas wc -l < somepipe & will give the no. of lines in the ls -ltr listing...that is just the way wc -l works...
This User Gave Thanks to shamrock For This Post:
Corona688
# 12  
Old 03-19-2014
Interesting... I'm guessing the strange hangs I've had trying to do that with stdin/stdout are effects of backgrounding something talking to the terminal, then.
# 13  
Old 03-19-2014
Quote:
Originally Posted by Corona688
Interesting... I'm guessing the strange hangs I've had trying to do that with stdin/stdout are effects of backgrounding something talking to the terminal, then.
I tested your and my versions and that is the only difference I saw...
# 14  
Old 03-19-2014
Quote:
Originally Posted by Corona688
Interesting... I'm guessing the strange hangs I've had trying to do that with stdin/stdout are effects of backgrounding something talking to the terminal, then.
Probably. & implies a subshell.

Regards,
Alister
 
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