Etydhv

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

What exploit Are these user agents trying to use?

What do you call someone who asks many questions?

What killed these X2 caps?

Is there a hemisphere-neutral way of specifying a season?

Unlock My Phone! February 2018

Expand and Contract

Can my sorcerer use a spellbook only to collect spells and scribe scrolls, not cast?

What does the expression "A Mann!" means

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

A category-like structure without composition?

How could indestructible materials be used in power generation?

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

Can I run a new neutral wire to repair a broken circuit?

What is the most common color to indicate the input-field is disabled?

How does a predictive coding aid in lossless compression?

iPad being using in wall mount battery swollen

Intersection Puzzle

Why was the shrinking from 8″ made only to 5.25″ and not smaller (4″ or less)?

Mathematica command that allows it to read my intentions

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

Forgetting the musical notes while performing in concert

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

How can I determine if the org that I'm currently connected to is a scratch org?

Watching something be piped to a file live with tail

How to continue running a program inspite of killing the shell which invoked itLinux terminal, my program only resumes after scrolling the printoutIs there a command in Linux which waits till it will be terminated?Ctrl+c in a sub process is killing a nohup'ed process earlier in the scriptCtrl-C'd an in-place recursive gzip - is this likely to have broken anything?Getting output of another script while preserving line-breakssocat and rich terminal againScript executed as other user from root creates files in wrong directory (root)Dealing with Ctrl+Z in UnixHow to pass Ctrl+C to script called from batch job

3

I have a python program which is, slowly, generating some output.

I want to capture that in a file, but I also thought I could watch it live with tail.

So in one terminal I'm doing :

python myprog.py > output.txt

and in another terminal :

tail -f output.txt

But it seems like the tail isn't showing me anything while the python program is running. If I hit ctrl-c to kill the python script, suddenly the tail of output.txt starts filling up. But not while the python is running.

What am I doing wrong?

linux command-line pipe

share|improve this question

asked 4 hours ago

interstarinterstar

325311

• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  How about python myprog.py | tee output.txt instead?
  
  – n8te
  4 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @n8te tee might show the same problem if the program isn't flushing the output buffer regularly. This needs flush() and tee.
  
  – JPhi1618
  1 hour ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @JPhi1618 - you could be right but I just tested it with 800MB of data output and it didn't skip a beat.
  
  – n8te
  1 hour ago
  

  
  
  
  

add a comment | 

3

I have a python program which is, slowly, generating some output.

I want to capture that in a file, but I also thought I could watch it live with tail.

So in one terminal I'm doing :

python myprog.py > output.txt

and in another terminal :

tail -f output.txt

But it seems like the tail isn't showing me anything while the python program is running. If I hit ctrl-c to kill the python script, suddenly the tail of output.txt starts filling up. But not while the python is running.

What am I doing wrong?

linux command-line pipe

share|improve this question

asked 4 hours ago

interstarinterstar

325311

• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  How about python myprog.py | tee output.txt instead?
  
  – n8te
  4 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @n8te tee might show the same problem if the program isn't flushing the output buffer regularly. This needs flush() and tee.
  
  – JPhi1618
  1 hour ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @JPhi1618 - you could be right but I just tested it with 800MB of data output and it didn't skip a beat.
  
  – n8te
  1 hour ago
  

  
  
  
  

add a comment | 

I have a python program which is, slowly, generating some output.

I want to capture that in a file, but I also thought I could watch it live with tail.

So in one terminal I'm doing :

python myprog.py > output.txt

and in another terminal :

tail -f output.txt

But it seems like the tail isn't showing me anything while the python program is running. If I hit ctrl-c to kill the python script, suddenly the tail of output.txt starts filling up. But not while the python is running.

What am I doing wrong?

linux command-line pipe

share|improve this question

asked 4 hours ago

interstarinterstar

325311

python myprog.py > output.txt

tail -f output.txt

share|improve this question

asked 4 hours ago

interstarinterstar

325311

share|improve this question

asked 4 hours ago

interstarinterstar

325311

asked 4 hours ago

interstarinterstar

325311

asked 4 hours ago

interstarinterstar

325311

2 Answers
2

active

oldest

votes

5

You may also need to explicitly flush the buffer for it to get piped upon generation. This is because output is typically only printed when the pipe's buffer fills up (which is in kilobytes I belive), and when the stdin message ends. This is probably to save on read/writes. You could do this after every print, or if you are looping, after the last print within the loop.

import sys
...
print('Some message')
sys.stdout.flush()

share|improve this answer

answered 4 hours ago

DaveyDavey

661

New contributor

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

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  If you have read this far, please don't be thinking of closing and re-opening the file to do this, the seeks will be a problem, especially for very large files. (I've seen this done!).
  
  – mckenzm
  42 mins ago
  

  
  
  
  

add a comment | 

4

Instead of trying to tail a live file, use tee instead. It was made to do exactly what you're trying to do.

From man tee:

tee(1) - Linux man page

Name tee - read from standard input and write to standard output and files

Synopsis

tee [OPTION]... [FILE]...

