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Write to file using tail -f through a pipe to grep

 
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# 1  
Old 02-16-2011
Write to file using tail -f through a pipe to grep
Hi -- I'm looking to write to a file after piping output from tail -f through to grep:

#write to a file for all lines with "searchtext" within in error_log:

Expand|Select|Wrap|Line Numbers
  1. tail -f /var/error_log | grep searchtext > output.txt


The above command fails even though the following command::
Expand|Select|Wrap|Line Numbers
  1. tail -f /var/error_log | grep searchtext


does produce output, as expected. Nothing is getting through while redirecting to files after a tail -f... The following work:
Expand|Select|Wrap|Line Numbers
  1. tail -f /var/error_log > output.txt //without grep
  2. tail /var/error_log | grep searchtext > output.txt //without -f


So it's just the combination of grep and tail with specifically the -f (following) option that causes the problem. It would be ideal to be able to do this with a single inline command. Any ideas?

thanks for the help!!
# 2  
Old 02-16-2011
You just have to wait for the buffer to be filed ...,
or use --line-buffered, if your grep implementation supports this (or a similar) option.

The stdout is buffered, when passed through a pipe.
# 3  
Old 02-17-2011
man 1 unbuffer
 
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