Why isn't `tail -f … | grep -q …` quitting when it finds a match?












1















I'm doing a pretty standard tail + grep:



tail -f some_log_file.txt | grep -q known-string


If I run the command without the -q:



tail -f some_log_file.txt | grep known-string


I see output:



[Tue Feb 12 11:32:45 2019] known-string.


so I know the grep is matching. However when I add -q the grep command doesn't exit, it just hangs there waiting for more output ... even though the man page says it will "Exit immediately with zero status if any match is found":




   -q, --quiet, --silent
Quiet; do not write anything to standard output. Exit
immediately with zero status if any match is found, even if an
error was detected. Also see the -s or --no-messages option.



Can anyone explain why -q isn't causing my grep to exit? I'm trying to chain a && beep to the end so the grep beeps when a match is found, but unless I can make it exit that won't work.










share|improve this question





























    1















    I'm doing a pretty standard tail + grep:



    tail -f some_log_file.txt | grep -q known-string


    If I run the command without the -q:



    tail -f some_log_file.txt | grep known-string


    I see output:



    [Tue Feb 12 11:32:45 2019] known-string.


    so I know the grep is matching. However when I add -q the grep command doesn't exit, it just hangs there waiting for more output ... even though the man page says it will "Exit immediately with zero status if any match is found":




       -q, --quiet, --silent
    Quiet; do not write anything to standard output. Exit
    immediately with zero status if any match is found, even if an
    error was detected. Also see the -s or --no-messages option.



    Can anyone explain why -q isn't causing my grep to exit? I'm trying to chain a && beep to the end so the grep beeps when a match is found, but unless I can make it exit that won't work.










    share|improve this question



























      1












      1








      1








      I'm doing a pretty standard tail + grep:



      tail -f some_log_file.txt | grep -q known-string


      If I run the command without the -q:



      tail -f some_log_file.txt | grep known-string


      I see output:



      [Tue Feb 12 11:32:45 2019] known-string.


      so I know the grep is matching. However when I add -q the grep command doesn't exit, it just hangs there waiting for more output ... even though the man page says it will "Exit immediately with zero status if any match is found":




         -q, --quiet, --silent
      Quiet; do not write anything to standard output. Exit
      immediately with zero status if any match is found, even if an
      error was detected. Also see the -s or --no-messages option.



      Can anyone explain why -q isn't causing my grep to exit? I'm trying to chain a && beep to the end so the grep beeps when a match is found, but unless I can make it exit that won't work.










      share|improve this question
















      I'm doing a pretty standard tail + grep:



      tail -f some_log_file.txt | grep -q known-string


      If I run the command without the -q:



      tail -f some_log_file.txt | grep known-string


      I see output:



      [Tue Feb 12 11:32:45 2019] known-string.


      so I know the grep is matching. However when I add -q the grep command doesn't exit, it just hangs there waiting for more output ... even though the man page says it will "Exit immediately with zero status if any match is found":




         -q, --quiet, --silent
      Quiet; do not write anything to standard output. Exit
      immediately with zero status if any match is found, even if an
      error was detected. Also see the -s or --no-messages option.



      Can anyone explain why -q isn't causing my grep to exit? I'm trying to chain a && beep to the end so the grep beeps when a match is found, but unless I can make it exit that won't work.







      linux grep tail






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      share|improve this question




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      edited Feb 12 at 21:17









      Kamil Maciorowski

      28.3k156185




      28.3k156185










      asked Feb 12 at 19:51









      machineghostmachineghost

      3372825




      3372825






















          2 Answers
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          active

          oldest

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          3














          From StackOverflow post
          'grep -q' not exiting with 'tail -f':




          tail -f will read a file and display lines later added, it will not
          terminate (unless a signal like SIGTERM is sent). grep is not the
          blocking part here, tail -f is. grep will read from the pipe until
          it is closed, but it never is because tail -f does not quit and keep
          the pipe open.





          A solution to your problem would probably be (not tested and very
          likely to perform badly):



          tail -f logfile | while read line; do
          echo $line | grep -q 'find me to quit' && break;
          done



          You will find more information and solutions in the linked post.






          share|improve this answer































            2














            grep does exit and the pipe goes away, although tail keeps running. This bug report log starts with a use case very similar to yours:




            I want to use tail and grep to follow a file until a particular pattern appears. But tail does not exit when grep is finished.



