Mrthdrb

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Roll the carpet

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Watching something be written to a file live with tail

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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

.everyoneloves__top-leaderboard:empty,.everyoneloves__mid-leaderboard:empty,.everyoneloves__bot-mid-leaderboard:empty margin-bottom:0;

5

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...

linux command-line tail

share|improve this question

asked Apr 3 at 0:36

ridthyselfridthyself

1284

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
  

  
  
  
  
  
  

  
  
  I'm not sure what you mean. When line 31 is written you want line 1 to be printed? So you want a delay? That's not what tail does.
  
  – pipe
  2 days ago
  

  
  
  
  

add a comment | 

5

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...

linux command-line tail

share|improve this question

asked Apr 3 at 0:36

ridthyselfridthyself

1284

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
  

  
  
  
  
  
  

  
  
  I'm not sure what you mean. When line 31 is written you want line 1 to be printed? So you want a delay? That's not what tail does.
  
  – pipe
  2 days ago
  

  
  
  
  

add a comment | 

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

asked Apr 3 at 0:36

ridthyselfridthyself

1284

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

asked Apr 3 at 0:36

ridthyselfridthyself

1284

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

asked Apr 3 at 0:36

ridthyselfridthyself

1284

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.

asked Apr 3 at 0:36

ridthyselfridthyself

1284

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.

asked Apr 3 at 0:36

ridthyselfridthyself

1284

3 Answers
3

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6

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

edited 2 days ago

answered Apr 3 at 1:28

murumuru

37.2k589164

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  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
  2 days ago
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

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

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  @ridthyself, your awk must be mawk, Try switching to gawk or pass the -W interactive option.
  
  – Stéphane Chazelas
  2 days ago
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Once I switched to gawk, it worked perfectly. Thank you! Using tmux to split my wide screen into two columns, I can log the terminal to a file using script, then use this code in the other window to create an console overflow window -- works like a dream.
  
  – ridthyself
  yesterday
  

  
  
  
  

add a comment | 

7

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
 ENDfor (i = NR - n + 1; i <= NR; i++) print s[i % n]'

share|improve this answer

answered 2 days ago

Stéphane ChazelasStéphane Chazelas

313k57592948

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Does this keep every line in s until awk finishes?
  
  – l0b0
  2 days ago
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  @l0b0, that keeps n lines in s, NR%n has values ranging from 0 to n-1
  
  – Stéphane Chazelas
  2 days ago
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Ah, of course, saw the modulo now.
  
  – l0b0
  2 days ago
  

  
  
  
  

add a comment | 

3

This isn't very efficient, because it will re-read the file two seconds after reading it last time, and you will miss lines if the output is coming too fast, but will otherwise do the job:

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

share|improve this answer

edited 2 days ago

answered Apr 3 at 1:04

l0b0l0b0

28.8k19122249

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  What would thi behaviour of this look like if the file takes more than 2 seconds to read?
  
  – Darren H
  2 days ago
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  @DarrenH With watch --precise, I'm not sure, but I would guess it runs the command back-to-back. With plain watch, it should run the tail/head pipe, wait two seconds, run it again, wait another two seconds, and so on.
  
  – a CVn
  2 days ago
  
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Will this meet OP's requirements if more than 30 lines are added to the file per watch interval?
  
  – a CVn
  2 days ago
  

  
  
  
  

add a comment | 

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6

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

edited 2 days ago

answered Apr 3 at 1:28

murumuru

37.2k589164

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  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
  2 days ago
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

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

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  @ridthyself, your awk must be mawk, Try switching to gawk or pass the -W interactive option.
  
  – Stéphane Chazelas
  2 days ago
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Once I switched to gawk, it worked perfectly. Thank you! Using tmux to split my wide screen into two columns, I can log the terminal to a file using script, then use this code in the other window to create an console overflow window -- works like a dream.
  
  – ridthyself
  yesterday
  

  
  
  
  

add a comment | 

6

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

edited 2 days ago

answered Apr 3 at 1:28

murumuru

37.2k589164

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  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
  2 days ago
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

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

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  @ridthyself, your awk must be mawk, Try switching to gawk or pass the -W interactive option.
  
  – Stéphane Chazelas
  2 days ago
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Once I switched to gawk, it worked perfectly. Thank you! Using tmux to split my wide screen into two columns, I can log the terminal to a file using script, then use this code in the other window to create an console overflow window -- works like a dream.
  
