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Using “tail” to follow a file without displaying the most recent lines


How can I do the equivalent of tail -f with ls?Observe multiple log files in one outputMaking less's follow option show line movementtail -f but suck in content of the file first (aka `cat -f`)Using tail to follow daily log file in BashTail -f the most recent log fileOnly output most recent 10 (or n) lines of a lengthy command outputtail display whole file and then only changesFor a given directory, how do I concatenate the tail end of recently modified files to a new file?Using head and tail to grab different sets of lines and saving into same file













2















I would like use a program like tail to follow a file as it's being written to, but not display the most recent lines.



For instance, when following a new file, no text will be displayed while the file is less than 30 lines. After more than 30 lines are written to the file, lines will be written to the screen starting at line 1.



So as lines 31-40 are written to the file, lines 1-10 will be written to the screen.



If there is no easy way to do this with tail, maybe a there's a way to write to a new file a prior line from the first file each time the first file is extended by a line, and the tail that new file...










share|improve this question







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    2















    I would like use a program like tail to follow a file as it's being written to, but not display the most recent lines.



    For instance, when following a new file, no text will be displayed while the file is less than 30 lines. After more than 30 lines are written to the file, lines will be written to the screen starting at line 1.



    So as lines 31-40 are written to the file, lines 1-10 will be written to the screen.



    If there is no easy way to do this with tail, maybe a there's a way to write to a new file a prior line from the first file each time the first file is extended by a line, and the tail that new file...










    share|improve this question







    New contributor




    ridthyself is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
    Check out our Code of Conduct.






















      2












      2








      2


      1






      I would like use a program like tail to follow a file as it's being written to, but not display the most recent lines.



      For instance, when following a new file, no text will be displayed while the file is less than 30 lines. After more than 30 lines are written to the file, lines will be written to the screen starting at line 1.



      So as lines 31-40 are written to the file, lines 1-10 will be written to the screen.



      If there is no easy way to do this with tail, maybe a there's a way to write to a new file a prior line from the first file each time the first file is extended by a line, and the tail that new file...










      share|improve this question







      New contributor




      ridthyself is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
      Check out our Code of Conduct.












      I would like use a program like tail to follow a file as it's being written to, but not display the most recent lines.



      For instance, when following a new file, no text will be displayed while the file is less than 30 lines. After more than 30 lines are written to the file, lines will be written to the screen starting at line 1.



      So as lines 31-40 are written to the file, lines 1-10 will be written to the screen.



      If there is no easy way to do this with tail, maybe a there's a way to write to a new file a prior line from the first file each time the first file is extended by a line, and the tail that new file...







      linux command-line tail






      share|improve this question







      New contributor




      ridthyself is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
      Check out our Code of Conduct.











      share|improve this question







      New contributor




      ridthyself is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
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      share|improve this question




      share|improve this question






      New contributor




      ridthyself is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
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      asked 2 hours ago









      ridthyselfridthyself

      111




      111




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      New contributor





      ridthyself is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
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          2 Answers
          2






          active

          oldest

          votes


















          2














          Maybe buffer with awk:



          tail -n +0 -f some/file | awk 'b[NR] = $0 NR > 30 print b[NR-30]; delete b[NR-30] END for (i = NR - 29; i <= NR; i++) print b[i]'


          The awk code, expanded:




          b[NR] = $0 # save the current line in a buffer array

          NR > 30 # once we have more than 30 lines
          print b[NR-30]; # print the line from 30 lines ago
          delete b[NR-30]; # and delete it

          END # once the pipe closes, print the rest
          for (i = NR - 29; i <= NR; i++)
          print b[i]






          share|improve this answer

























          • This works, but form the script I would expect it to work like tail, printing out a previous line as each new line is added to the file. Instead it prints out in spurts of ~70 lines after ~100 lines are added to the file. It does not print the most recent 30 lines, so it's pretty close...

            – ridthyself
            58 mins ago











          • @ridthyself if you have GNU awk, try adding a fflush(); after the print b[NR-30];. Maybe the output is being buffered.

            – muru
            48 mins ago


















          1














          This isn't very efficient, because it will re-read the file every two seconds, but will do the job:



          watch 'tail -n40 /path/to/file | head -n10'





          share|improve this answer























            Your Answer








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            2 Answers
            2






            active

            oldest

            votes








            2 Answers
            2






            active

            oldest

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            active

            oldest

            votes






            active

            oldest

            votes









            2














            Maybe buffer with awk:



            tail -n +0 -f some/file | awk 'b[NR] = $0 NR > 30 print b[NR-30]; delete b[NR-30] END for (i = NR - 29; i <= NR; i++) print b[i]'


            The awk code, expanded:




            b[NR] = $0 # save the current line in a buffer array

            NR > 30 # once we have more than 30 lines
            print b[NR-30]; # print the line from 30 lines ago
            delete b[NR-30]; # and delete it

            END # once the pipe closes, print the rest
            for (i = NR - 29; i <= NR; i++)
            print b[i]






            share|improve this answer

























            • This works, but form the script I would expect it to work like tail, printing out a previous line as each new line is added to the file. Instead it prints out in spurts of ~70 lines after ~100 lines are added to the file. It does not print the most recent 30 lines, so it's pretty close...

