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Output redirection is not working with top piped to grep

3

1

When I run this command in Linux:

$ top -b -d 20 | grep "load average" -A 20 > top.log

top.log is always empty.

But when I run this command:

$ top -b -d 20 | grep "load average" -A 20 | tee top.log

then top.log has contents.

If I don't want to use tee (because I don't want the output to be displayed onto the console), how do I correct the first command so that top.log is updated?

My machine uses CentOS Linux 7 (Core).

centos pipe io-redirection top

share|improve this question

asked yesterday

kurtkurt

1183

New contributor

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

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  weird. In my Cento7 machine, both commands work well.
  
  – JinChin
  yesterday
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  if you want only load average, why not use uptime ?
  
  – Archemar
  yesterday
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @Archemar What is the -A option of grep for?
  
  – Uncle Billy
  yesterday
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @Archemar @UncleBilly, i'm using a trick for limiting the amount of entries returned by top.
  
  – kurt
  yesterday
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @UncleBilly Ah, yes that is the catch ...
  
  – Archemar
  yesterday
  

  
  
  
  

add a comment | 

3

1

When I run this command in Linux:

$ top -b -d 20 | grep "load average" -A 20 > top.log

top.log is always empty.

But when I run this command:

$ top -b -d 20 | grep "load average" -A 20 | tee top.log

then top.log has contents.

If I don't want to use tee (because I don't want the output to be displayed onto the console), how do I correct the first command so that top.log is updated?

My machine uses CentOS Linux 7 (Core).

centos pipe io-redirection top

share|improve this question

asked yesterday

kurtkurt

1183

New contributor

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

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  weird. In my Cento7 machine, both commands work well.
  
  – JinChin
  yesterday
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  if you want only load average, why not use uptime ?
  
  – Archemar
  yesterday
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @Archemar What is the -A option of grep for?
  
  – Uncle Billy
  yesterday
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @Archemar @UncleBilly, i'm using a trick for limiting the amount of entries returned by top.
  
  – kurt
  yesterday
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @UncleBilly Ah, yes that is the catch ...
  
  – Archemar
  yesterday
  

  
  
  
  

add a comment | 

When I run this command in Linux:

$ top -b -d 20 | grep "load average" -A 20 > top.log

top.log is always empty.

But when I run this command:

$ top -b -d 20 | grep "load average" -A 20 | tee top.log

then top.log has contents.

If I don't want to use tee (because I don't want the output to be displayed onto the console), how do I correct the first command so that top.log is updated?

My machine uses CentOS Linux 7 (Core).

centos pipe io-redirection top

share|improve this question

asked yesterday

kurtkurt

1183

New contributor

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

$ top -b -d 20 | grep "load average" -A 20 > top.log

$ top -b -d 20 | grep "load average" -A 20 | tee top.log

share|improve this question

asked yesterday

kurtkurt

1183

New contributor

kurt 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

kurtkurt

1183

New contributor

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

asked yesterday

kurtkurt

1183

New contributor

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

asked yesterday

kurtkurt

1183

2 Answers
2

active

oldest

votes

3

You should use the --line-buffered option of grep (since your question is tagged "centos", you're most certainly using GNU grep).

By default, grep will only use line buffering when the output is a terminal (just like stdio functions: printf, puts, etc). The --line-buffered option is overriding that. GNU coreutils also has a stdbuf(1) wrapper that should work with any dynamically linked program which is using stdio.

share|improve this answer

edited yesterday

answered yesterday

Uncle BillyUncle Billy

8778

add a comment | 

-1

You can use cat /proc/loadavg at intervals. Why pull it from top? It is intended for active monitoring (mostly), and will always use more resources than a fast read of loadavg. For the process listing, just use ps, and sort with the —sort flag. E.g.

