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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 302868783 by RudiC on Monday 28th of October 2013 06:52:46 PM
Old 10-28-2013
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.
 

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LEARN ABOUT OPENSOLARIS

tac

TAC(1)								   User Commands							    TAC(1)

NAME

       tac - concatenate and print files in reverse

SYNOPSIS

       tac [OPTION]... [FILE]...

DESCRIPTION

       Write each FILE to standard output, last line first.  With no FILE, or when FILE is -, read standard input.

       Mandatory arguments to long options are mandatory for short options too.

       -b, --before
	      attach the separator before instead of after

       -r, --regex
	      interpret the separator as a regular expression

       -s, --separator=STRING
	      use STRING as the separator instead of newline

       --help display this help and exit

       --version
	      output version information and exit

AUTHOR

       Written by Jay Lepreau and David MacKenzie.

REPORTING BUGS

       Report bugs to <bug-coreutils@gnu.org>.

COPYRIGHT

       Copyright (C) 2006 Free Software Foundation, Inc.
       This   is   free   software.    You   may   redistribute   copies   of	it   under   the   terms   of	the  GNU  General  Public  License
       <http://www.gnu.org/licenses/gpl.html>.	There is NO WARRANTY, to the extent permitted by law.

SEE ALSO

       The full documentation for tac is maintained as a Texinfo manual.  If the info and tac programs are properly installed at  your	site,  the
       command

	      info tac

       should give you access to the complete manual.

ATTRIBUTES

       See attributes(5) for descriptions of the following attributes:

       +--------------------+--------------------+
       |  ATTRIBUTE TYPE    |  ATTRIBUTE VALUE	 |
       +--------------------+--------------------+
       |Availability	    | SUNWgnu-coreutils  |
       +--------------------+--------------------+
       |Interface Stability | Uncommitted	 |
       +--------------------+--------------------+
NOTES

       Source for GNU coreutils is available on http://opensolaris.org.

tac 6.7 							   December 2006							    TAC(1)
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