  – ridthyself
  yesterday
  

  
  
  
  

add a comment | 

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

edited 2 days ago

answered Apr 3 at 1:28

murumuru

37.2k589164

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]'

 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

edited 2 days ago

answered Apr 3 at 1:28

murumuru

37.2k589164

answered Apr 3 at 1:28

murumuru

37.2k589164

answered Apr 3 at 1:28

murumuru

37.2k589164

7

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
 ENDfor (i = NR - n + 1; i <= NR; i++) print s[i % n]'

share|improve this answer

answered 2 days ago

Stéphane ChazelasStéphane Chazelas

313k57592948

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Does this keep every line in s until awk finishes?
  
  – l0b0
  2 days ago
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  @l0b0, that keeps n lines in s, NR%n has values ranging from 0 to n-1
  
  – Stéphane Chazelas
  2 days ago
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Ah, of course, saw the modulo now.
  
  – l0b0
  2 days ago
  

  
  
  
  

add a comment | 

7

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
 ENDfor (i = NR - n + 1; i <= NR; i++) print s[i % n]'

share|improve this answer

answered 2 days ago

Stéphane ChazelasStéphane Chazelas

313k57592948

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Does this keep every line in s until awk finishes?
  
  – l0b0
  2 days ago
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  @l0b0, that keeps n lines in s, NR%n has values ranging from 0 to n-1
  
  – Stéphane Chazelas
  2 days ago
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Ah, of course, saw the modulo now.
  
  – l0b0
  2 days ago
  

  
  
  
  

add a comment | 

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
 ENDfor (i = NR - n + 1; i <= NR; i++) print s[i % n]'

share|improve this answer

answered 2 days ago

Stéphane ChazelasStéphane Chazelas

313k57592948

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

share|improve this answer

answered 2 days ago

Stéphane ChazelasStéphane Chazelas

313k57592948

answered 2 days ago

Stéphane ChazelasStéphane Chazelas

313k57592948

answered 2 days ago

Stéphane ChazelasStéphane Chazelas

313k57592948

3

This isn't very efficient, because it will re-read the file two seconds after reading it last time, and you will miss lines if the output is coming too fast, but will otherwise do the job:

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

share|improve this answer

edited 2 days ago

answered Apr 3 at 1:04

l0b0l0b0

28.8k19122249

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  What would thi behaviour of this look like if the file takes more than 2 seconds to read?
  
  – Darren H
  2 days ago
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  @DarrenH With watch --precise, I'm not sure, but I would guess it runs the command back-to-back. With plain watch, it should run the tail/head pipe, wait two seconds, run it again, wait another two seconds, and so on.
  
  – a CVn
  2 days ago
  
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Will this meet OP's requirements if more than 30 lines are added to the file per watch interval?
  
  – a CVn
  2 days ago
  

  
  
  
  

add a comment | 

3

This isn't very efficient, because it will re-read the file two seconds after reading it last time, and you will miss lines if the output is coming too fast, but will otherwise do the job:

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

share|improve this answer

edited 2 days ago

answered Apr 3 at 1:04

l0b0l0b0

28.8k19122249

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  What would thi behaviour of this look like if the file takes more than 2 seconds to read?
  
  – Darren H
  2 days ago
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  @DarrenH With watch --precise, I'm not sure, but I would guess it runs the command back-to-back. With plain watch, it should run the tail/head pipe, wait two seconds, run it again, wait another two seconds, and so on.
  
  – a CVn
  2 days ago
  
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Will this meet OP's requirements if more than 30 lines are added to the file per watch interval?
  
  – a CVn
  2 days ago
  

  
  
  
  

add a comment | 

This isn't very efficient, because it will re-read the file two seconds after reading it last time, and you will miss lines if the output is coming too fast, but will otherwise do the job:

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

share|improve this answer

edited 2 days ago

answered Apr 3 at 1:04

l0b0l0b0

28.8k19122249

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

share|improve this answer

edited 2 days ago

answered Apr 3 at 1:04

l0b0l0b0

28.8k19122249

answered Apr 3 at 1:04

l0b0l0b0

28.8k19122249

answered Apr 3 at 1:04

l0b0l0b0

28.8k19122249