Description

Copy standard input to each FILE, and also to standard output.

-a, --append 
 append to the given FILEs, do not overwrite 
-i, --ignore-interrupts 
 ignore interrupt signals 
--help 
 display this help and exit 
--version
 output version information and exit

If a FILE is -, copy again to standard output.

So in your case you'd run:

python myprog.py | tee output.txt

share|improve this answer

answered 4 hours ago

n8ten8te

5,18272234

add a comment | 

Your Answer

StackExchange.ready(function()
var channelOptions =
tags: "".split(" "),
id: "3"
;
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: true,
noModals: true,
showLowRepImageUploadWarning: true,
reputationToPostImages: 10,
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%2fsuperuser.com%2fquestions%2f1421123%2fwatching-something-be-piped-to-a-file-live-with-tail%23new-answer', 'question_page');

);

Post as a guest

Name

Email

Required, but never shown

5

You may also need to explicitly flush the buffer for it to get piped upon generation. This is because output is typically only printed when the pipe's buffer fills up (which is in kilobytes I belive), and when the stdin message ends. This is probably to save on read/writes. You could do this after every print, or if you are looping, after the last print within the loop.

import sys
...
print('Some message')
sys.stdout.flush()

share|improve this answer

answered 4 hours ago

DaveyDavey

661

New contributor

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

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  If you have read this far, please don't be thinking of closing and re-opening the file to do this, the seeks will be a problem, especially for very large files. (I've seen this done!).
  
  – mckenzm
  42 mins ago
  

  
  
  
  

add a comment | 

5

You may also need to explicitly flush the buffer for it to get piped upon generation. This is because output is typically only printed when the pipe's buffer fills up (which is in kilobytes I belive), and when the stdin message ends. This is probably to save on read/writes. You could do this after every print, or if you are looping, after the last print within the loop.

import sys
...
print('Some message')
sys.stdout.flush()

share|improve this answer

answered 4 hours ago

DaveyDavey

661

New contributor

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

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  If you have read this far, please don't be thinking of closing and re-opening the file to do this, the seeks will be a problem, especially for very large files. (I've seen this done!).
  
  – mckenzm
  42 mins ago
  

  
  
  
  

add a comment | 

You may also need to explicitly flush the buffer for it to get piped upon generation. This is because output is typically only printed when the pipe's buffer fills up (which is in kilobytes I belive), and when the stdin message ends. This is probably to save on read/writes. You could do this after every print, or if you are looping, after the last print within the loop.

import sys
...
print('Some message')
sys.stdout.flush()

share|improve this answer

answered 4 hours ago

DaveyDavey

661

New contributor

Davey 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 answer

answered 4 hours ago

DaveyDavey

661

New contributor

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

answered 4 hours ago

DaveyDavey

661

New contributor

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

answered 4 hours ago

DaveyDavey

661

4

Instead of trying to tail a live file, use tee instead. It was made to do exactly what you're trying to do.

From man tee:

tee(1) - Linux man page

Name tee - read from standard input and write to standard output and files

Synopsis

tee [OPTION]... [FILE]...

Description

Copy standard input to each FILE, and also to standard output.

-a, --append 
 append to the given FILEs, do not overwrite 
-i, --ignore-interrupts 
 ignore interrupt signals 
--help 
 display this help and exit 
--version
 output version information and exit

If a FILE is -, copy again to standard output.

So in your case you'd run:

python myprog.py | tee output.txt

share|improve this answer

answered 4 hours ago

n8ten8te

5,18272234

add a comment | 

4

Instead of trying to tail a live file, use tee instead. It was made to do exactly what you're trying to do.

From man tee:

tee(1) - Linux man page

Name tee - read from standard input and write to standard output and files

Synopsis

tee [OPTION]... [FILE]...

Description

Copy standard input to each FILE, and also to standard output.

-a, --append 
 append to the given FILEs, do not overwrite 
-i, --ignore-interrupts 
 ignore interrupt signals 
--help 
 display this help and exit 
--version
 output version information and exit

If a FILE is -, copy again to standard output.

So in your case you'd run:

python myprog.py | tee output.txt

share|improve this answer

answered 4 hours ago

n8ten8te

5,18272234

add a comment | 

Instead of trying to tail a live file, use tee instead. It was made to do exactly what you're trying to do.

From man tee:

tee(1) - Linux man page

Name tee - read from standard input and write to standard output and files

Synopsis

tee [OPTION]... [FILE]...

Description

Copy standard input to each FILE, and also to standard output.

-a, --append 
 append to the given FILEs, do not overwrite 
-i, --ignore-interrupts 
 ignore interrupt signals 
--help 
 display this help and exit 
--version
 output version information and exit

If a FILE is -, copy again to standard output.

So in your case you'd run:

python myprog.py | tee output.txt

share|improve this answer

answered 4 hours ago

n8ten8te

5,18272234

python myprog.py | tee output.txt

share|improve this answer

answered 4 hours ago

n8ten8te

5,18272234

answered 4 hours ago

n8ten8te

5,18272234

answered 4 hours ago

n8ten8te

5,18272234