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Full Discussion: Want to remove all lines but not latest 50 lines from a file
Top Forums UNIX for Dummies Questions & Answers Want to remove all lines but not latest 50 lines from a file Post 302868901 by alister on Tuesday 29th of October 2013 08:24:39 AM
Old 10-29-2013
Quote:
Originally Posted by RudiC
For discussion:
Code:
tac < file | head -5 | tac | tee file >/dev/null

will keep the data stream within the pipes, by the input redirection file is opened for read early by the shell, before tee will write to it. The whole thing is pretty fast, although it will read the entire file. It keeps the inode of file. I'm not sure if and where the caveats are. I tested it with 10 million lines to get them down to 5000 on my Linux system.
As drl demonstrated, this solution is not reliable.

The problem with this approach is that we cannot make any assumptions about when tee will truncate the file. The elements of a pipeline need not be created and scheduled sequentially. Even if they are, the number and size of the pipeline's buffers (both in the kernel and userspace) impose an upper limit on the amount of data that can be moved before truncation.
Code:
$ seq 100000 > data
$ wc -l < data
100000

$ cp data data.bkp
$ tac data | tee data >/dev/null
tac: data: read error
$ wc -l < data
24420

$ cp data.bkp data
$ tac data | head -n 100000 | tee data >/dev/null
tac: data: read error
$ wc -l < data
46266

$ cp data.bkp data
$ tac data | head -n 100000 | head -n 100000 | tee data >/dev/null
tac: data: read error
$ wc -l < data
68111

$ cp data.bkp data
$ tac data | head -n 100000 | head -n 100000 | head -n 100000 | tee data >/dev/null
tac: data: read error
$ wc -l < data
98148

With enough buffering, you might get lucky ...
Code:
$ cp data.bkp data
$ tac data | head -n 100000 | head -n 100000 | head -n 100000 | head -n 100000 | tee data >/dev/null
$ wc -l < data
100000

... or not.
Code:
$ cp data.bkp data
$ tac data | head -n 100000 | head -n 100000 | head -n 100000 | head -n 100000 | tee data >/dev/null
tac: data: read error
$ wc -l < data
80399

Regards,
Alister
This User Gave Thanks to alister For This Post:
RudiC
 

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