              – ridthyself
              58 mins ago











            • @ridthyself if you have GNU awk, try adding a fflush(); after the print b[NR-30];. Maybe the output is being buffered.

              – muru
              48 mins ago















            2














            Maybe buffer with awk:



            tail -n +0 -f some/file | awk 'b[NR] = $0 NR > 30 print b[NR-30]; delete b[NR-30] END for (i = NR - 29; i <= NR; i++) print b[i]'


            The awk code, expanded:




            b[NR] = $0 # save the current line in a buffer array

            NR > 30 # once we have more than 30 lines
            print b[NR-30]; # print the line from 30 lines ago
            delete b[NR-30]; # and delete it

            END # once the pipe closes, print the rest
            for (i = NR - 29; i <= NR; i++)
            print b[i]






            share|improve this answer

























            • This works, but form the script I would expect it to work like tail, printing out a previous line as each new line is added to the file. Instead it prints out in spurts of ~70 lines after ~100 lines are added to the file. It does not print the most recent 30 lines, so it's pretty close...

              – ridthyself
              58 mins ago











            • @ridthyself if you have GNU awk, try adding a fflush(); after the print b[NR-30];. Maybe the output is being buffered.

              – muru
              48 mins ago













            2












            2








            2







            Maybe buffer with awk:



            tail -n +0 -f some/file | awk 'b[NR] = $0 NR > 30 print b[NR-30]; delete b[NR-30] END for (i = NR - 29; i <= NR; i++) print b[i]'


            The awk code, expanded:




            b[NR] = $0 # save the current line in a buffer array

            NR > 30 # once we have more than 30 lines
            print b[NR-30]; # print the line from 30 lines ago
            delete b[NR-30]; # and delete it

            END # once the pipe closes, print the rest
            for (i = NR - 29; i <= NR; i++)
            print b[i]






            share|improve this answer















            Maybe buffer with awk:



            tail -n +0 -f some/file | awk 'b[NR] = $0 NR > 30 print b[NR-30]; delete b[NR-30] END for (i = NR - 29; i <= NR; i++) print b[i]'


            The awk code, expanded:




            b[NR] = $0 # save the current line in a buffer array

            NR > 30 # once we have more than 30 lines
            print b[NR-30]; # print the line from 30 lines ago
            delete b[NR-30]; # and delete it

            END # once the pipe closes, print the rest
            for (i = NR - 29; i <= NR; i++)
            print b[i]







            share|improve this answer














            share|improve this answer



            share|improve this answer








            edited 49 mins ago

























            answered 1 hour ago









            murumuru

            36.8k589163




            36.8k589163












            • This works, but form the script I would expect it to work like tail, printing out a previous line as each new line is added to the file. Instead it prints out in spurts of ~70 lines after ~100 lines are added to the file. It does not print the most recent 30 lines, so it's pretty close...

              – ridthyself
              58 mins ago











            • @ridthyself if you have GNU awk, try adding a fflush(); after the print b[NR-30];. Maybe the output is being buffered.

              – muru
              48 mins ago

















            • This works, but form the script I would expect it to work like tail, printing out a previous line as each new line is added to the file. Instead it prints out in spurts of ~70 lines after ~100 lines are added to the file. It does not print the most recent 30 lines, so it's pretty close...

              – ridthyself
              58 mins ago











            • @ridthyself if you have GNU awk, try adding a fflush(); after the print b[NR-30];. Maybe the output is being buffered.

              – muru
              48 mins ago
















            This works, but form the script I would expect it to work like tail, printing out a previous line as each new line is added to the file. Instead it prints out in spurts of ~70 lines after ~100 lines are added to the file. It does not print the most recent 30 lines, so it's pretty close...

            – ridthyself
            58 mins ago





            This works, but form the script I would expect it to work like tail, printing out a previous line as each new line is added to the file. Instead it prints out in spurts of ~70 lines after ~100 lines are added to the file. It does not print the most recent 30 lines, so it's pretty close...

            – ridthyself
            58 mins ago













            @ridthyself if you have GNU awk, try adding a fflush(); after the print b[NR-30];. Maybe the output is being buffered.

            – muru
            48 mins ago





            @ridthyself if you have GNU awk, try adding a fflush(); after the print b[NR-30];. Maybe the output is being buffered.

            – muru
            48 mins ago













            1














            This isn't very efficient, because it will re-read the file every two seconds, but will do the job:



            watch 'tail -n40 /path/to/file | head -n10'





            share|improve this answer



























              1














              This isn't very efficient, because it will re-read the file every two seconds, but will do the job:



              watch 'tail -n40 /path/to/file | head -n10'





              share|improve this answer

























                1












                1








                1







                This isn't very efficient, because it will re-read the file every two seconds, but will do the job:



                watch 'tail -n40 /path/to/file | head -n10'





                share|improve this answer













                This isn't very efficient, because it will re-read the file every two seconds, but will do the job:



                watch 'tail -n40 /path/to/file | head -n10'






                share|improve this answer












                share|improve this answer



                share|improve this answer










                answered 2 hours ago









                l0b0l0b0

                28.7k19121249




                28.7k19121249




















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