            $ echo xxx > /tmp/blabla
            $ tail -f /tmp/blabla |grep -m1 --line-buffered "xxx"
            xxx


            Now tail still tries to read and exits only if I write again into /tmp/blabla.



            Is this how it's supposed to be?




            The explanation there:




            tail does exit on SIGPIPE, however it will only get the signal on write(), and so you need to get more data in the file before tail will exit.




            As far as I know, this exact mechanism is very common. Many tools exit after they try to write something to a broken pipe, it's not a bug.



            Then this wish came:




            It's a fair point though that tail, since it can hang around forever should take special steps to be responsive to the other end of the pipe going away.




            And finally:




            Implemented in:
            https://git.sv.gnu.org/cgit/coreutils.git/commit/?id=v8.27-42-gce0415f




            And indeed, when I try to reproduce your problem with tail from GNU coreutils 8.28, I cannot. The tool exits immediately.






            share|improve this answer























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              2 Answers
              2






              active

              oldest

              votes








              2 Answers
              2






              active

              oldest

              votes









              active

              oldest

              votes






              active

              oldest

              votes









              3














              From StackOverflow post
              'grep -q' not exiting with 'tail -f':




              tail -f will read a file and display lines later added, it will not
              terminate (unless a signal like SIGTERM is sent). grep is not the
              blocking part here, tail -f is. grep will read from the pipe until
              it is closed, but it never is because tail -f does not quit and keep
              the pipe open.





              A solution to your problem would probably be (not tested and very
              likely to perform badly):



              tail -f logfile | while read line; do
              echo $line | grep -q 'find me to quit' && break;
              done



              You will find more information and solutions in the linked post.






              share|improve this answer




























                3














                From StackOverflow post
                'grep -q' not exiting with 'tail -f':




                tail -f will read a file and display lines later added, it will not
                terminate (unless a signal like SIGTERM is sent). grep is not the
                blocking part here, tail -f is. grep will read from the pipe until
                it is closed, but it never is because tail -f does not quit and keep
                the pipe open.





                A solution to your problem would probably be (not tested and very
                likely to perform badly):



                tail -f logfile | while read line; do
                echo $line | grep -q 'find me to quit' && break;
                done



                You will find more information and solutions in the linked post.






                share|improve this answer


























                  3












                  3








                  3







                  From StackOverflow post
                  'grep -q' not exiting with 'tail -f':




                  tail -f will read a file and display lines later added, it will not
                  terminate (unless a signal like SIGTERM is sent). grep is not the
                  blocking part here, tail -f is. grep will read from the pipe until
                  it is closed, but it never is because tail -f does not quit and keep
                  the pipe open.





                  A solution to your problem would probably be (not tested and very
                  likely to perform badly):



                  tail -f logfile | while read line; do
                  echo $line | grep -q 'find me to quit' && break;
                  done



                  You will find more information and solutions in the linked post.






                  share|improve this answer













                  From StackOverflow post
                  'grep -q' not exiting with 'tail -f':




                  tail -f will read a file and display lines later added, it will not
                  terminate (unless a signal like SIGTERM is sent). grep is not the
                  blocking part here, tail -f is. grep will read from the pipe until
                  it is closed, but it never is because tail -f does not quit and keep
                  the pipe open.





                  A solution to your problem would probably be (not tested and very
                  likely to perform badly):



                  tail -f logfile | while read line; do
                  echo $line | grep -q 'find me to quit' && break;
                  done



                  You will find more information and solutions in the linked post.







                  share|improve this answer












                  share|improve this answer



                  share|improve this answer










                  answered Feb 12 at 20:10









                  harrymcharrymc

                  261k14271578




                  261k14271578

























                      2














                      grep does exit and the pipe goes away, although tail keeps running. This bug report log starts with a use case very similar to yours:




                      I want to use tail and grep to follow a file until a particular pattern appears. But tail does not exit when grep is finished.



                      $ echo xxx > /tmp/blabla
                      $ tail -f /tmp/blabla |grep -m1 --line-buffered "xxx"
                      xxx


                      Now tail still tries to read and exits only if I write again into /tmp/blabla.



                      Is this how it's supposed to be?




                      The explanation there:




                      tail does exit on SIGPIPE, however it will only get the signal on write(), and so you need to get more data in the file before tail will exit.




