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Full Discussion: retain last 1000 line in a file
Top Forums Shell Programming and Scripting retain last 1000 line in a file Post 302420445 by methyl on Tuesday 11th of May 2010 04:28:39 PM
Old 05-11-2010
The approach will depend on the Operating System and the related tools, whether the file is currently open by a process (like syslogd), and whether it is a normal unix text file suitable for processing with shell tools.

Worth looking at the actual size of 1000 records because most versions of "tail" are limited in how much data they will buffer (you could for example ask for 1000 lines and only get 300).

If we assume that the file is a static normal unix text file and not open by another process, my first inclination would be to use "wc" to count the records then "sed" to output the required number of records to a temporary file.
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LEARN ABOUT HPUX

scroll_lines

scroll_lines(5) 						File Formats Manual						   scroll_lines(5)

NAME

       scroll_lines - number of scrollable lines used by the Internal Terminal Emulator

VALUES

   Failsafe
   Default
   Allowed values
   Recommended values
       No larger than is sufficient for a user's purposes.

DESCRIPTION

       This  tunable  specifies the total number of scroll buffer lines used by the HP-UX graphics console Internal Terminal Emulator (ITE).  This
       value is the sum of both on-screen and off-screen lines.  For instance, if the ITE has 68 on-screen lines, and the value of scroll_lines is
       128, then there will be 60 off-screen lines that can be scrolled back into view.

       During  boot, the system may adjust the value of upward, depending upon the installed graphics hardware.  On graphics cards capable of run-
       ning at more than one resolution, the ITE will first determine the maximum number of on-screen lines across all possible resolutions.   The
       ITE will then ensure that is at least equal to this number of on-screen lines.  For example, assume the graphics card supports two possible
       resolutions which result in one ITE resolution of 160 characters by 62 lines and another ITE resolution of 120 characters by 78 lines.	If
       a value of 62 had been specified for in the system file, the ITE would then adjust this value upwards to 78.

   Who Is Expected to Change This Tunable?
       Anyone.

   Restrictions on Changing
       Changes to this tunable take effect at the next reboot.

   When Should the Value of This Tunable Be Raised?
       The value of can be increased if the user would like to retain additional lines in the scroll buffer.

   What Are the Side Effects of Raising the Value of This Tunable?
       Increasing  the	size  of the ITE scroll buffer consumes kernel memory which will be dedicated to the ITE and cannot be used for other pur-
       poses by the rest of the system.  Memory is used at the rate of 2 bytes per off-screen character.

   When Should the Value of This Tunable Be Lowered?
       If the additional off-screen lines in the scroll buffer are not required, can be decreased to free up a small amount of memory.	 The  sav-
       ings  will  be two bytes per off-screen character.  For instance, if the display is 160 characters wide, decreasing by 10 will free up 3200
       bytes.

   What Are the Side Effects of Lowering the Value of This Tunable?
       If is set below or equal to the number of lines displayed on the ITE screen, no off-screen text will be accessible.

   What Other Tunables Should Be Changed at the Same Time?
       None.

WARNINGS

       All HP-UX kernel tunable parameters are release specific.  This parameter may be removed or have its meaning changed in future releases	of
       HP-UX.

       Installation  of  optional  kernel  software, from HP or other vendors, may cause changes to tunable parameter values.  After installation,
       some tunable parameters may no longer be at the default or recommended values.  For information about the effects of installation  on  tun-
       able  values,  consult  the documentation for the kernel software being installed.  For information about optional kernel software that was
       factory installed on your system, see at

AUTHOR

       was developed by HP.

							     Tunable Kernel Parameters						   scroll_lines(5)
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