( while true ; do cat /proc/loadavg ; ps -aux | sort -nrk 3,3 | head -n 20 ; sleep 3 ; done ; ) | pipe ...

or

( while true ; do cat /proc/loadavg ; ps -Ao user,uid,pid,comm,pcpu,tty —sort=-pcpu —no-headers | head -n 20 ; sleep 3 ; done ; ) | pipe ...

share|improve this answer

edited 23 hours ago

answered yesterday

user2497user2497

588137

add a comment | 

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3

You should use the --line-buffered option of grep (since your question is tagged "centos", you're most certainly using GNU grep).

By default, grep will only use line buffering when the output is a terminal (just like stdio functions: printf, puts, etc). The --line-buffered option is overriding that. GNU coreutils also has a stdbuf(1) wrapper that should work with any dynamically linked program which is using stdio.

share|improve this answer

edited yesterday

answered yesterday

Uncle BillyUncle Billy

8778

add a comment | 

3

You should use the --line-buffered option of grep (since your question is tagged "centos", you're most certainly using GNU grep).

By default, grep will only use line buffering when the output is a terminal (just like stdio functions: printf, puts, etc). The --line-buffered option is overriding that. GNU coreutils also has a stdbuf(1) wrapper that should work with any dynamically linked program which is using stdio.

share|improve this answer

edited yesterday

answered yesterday

Uncle BillyUncle Billy

8778

add a comment | 

You should use the --line-buffered option of grep (since your question is tagged "centos", you're most certainly using GNU grep).

By default, grep will only use line buffering when the output is a terminal (just like stdio functions: printf, puts, etc). The --line-buffered option is overriding that. GNU coreutils also has a stdbuf(1) wrapper that should work with any dynamically linked program which is using stdio.

share|improve this answer

edited yesterday

answered yesterday

Uncle BillyUncle Billy

8778

share|improve this answer

edited yesterday

answered yesterday

Uncle BillyUncle Billy

8778

answered yesterday

Uncle BillyUncle Billy

8778

answered yesterday

Uncle BillyUncle Billy

8778

-1

You can use cat /proc/loadavg at intervals. Why pull it from top? It is intended for active monitoring (mostly), and will always use more resources than a fast read of loadavg. For the process listing, just use ps, and sort with the —sort flag. E.g.

( while true ; do cat /proc/loadavg ; ps -aux | sort -nrk 3,3 | head -n 20 ; sleep 3 ; done ; ) | pipe ...

or

( while true ; do cat /proc/loadavg ; ps -Ao user,uid,pid,comm,pcpu,tty —sort=-pcpu —no-headers | head -n 20 ; sleep 3 ; done ; ) | pipe ...

share|improve this answer

edited 23 hours ago

answered yesterday

user2497user2497

588137

add a comment | 

-1

You can use cat /proc/loadavg at intervals. Why pull it from top? It is intended for active monitoring (mostly), and will always use more resources than a fast read of loadavg. For the process listing, just use ps, and sort with the —sort flag. E.g.

( while true ; do cat /proc/loadavg ; ps -aux | sort -nrk 3,3 | head -n 20 ; sleep 3 ; done ; ) | pipe ...

or

( while true ; do cat /proc/loadavg ; ps -Ao user,uid,pid,comm,pcpu,tty —sort=-pcpu —no-headers | head -n 20 ; sleep 3 ; done ; ) | pipe ...

share|improve this answer

edited 23 hours ago

answered yesterday

user2497user2497

588137

add a comment | 

You can use cat /proc/loadavg at intervals. Why pull it from top? It is intended for active monitoring (mostly), and will always use more resources than a fast read of loadavg. For the process listing, just use ps, and sort with the —sort flag. E.g.

( while true ; do cat /proc/loadavg ; ps -aux | sort -nrk 3,3 | head -n 20 ; sleep 3 ; done ; ) | pipe ...

or

( while true ; do cat /proc/loadavg ; ps -Ao user,uid,pid,comm,pcpu,tty —sort=-pcpu —no-headers | head -n 20 ; sleep 3 ; done ; ) | pipe ...

share|improve this answer

edited 23 hours ago

answered yesterday

user2497user2497

588137

share|improve this answer

edited 23 hours ago

answered yesterday

user2497user2497

588137

answered yesterday

user2497user2497

588137

answered yesterday

user2497user2497

588137