                      As far as I know, this exact mechanism is very common. Many tools exit after they try to write something to a broken pipe, it's not a bug.



                      Then this wish came:




                      It's a fair point though that tail, since it can hang around forever should take special steps to be responsive to the other end of the pipe going away.




                      And finally:




                      Implemented in:
                      https://git.sv.gnu.org/cgit/coreutils.git/commit/?id=v8.27-42-gce0415f




                      And indeed, when I try to reproduce your problem with tail from GNU coreutils 8.28, I cannot. The tool exits immediately.






                      share|improve this answer




























                        2














                        grep does exit and the pipe goes away, although tail keeps running. This bug report log starts with a use case very similar to yours:




                        I want to use tail and grep to follow a file until a particular pattern appears. But tail does not exit when grep is finished.



                        $ echo xxx > /tmp/blabla
                        $ tail -f /tmp/blabla |grep -m1 --line-buffered "xxx"
                        xxx


                        Now tail still tries to read and exits only if I write again into /tmp/blabla.



                        Is this how it's supposed to be?




                        The explanation there:




                        tail does exit on SIGPIPE, however it will only get the signal on write(), and so you need to get more data in the file before tail will exit.




                        As far as I know, this exact mechanism is very common. Many tools exit after they try to write something to a broken pipe, it's not a bug.



                        Then this wish came:




                        It's a fair point though that tail, since it can hang around forever should take special steps to be responsive to the other end of the pipe going away.




                        And finally:




                        Implemented in:
                        https://git.sv.gnu.org/cgit/coreutils.git/commit/?id=v8.27-42-gce0415f




                        And indeed, when I try to reproduce your problem with tail from GNU coreutils 8.28, I cannot. The tool exits immediately.






                        share|improve this answer


























                          2












                          2








                          2







                          grep does exit and the pipe goes away, although tail keeps running. This bug report log starts with a use case very similar to yours:




                          I want to use tail and grep to follow a file until a particular pattern appears. But tail does not exit when grep is finished.



                          $ echo xxx > /tmp/blabla
                          $ tail -f /tmp/blabla |grep -m1 --line-buffered "xxx"
                          xxx


                          Now tail still tries to read and exits only if I write again into /tmp/blabla.



                          Is this how it's supposed to be?




                          The explanation there:




                          tail does exit on SIGPIPE, however it will only get the signal on write(), and so you need to get more data in the file before tail will exit.




                          As far as I know, this exact mechanism is very common. Many tools exit after they try to write something to a broken pipe, it's not a bug.



                          Then this wish came:




                          It's a fair point though that tail, since it can hang around forever should take special steps to be responsive to the other end of the pipe going away.




                          And finally:




                          Implemented in:
                          https://git.sv.gnu.org/cgit/coreutils.git/commit/?id=v8.27-42-gce0415f




                          And indeed, when I try to reproduce your problem with tail from GNU coreutils 8.28, I cannot. The tool exits immediately.






                          share|improve this answer













                          grep does exit and the pipe goes away, although tail keeps running. This bug report log starts with a use case very similar to yours:




                          I want to use tail and grep to follow a file until a particular pattern appears. But tail does not exit when grep is finished.



                          $ echo xxx > /tmp/blabla
                          $ tail -f /tmp/blabla |grep -m1 --line-buffered "xxx"
                          xxx


                          Now tail still tries to read and exits only if I write again into /tmp/blabla.



                          Is this how it's supposed to be?




                          The explanation there:




                          tail does exit on SIGPIPE, however it will only get the signal on write(), and so you need to get more data in the file before tail will exit.




                          As far as I know, this exact mechanism is very common. Many tools exit after they try to write something to a broken pipe, it's not a bug.



                          Then this wish came:




                          It's a fair point though that tail, since it can hang around forever should take special steps to be responsive to the other end of the pipe going away.




                          And finally:




                          Implemented in:
                          https://git.sv.gnu.org/cgit/coreutils.git/commit/?id=v8.27-42-gce0415f




                          And indeed, when I try to reproduce your problem with tail from GNU coreutils 8.28, I cannot. The tool exits immediately.







                          share|improve this answer












                          share|improve this answer



                          share|improve this answer










                          answered Feb 12 at 21:08









                          Kamil MaciorowskiKamil Maciorowski

                          28.3k156185




                          28.3k156185






























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