Ttdfjt

Should I tell management that I intend to leave due to bad software development practices?

One verb to replace 'be a member of' a club

Alternative to sending password over mail?

Am I breaking OOP practice with this architecture?

Why is this clock signal connected to a capacitor to gnd?

How would I stat a creature to be immune to everything but the Magic Missile spell? (just for fun)

How dangerous is XSS?

If human space travel is limited by the G force vulnerability, is there a way to counter G forces?

What about the virus in 12 Monkeys?

Do UK voters know if their MP will be the Speaker of the House?

A category-like structure without composition?

How can saying a song's name be a copyright violation?

Size of subfigure fitting its content (tikzpicture)

Short story with a alien planet, government officials must wear exploding medallions

GFCI outlets - can they be repaired? Are they really needed at the end of a circuit?

How do conventional missiles fly?

How could indestructible materials be used in power generation?

How do I gain back my faith in my PhD degree?

What method can I use to design a dungeon difficult enough that the PCs can't make it through without killing them?

Why didn't Miles's spider sense work before?

In 'Revenger,' what does 'cove' come from?

Ambiguity in the definition of entropy

Avoiding the "not like other girls" trope?

How much of data wrangling is a data scientist's job?

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

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 yesterday

ridthyselfridthyself

1263

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

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  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
  11 hours 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 yesterday

ridthyselfridthyself

1263

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.

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  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
  11 hours 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 yesterday

ridthyselfridthyself

1263

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 yesterday

ridthyselfridthyself

1263

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 yesterday

ridthyselfridthyself

1263

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 yesterday

ridthyselfridthyself

1263

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 yesterday

ridthyselfridthyself

1263

3 Answers
3

active

oldest

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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 23 hours ago

answered yesterday

murumuru

37k589164

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  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
  23 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

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

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

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

  
  
  
  

add a comment | 

6

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 22 hours ago

Stéphane ChazelasStéphane Chazelas

312k57592948

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Does this keep every line in s until awk finishes?
  
  – l0b0
  7 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

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

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Ah, of course, saw the modulo now.
  
  – l0b0
  6 hours 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 7 hours ago

answered yesterday

l0b0l0b0

28.8k19121249

• 
  
  
  

  
  

  
  
  
  
  
  

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

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @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
  11 hours ago
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Will this meet OP's requirements if more than 30 lines are added to the file per watch interval?
  
  – a CVn
  11 hours 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 23 hours ago

answered yesterday

murumuru

37k589164

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  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
  23 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

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

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

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

  
  
  
  

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 23 hours ago

answered yesterday

murumuru

37k589164

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  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
  23 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

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

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

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

  
  
  
  

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 23 hours ago

answered yesterday

murumuru

37k589164

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 23 hours ago

answered yesterday

murumuru

37k589164

answered yesterday

murumuru

37k589164

answered yesterday

murumuru

37k589164

6

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 22 hours ago

Stéphane ChazelasStéphane Chazelas

312k57592948

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Does this keep every line in s until awk finishes?
  
  – l0b0
  7 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

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

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Ah, of course, saw the modulo now.
  
  – l0b0
  6 hours ago
  

  
  
  
  

add a comment | 

6

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 22 hours ago

Stéphane ChazelasStéphane Chazelas

312k57592948

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Does this keep every line in s until awk finishes?
  
  – l0b0
  7 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

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

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Ah, of course, saw the modulo now.
  
  – l0b0
  6 hours 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 22 hours ago

Stéphane ChazelasStéphane Chazelas

312k57592948

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 22 hours ago

Stéphane ChazelasStéphane Chazelas

312k57592948

answered 22 hours ago

Stéphane ChazelasStéphane Chazelas

312k57592948

answered 22 hours ago

Stéphane ChazelasStéphane Chazelas

312k57592948

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 7 hours ago

answered yesterday

l0b0l0b0

28.8k19121249

• 
  
  
  

  
  

  
  
  
  
  
  

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

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @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
  11 hours ago
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Will this meet OP's requirements if more than 30 lines are added to the file per watch interval?
  
  – a CVn
  11 hours 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 7 hours ago

answered yesterday

l0b0l0b0

28.8k19121249

• 
  
  
  

  
  

  
  
  
  
  
  

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

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @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
  11 hours ago
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Will this meet OP's requirements if more than 30 lines are added to the file per watch interval?
  
  – a CVn
  11 hours 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 7 hours ago

answered yesterday

l0b0l0b0

28.8k19121249

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

share|improve this answer

edited 7 hours ago

answered yesterday

l0b0l0b0

28.8k19121249

answered yesterday

l0b0l0b0

28.8k19121249

answered yesterday

l0b0l0b0

28.8k19121249