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












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...










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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







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      share|improve this question







      New contributor




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      asked 3 hours ago









      ridthyselfridthyself

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          3 Answers
          3






          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
            1 hour ago











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

            – muru
            1 hour ago











          • @ridthyself, your awk must be mawk, Try switching to gawk or pass the -W interactive option.

            – Stéphane Chazelas
            17 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































            0














            Same as @muru's but using the modulo operator instead of storing and deleting:



            tail -fn+1 some/file | awk -v n=30 '
            NR > n {print s[NR % n]}
            {s[NR % n] = $0}
            END{for (i = NR - n + 1; i <= NR; i++) print s[i % n]}'





            share|improve this answer
























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              3 Answers
              3






              active

              oldest

              votes








              3 Answers
              3






              active

              oldest

              votes









              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
                1 hour ago











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

                – muru
                1 hour ago











              • @ridthyself, your awk must be mawk, Try switching to gawk or pass the -W interactive option.

                – Stéphane Chazelas
                17 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
                1 hour ago











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

                – muru
                1 hour ago











              • @ridthyself, your awk must be mawk, Try switching to gawk or pass the -W interactive option.

                – Stéphane Chazelas
                17 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 1 hour ago

























              answered 2 hours 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
                1 hour ago











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

                – muru
                1 hour ago











              • @ridthyself, your awk must be mawk, Try switching to gawk or pass the -W interactive option.

                – Stéphane Chazelas
                17 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
                1 hour ago











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

                – muru
                1 hour ago











              • @ridthyself, your awk must be mawk, Try switching to gawk or pass the -W interactive option.

                – Stéphane Chazelas
                17 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
              1 hour 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
              1 hour ago













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

              – muru
              1 hour ago





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

              – muru
              1 hour ago













              @ridthyself, your awk must be mawk, Try switching to gawk or pass the -W interactive option.

              – Stéphane Chazelas
              17 mins ago





              @ridthyself, your awk must be mawk, Try switching to gawk or pass the -W interactive option.

              – Stéphane Chazelas
              17 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























                      0














                      Same as @muru's but using the modulo operator instead of storing and deleting:



                      tail -fn+1 some/file | awk -v n=30 '
                      NR > n {print s[NR % n]}
                      {s[NR % n] = $0}
                      END{for (i = NR - n + 1; i <= NR; i++) print s[i % n]}'





                      share|improve this answer




























                        0














                        Same as @muru's but using the modulo operator instead of storing and deleting:



                        tail -fn+1 some/file | awk -v n=30 '
                        NR > n {print s[NR % n]}
                        {s[NR % n] = $0}
                        END{for (i = NR - n + 1; i <= NR; i++) print s[i % n]}'





                        share|improve this answer


























                          0












                          0








                          0







                          Same as @muru's but using the modulo operator instead of storing and deleting:



                          tail -fn+1 some/file | awk -v n=30 '
                          NR > n {print s[NR % n]}
                          {s[NR % n] = $0}
                          END{for (i = NR - n + 1; i <= NR; i++) print s[i % n]}'





                          share|improve this answer













                          Same as @muru's but using the modulo operator instead of storing and deleting:



                          tail -fn+1 some/file | awk -v n=30 '
                          NR > n {print s[NR % n]}
                          {s[NR % n] = $0}
                          END{for (i = NR - n + 1; i <= NR; i++) print s[i % n]}'






                          share|improve this answer












                          share|improve this answer



                          share|improve this answer










                          answered 19 mins ago









                          Stéphane ChazelasStéphane Chazelas

                          312k57592948




                          312k57592948






















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