5 ms·

Taming non-terminating Bash processes

16 points by pstadler 12y ago

fragmede 12y ago
That's all well and good, but the real issue is that dns-sd is really meant to be used from a shell script. It should be noted that Linux's command line program to enumerate network services, avahi-browse, includes a '-t' flag so that terminates after dumping service entries. https://developer.apple.com/library/mac/documentation/Darwin/Reference/ManPages/man1/dns-sd.1.html https://developer.apple.com/library/mac/documentation/Darwin...
- duskwuff 12y ago
  ITYM "really not meant to be used from a shell script".
fensipens 12y ago
Sorry Pädu but this smells for many reasons.. first: The dns-sd command is primarily intended for interactive use. Because its command-line arguments and output format are subject to change, invoking it from a shell script will generally be fragile. Additionally, the asynchronous nature of DNS Service Discovery does not lend itself easily to script-oriented programming. For example, calls like "browse" never complete; Second, "SIGKILL is too verbose, SIGINT is too soft".. how about SIGTERM? Its very likely that you're doing it wrong if you need to use -KILL in your scripts. Third, "[1] 58181 killed ./discover-vnc.sh --- It's crucial to suppress this line." again, if you think you must suppress this, you're doing it wrong. This shouldn't pop up in the first place. Fourth, what about fifos and `$!'? Fifth, why bash? Use the sh-subset. etc. It's cute that you managed to solve your problem but this is nothing more than an unreliable hack, pompously presented ("Taming non-terminating Bash processes", "Kill the children"..)
- pstadler 12y ago
  Thanks man, your feedback is appreciated. I'm well aware of the dirty nature of this solution[1] but it seems to be the only way to achieve this within context (OS X, Alfred). [1] https://github.com/pstadler/non-terminating-bash-processes#conclusion https://github.com/pstadler/non-terminating-bash-processes#c... Edit: $! is not working with the distributed Bash version (currently 3.2) on OS X. Unfortunately, dns-sd is the only utility to discover network services on OS X. There are many projects which use the same hack involving dns-sd. I invite you to find a better solution to this problem.
  - fensipens 12y ago
    Try something like this (untested): #! /bin/sh fifo=/tmp/discover-vnc.fifo rm -f $fifo mkfifo $fifo dns-sd -B _rfb._tcp >$fifo & pid=$! while read line; do case $line in *_rfb._tcp.*) # do something here.. then test # if flag=3 and break otherwise esac done <$fifo kill $pid
    - pstadler 12y ago
      Results from dns-sd are not necessarily being delivered instantly and there might be no lines which contain this flag to check of it's the last entry. It's also rather ugly to create a named pipe for this purpose, don't you think?
      - fensipens 12y ago
        there might be no lines which contain this flag Again, untested: #! /bin/sh timeout=5 fifo=/tmp/discover-vnc.fifo rm -f $fifo mkfifo $fifo dns-sd -B _rfb._tcp >$fifo & pid1=$! while read line; do case $line in *_rfb._tcp.*) # do something here.. then test # if flag=3 and break otherwise esac done <$fifo & pid2=$! sleep $timeout && kill $pid1 $pid2 & pid3=$! wait $pid2 kill $pid3 It's also rather ugly to create a named pipe for this purpose Because it creates a file in /tmp? Who cares? The alternative is bash voodoo magic.. the same magic that gave us shellshock.
        pstadler 12y ago
        The alternative is bash voodoo magic.. the same magic that gave us shellshock. I love this one.
  - fensipens 12y ago
    $! is not working with the distributed Bash version (currently 3.2) on OS X. $ bash --version GNU bash, version 3.2.48(1)-release (x86_64-apple-darwin10.0) Copyright (C) 2007 Free Software Foundation, Inc. $ : & echo $! [1] 329 329
ttwwmm 12y ago
The use of <(..) is really weird here... why not just pipe dns-sd to the while loop? dns-sd | while read -r line ; do ... done Sure, you won't be able to accumulate the output for the trap, but you shouldn't need to---this is stream processing! Consider replacing the while loop with awk. This sort of thing is exactly what awk is good at: BEGIN { FS = " "; count = 0; } NR > 5 { count++ print if ($3 != "3") { exit; } } END { printf("%d hosts found\n", count); } You could also do it with GNU sed (you'll need the q or Q command for early exit, and that's a GNU extension---no idea if it's available on OS X).
- pstadler 12y ago
  This will stay in the loop if there's no output or if no flag == 3 is received.

