Trjtdtk

What does it mean that physics no longer uses mechanical models to describe phenomena?

Is there any word for a place full of confusion?

How to unroll a parameter pack from right to left

How to run automated tests after each commit?

Sum letters are not two different

Can you explain what "processes and tools" means in the first Agile principle?

Did any compiler fully use 80-bit floating point?

What do you call the main part of a joke?

Would it be easier to apply for a UK visa if there is a host family to sponsor for you in going there?

Who can remove European Commissioners?

Strange behavior of Object.defineProperty() in JavaScript

Movie where a circus ringmaster turns people into animals

How to compare two different files line by line in unix?

How to pronounce 伝統色

A letter with no particular backstory

How much damage would a cupful of neutron star matter do to the Earth?

Is there public access to the Meteor Crater in Arizona?

What does the distribution of bootstrapped values in this Cullen and Frey Graph tell me?

How does light 'choose' between wave and particle behaviour?

Was this incident in Captain America: Civil War officially confirmed to be a consequence of Spider-Man’s Spidey sense?

In musical terms, what properties are varied by the human voice to produce different words / syllables?

How often does castling occur in grandmaster games?

How do I tell what width chain my used chainring needs?

Central Vacuuming: Is it worth it, and how does it compare to normal vacuuming?

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

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

6

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

1334

• 
  
  
  

  
  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
  Apr 3 at 14:58
  

  
  
  
  

add a comment | 

6

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

1334

• 
  
  
  

  
  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
  Apr 3 at 14:58
  

  
  
  
  

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

1334

share|improve this question

asked Apr 3 at 0:36

ridthyselfridthyself

1334

share|improve this question

asked Apr 3 at 0:36

ridthyselfridthyself

1334

asked Apr 3 at 0:36

ridthyselfridthyself

1334

asked Apr 3 at 0:36

ridthyselfridthyself

1334

3 Answers
3

active

oldest

votes

7

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 Apr 3 at 2:35

answered Apr 3 at 1:28

murumuru

38k590166

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  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
  Apr 3 at 2:25
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  @ridthyself if you have GNU awk, try adding a fflush(); after the print b[NR-30];. Maybe the output is being buffered.
  
  – muru
  Apr 3 at 2:35
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  @ridthyself, your awk must be mawk, Try switching to gawk or pass the -W interactive option.
  
  – Stéphane Chazelas
  Apr 3 at 3:31
  

  
  
  
  


• 
  
  
  

  
  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
  Apr 4 at 22:02
  

  
  
  
  

add a comment | 

8

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 Apr 3 at 3:28

Stéphane ChazelasStéphane Chazelas

315k57597955

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Does this keep every line in s until awk finishes?
  
  – l0b0
  Apr 3 at 18:31
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  @l0b0, that keeps n lines in s, NR%n has values ranging from 0 to n-1
  
  – Stéphane Chazelas
  Apr 3 at 19:05
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Ah, of course, saw the modulo now.
  
  – l0b0
  Apr 3 at 19:36
  

  
  
  
  

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 Apr 3 at 18:27

answered Apr 3 at 1:04

l0b0l0b0

28.9k20122249

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  What would thi behaviour of this look like if the file takes more than 2 seconds to read?
  
  – Darren H
  Apr 3 at 6:10
  

  
  
  
  


• 
  
  
  

  
  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
  Apr 3 at 14:54
  
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Will this meet OP's requirements if more than 30 lines are added to the file per watch interval?
  
  – a CVn
  Apr 3 at 14:56
  

  
  
  
  

add a comment | 

Your Answer

StackExchange.ready(function()
var channelOptions =
tags: "".split(" "),
id: "106"
;
initTagRenderer("".split(" "), "".split(" "), channelOptions);

StackExchange.using("externalEditor", function()
// Have to fire editor after snippets, if snippets enabled
if (StackExchange.settings.snippets.snippetsEnabled)
StackExchange.using("snippets", function()
createEditor();
);

else
createEditor();

);

function createEditor()
StackExchange.prepareEditor(
heartbeatType: 'answer',
autoActivateHeartbeat: false,
convertImagesToLinks: false,
noModals: true,
showLowRepImageUploadWarning: true,
reputationToPostImages: null,
bindNavPrevention: true,
postfix: "",
imageUploader:
brandingHtml: "Powered by u003ca class="icon-imgur-white" href="https://imgur.com/"u003eu003c/au003e",
contentPolicyHtml: "User contributions licensed under u003ca href="https://creativecommons.org/licenses/by-sa/3.0/"u003ecc by-sa 3.0 with attribution requiredu003c/au003e u003ca href="https://stackoverflow.com/legal/content-policy"u003e(content policy)u003c/au003e",
allowUrls: true
,
onDemand: true,
discardSelector: ".discard-answer"
,immediatelyShowMarkdownHelp:true
);



