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Top Forums UNIX for Dummies Questions & Answers Want to remove all lines but not latest 50 lines from a file Post 302868095 by drl on Friday 25th of October 2013 08:45:04 PM
Old 10-25-2013
Hi.

Modern shells can hold data in memory by using variables. Here's an example:
Code:
#!/usr/bin/env bash

# @(#) s1	Demonstrate keeping lines in shell memory, variables.

# Utility functions: print-as-echo, print-line-with-visual-space, debug.
# export PATH="/usr/local/bin:/usr/bin:/bin"
pe() { for _i;do printf "%s" "$_i";done; printf "\n"; }
pl() { pe;pe "-----" ;pe "$*"; }
db() { ( printf " db, ";for _i;do printf "%s" "$_i";done;printf "\n" ) >&2 ; }
db() { : ; }
C=$HOME/bin/context && [ -f $C ] && $C specimen tail

# Stage new sample data file.
cp data0 data1

FILE=${1-data1}

pl " Input data file $FILE:"
specimen 3 $FILE

pl " Results:"
v1=$( tail -3 $FILE )
rm $FILE
echo "$v1" > $FILE
specimen 3 $FILE

exit 0

producing:
Code:
$ ./s1

Environment: LC_ALL = C, LANG = C
(Versions displayed with local utility "version")
OS, ker|rel, machine: Linux, 2.6.26-2-amd64, x86_64
Distribution        : Debian GNU/Linux 5.0.8 (lenny, workstation) 
bash GNU bash 3.2.39
specimen (local) 1.17
tail (GNU coreutils) 6.10

-----
 Input data file data1:
Edges: 3:0:3 of 26 lines in file "data1"
Four score and seven years ago our fathers brought forth on this
continent a new nation, conceived in Liberty, and dedicated to
the proposition that all men are created equal.
   ---
under God, shall have a new birth of freedom--and that
government: of the people, by the people, for the people, shall
not perish from the earth.

-----
 Results:
Whole: 3:0:3 of 3 lines in file "data1"
under God, shall have a new birth of freedom--and that
government: of the people, by the people, for the people, shall
not perish from the earth.

So this holds the last 3 lines of Lincoln's Gettysburg address in memory, variable v1 , then removes the file, and finally writes the saved data to the file. One could also just truncate the file if one wanted to keep the inode.

This obviously has risks -- if the code is wrong or if something else bad happens, it's possible that the entire file could be lost. Of course, one has backups for such occasions.

This method works in the versions of Linux and bash as noted. Will it work in your system? I don't know, but you could test it, and, as usual, be prepared with a backup if things go awry.

Best wishes ... cheers, drl

( Originally posted by mistake in thread No Paging Space Available - The UNIX and Linux Forums )
 

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TAIL(1) 							   User Commands							   TAIL(1)

NAME
tail - output the last part of files SYNOPSIS
tail [OPTION]... [FILE]... DESCRIPTION
Print the last 10 lines of each FILE to standard output. With more than one FILE, precede each with a header giving the file name. With no FILE, or when FILE is -, read standard input. Mandatory arguments to long options are mandatory for short options too. -c, --bytes=N output the last N bytes; alternatively, use +N to output bytes starting with the Nth of each file -f, --follow[={name|descriptor}] output appended data as the file grows; -f, --follow, and --follow=descriptor are equivalent -F same as --follow=name --retry -n, --lines=N output the last N lines, instead of the last 10; or use +N to output lines starting with the Nth --max-unchanged-stats=N with --follow=name, reopen a FILE which has not changed size after N (default 5) iterations to see if it has been unlinked or renamed (this is the usual case of rotated log files) --pid=PID with -f, terminate after process ID, PID dies -q, --quiet, --silent never output headers giving file names --retry keep trying to open a file even when it is or becomes inaccessible; useful when following by name, i.e., with --follow=name -s, --sleep-interval=S with -f, sleep for approximately S seconds (default 1.0) between iterations -v, --verbose always output headers giving file names --help display this help and exit --version output version information and exit If the first character of N (the number of bytes or lines) is a `+', print beginning with the Nth item from the start of each file, other- wise, print the last N items in the file. N may have a multiplier suffix: b 512, kB 1000, K 1024, MB 1000*1000, M 1024*1024, GB 1000*1000*1000, G 1024*1024*1024, and so on for T, P, E, Z, Y. With --follow (-f), tail defaults to following the file descriptor, which means that even if a tail'ed file is renamed, tail will continue to track its end. This default behavior is not desirable when you really want to track the actual name of the file, not the file descrip- tor (e.g., log rotation). Use --follow=name in that case. That causes tail to track the named file by reopening it periodically to see if it has been removed and recreated by some other program. AUTHOR
Written by Paul Rubin, David MacKenzie, Ian Lance Taylor, and Jim Meyering. REPORTING BUGS
Report tail bugs to bug-coreutils@gnu.org GNU coreutils home page: <http://www.gnu.org/software/coreutils/> General help using GNU software: <http://www.gnu.org/gethelp/> COPYRIGHT
Copyright (C) 2009 Free Software Foundation, Inc. License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>. This is free software: you are free to change and redistribute it. There is NO WARRANTY, to the extent permitted by law. SEE ALSO
The full documentation for tail is maintained as a Texinfo manual. If the info and tail programs are properly installed at your site, the command info coreutils 'tail invocation' should give you access to the complete manual. GNU coreutils 7.1 July 2010 TAIL(1)
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