16 points by pstadler 12y ago

That's all well and good, but the real issue is that dns-sd is really meant to be used from a shell script. It should be noted that Linux's command line program to enumerate network services, avahi-browse, includes a '-t' flag so that terminates after dumping service entries. https://developer.apple.com/library/mac/documentation/Darwin/Reference/ManPages/man1/dns-sd.1.html https://developer.apple.com/library/mac/documentation/Darwin...

duskwuff 12y ago
ITYM "really not meant to be used from a shell script".

fensipens 12y ago

Sorry Pädu but this smells for many reasons.. first: The dns-sd command is primarily intended for interactive use. Because its command-line arguments and output format are subject to change, invoking it from a shell script will generally be fragile. Additionally, the asynchronous nature of DNS Service Discovery does not lend itself easily to script-oriented programming. For example, calls like "browse" never complete; Second, "SIGKILL is too verbose, SIGINT is too soft".. how about SIGTERM? Its very likely that you're doing it wrong if you need to use -KILL in your scripts. Third, "[1] 58181 killed ./discover-vnc.sh --- It's crucial to suppress this line." again, if you think you must suppress this, you're doing it wrong. This shouldn't pop up in the first place. Fourth, what about fifos and `$!'? Fifth, why bash? Use the sh-subset. etc. It's cute that you managed to solve your problem but this is nothing more than an unreliable hack, pompously presented ("Taming non-terminating Bash processes", "Kill the children"..)

pstadler 12y ago
Thanks man, your feedback is appreciated. I'm well aware of the dirty nature of this solution[1] but it seems to be the only way to achieve this within context (OS X, Alfred). [1] https://github.com/pstadler/non-terminating-bash-processes#conclusion https://github.com/pstadler/non-terminating-bash-processes#c... Edit: $! is not working with the distributed Bash version (currently 3.2) on OS X. Unfortunately, dns-sd is the only utility to discover network services on OS X. There are many projects which use the same hack involving dns-sd. I invite you to find a better solution to this problem.
- fensipens 12y ago
  Try something like this (untested): #! /bin/sh fifo=/tmp/discover-vnc.fifo rm -f $fifo mkfifo $fifo dns-sd -B _rfb._tcp >$fifo & pid=$! while read line; do case $line in *_rfb._tcp.*) # do something here.. then test # if flag=3 and break otherwise esac done <$fifo kill $pid
  - pstadler 12y ago
    Results from dns-sd are not necessarily being delivered instantly and there might be no lines which contain this flag to check of it's the last entry. It's also rather ugly to create a named pipe for this purpose, don't you think?
    - fensipens 12y ago
      there might be no lines which contain this flag Again, untested: #! /bin/sh timeout=5 fifo=/tmp/discover-vnc.fifo rm -f $fifo mkfifo $fifo dns-sd -B _rfb._tcp >$fifo & pid1=$! while read line; do case $line in *_rfb._tcp.*) # do something here.. then test # if flag=3 and break otherwise esac done <$fifo & pid2=$! sleep $timeout && kill $pid1 $pid2 & pid3=$! wait $pid2 kill $pid3 It's also rather ugly to create a named pipe for this purpose Because it creates a file in /tmp? Who cares? The alternative is bash voodoo magic.. the same magic that gave us shellshock.
      - pstadler 12y ago
        The alternative is bash voodoo magic.. the same magic that gave us shellshock. I love this one.
- fensipens 12y ago
  $! is not working with the distributed Bash version (currently 3.2) on OS X. $ bash --version GNU bash, version 3.2.48(1)-release (x86_64-apple-darwin10.0) Copyright (C) 2007 Free Software Foundation, Inc. $ : & echo $! [1] 329 329

ttwwmm 12y ago

The use of <(..) is really weird here... why not just pipe dns-sd to the while loop? dns-sd | while read -r line ; do ... done Sure, you won't be able to accumulate the output for the trap, but you shouldn't need to---this is stream processing! Consider replacing the while loop with awk. This sort of thing is exactly what awk is good at: BEGIN { FS = " "; count = 0; } NR > 5 { count++ print if ($3 != "3") { exit; } } END { printf("%d hosts found\n", count); } You could also do it with GNU sed (you'll need the q or Q command for early exit, and that's a GNU extension---no idea if it's available on OS X).

pstadler 12y ago
This will stay in the loop if there's no output or if no flag == 3 is received.