);

draft saved

draft discarded

Sign up or log in

StackExchange.ready(function ()
StackExchange.helpers.onClickDraftSave('#login-link');
);

Sign up using Google

Sign up using Facebook

Sign up using Email and Password

Post as a guest

Name

Email

Required, but never shown

StackExchange.ready(
function ()
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2funix.stackexchange.com%2fquestions%2f510178%2fusing-tail-to-follow-a-file-without-displaying-the-most-recent-lines%23new-answer', 'question_page');

);

Post as a guest

Name

Email

Required, but never shown

7

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 Apr 3 at 2:35

answered Apr 3 at 1:28

murumuru

38k590166

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  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
  Apr 3 at 2:25
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  @ridthyself if you have GNU awk, try adding a fflush(); after the print b[NR-30];. Maybe the output is being buffered.
  
  – muru
  Apr 3 at 2:35
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  @ridthyself, your awk must be mawk, Try switching to gawk or pass the -W interactive option.
  
  – Stéphane Chazelas
  Apr 3 at 3:31
  

  
  
  
  


• 
  
  
  

  
  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
  Apr 4 at 22:02
  

  
  
  
  

add a comment | 

7

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 Apr 3 at 2:35

answered Apr 3 at 1:28

murumuru

38k590166

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  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
  Apr 3 at 2:25
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  @ridthyself if you have GNU awk, try adding a fflush(); after the print b[NR-30];. Maybe the output is being buffered.
  
  – muru
  Apr 3 at 2:35
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  @ridthyself, your awk must be mawk, Try switching to gawk or pass the -W interactive option.
  
  – Stéphane Chazelas
  Apr 3 at 3:31
  

  
  
  
  


• 
  
  
  

  
  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
  Apr 4 at 22:02
  

  
  
  
  

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 Apr 3 at 2:35

answered Apr 3 at 1:28

murumuru

38k590166

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 Apr 3 at 2:35

answered Apr 3 at 1:28

murumuru

38k590166

answered Apr 3 at 1:28

murumuru

38k590166

answered Apr 3 at 1:28

murumuru

38k590166

8

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 Apr 3 at 3:28

Stéphane ChazelasStéphane Chazelas

315k57597955

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Does this keep every line in s until awk finishes?
  
  – l0b0
  Apr 3 at 18:31
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  @l0b0, that keeps n lines in s, NR%n has values ranging from 0 to n-1
  
  – Stéphane Chazelas
  Apr 3 at 19:05
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Ah, of course, saw the modulo now.
  
  – l0b0
  Apr 3 at 19:36
  

  
  
  
  

add a comment | 

8

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 Apr 3 at 3:28

Stéphane ChazelasStéphane Chazelas

315k57597955

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Does this keep every line in s until awk finishes?
  
  – l0b0
  Apr 3 at 18:31
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  @l0b0, that keeps n lines in s, NR%n has values ranging from 0 to n-1
  
  – Stéphane Chazelas
  Apr 3 at 19:05
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Ah, of course, saw the modulo now.
  
  – l0b0
  Apr 3 at 19:36
  

  
  
  
  

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 Apr 3 at 3:28

Stéphane ChazelasStéphane Chazelas

315k57597955

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 Apr 3 at 3:28

Stéphane ChazelasStéphane Chazelas

315k57597955

answered Apr 3 at 3:28

Stéphane ChazelasStéphane Chazelas

315k57597955

answered Apr 3 at 3:28

Stéphane ChazelasStéphane Chazelas

315k57597955

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 Apr 3 at 18:27

answered Apr 3 at 1:04

l0b0l0b0

28.9k20122249

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  What would thi behaviour of this look like if the file takes more than 2 seconds to read?
  
  – Darren H
  Apr 3 at 6:10
  

  
  
  
  


• 
  
  
  

  
  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
  Apr 3 at 14:54
  
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Will this meet OP's requirements if more than 30 lines are added to the file per watch interval?
  
  – a CVn
  Apr 3 at 14:56
  

  
  
  
  

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 Apr 3 at 18:27

answered Apr 3 at 1:04

l0b0l0b0

28.9k20122249

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  What would thi behaviour of this look like if the file takes more than 2 seconds to read?
  
  – Darren H
  Apr 3 at 6:10
  

  
  
  
  


• 
  
  
  

  
  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
  Apr 3 at 14:54
  
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Will this meet OP's requirements if more than 30 lines are added to the file per watch interval?
  
  – a CVn
  Apr 3 at 14:56
  

  
  
  
  

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 Apr 3 at 18:27

answered Apr 3 at 1:04

l0b0l0b0

28.9k20122249

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

share|improve this answer

edited Apr 3 at 18:27

answered Apr 3 at 1:04

l0b0l0b0

28.9k20122249

answered Apr 3 at 1:04

l0b0l0b0

28.9k20122249

answered Apr 3 at 1:04

l0b0l0b0

28.9k20122249