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Linux 2.6 - man page for tail (linux section 1posix)

TAIL(P) 			    POSIX Programmer's Manual				  TAIL(P)

NAME
       tail - copy the last part of a file

SYNOPSIS
       tail [-f][ -c number| -n number][file]

DESCRIPTION
       The  tail  utility  shall copy its input file to the standard output beginning at a desig-
       nated place.

       Copying shall begin at the point in the file indicated by  the  -c  number  or  -n  number
       options. The option-argument number shall be counted in units of lines or bytes, according
       to the options -n and -c. Both line and byte counts start from 1.

       Tails relative to the end of the file may be saved in an internal buffer, and thus may  be
       limited in length. Such a buffer, if any, shall be no smaller than {LINE_MAX}*10 bytes.

OPTIONS
       The  tail  utility  shall  conform to the Base Definitions volume of IEEE Std 1003.1-2001,
       Section 12.2, Utility Syntax Guidelines.

       The following options shall be supported:

       -c  number
	      The application shall ensure that the number option-argument is a  decimal  integer
	      whose  sign affects the location in the file, measured in bytes, to begin the copy-
	      ing:

				 Sign	Copying Starts
				 +	Relative to the beginning of the file.
				 -	Relative to the end of the file.
				 none	Relative to the end of the file.

       The origin for counting shall be 1; that is, -c +1 represents the first byte of the  file,
       -c -1 the last.

       -f     If the input file is a regular file or if the file operand specifies a FIFO, do not
	      terminate after the last line of the input file has been copied, but read and  copy
	      further bytes from the input file when they become available. If no file operand is
	      specified and standard input is a pipe, the -f option  shall  be	ignored.  If  the
	      input  file  is not a FIFO, pipe, or regular file, it is unspecified whether or not
	      the -f option shall be ignored.

       -n  number
	      This option shall be equivalent to -c number, except the starting location  in  the
	      file  shall be measured in lines instead of bytes. The origin for counting shall be
	      1; that is, -n +1 represents the first line of the file, -n -1 the last.

       If neither -c nor -n is specified, -n 10 shall be assumed.

OPERANDS
       The following operand shall be supported:

       file   A pathname of an input file. If no file operands are specified, the standard  input
	      shall be used.

STDIN
       The  standard  input  shall  be used only if no file operands are specified. See the INPUT
       FILES section.

INPUT FILES
       If the -c option is specified, the input file can contain arbitrary data;  otherwise,  the
       input file shall be a text file.

ENVIRONMENT VARIABLES
       The following environment variables shall affect the execution of tail:

       LANG   Provide  a  default  value for the internationalization variables that are unset or
	      null. (See the Base Definitions volume of IEEE Std 1003.1-2001, Section 8.2, Inter-
	      nationalization Variables for the precedence of internationalization variables used
	      to determine the values of locale categories.)

       LC_ALL If set to a non-empty string value, override the values of all the  other  interna-
	      tionalization variables.

       LC_CTYPE
	      Determine  the  locale for the interpretation of sequences of bytes of text data as
	      characters (for example, single-byte as opposed to multi-byte characters	in  argu-
	      ments and input files).

       LC_MESSAGES
	      Determine the locale that should be used to affect the format and contents of diag-
	      nostic messages written to standard error.

       NLSPATH
	      Determine the location of message catalogs for the processing of LC_MESSAGES .

ASYNCHRONOUS EVENTS
       Default.

STDOUT
       The designated portion of the input file shall be written to standard output.

STDERR
       The standard error shall be used only for diagnostic messages.

OUTPUT FILES
       None.

EXTENDED DESCRIPTION
       None.

EXIT STATUS
       The following exit values shall be returned:

	0     Successful completion.

       >0     An error occurred.

CONSEQUENCES OF ERRORS
       Default.

       The following sections are informative.

APPLICATION USAGE
       The -c option should be used with caution when the input is a text file containing  multi-
       byte characters; it may produce output that does not start on a character boundary.

       Although  the  input  file to tail can be any type, the results might not be what would be
       expected on some character special device files or on file types not described by the Sys-
       tem  Interfaces	volume of IEEE Std 1003.1-2001. Since this volume of IEEE Std 1003.1-2001
       does not specify the block size used when doing input, tail need not read all of the  data
       from devices that only perform block transfers.

EXAMPLES
       The  -f	option	can be used to monitor the growth of a file that is being written by some
       other process. For example, the command:

	      tail -f fred

       prints the last ten lines of the file fred, followed by any lines  that	are  appended  to
       fred between the time tail is initiated and killed. As another example, the command:

	      tail -f -c 15 fred

       prints the last 15 bytes of the file fred, followed by any bytes that are appended to fred
       between the time tail is initiated and killed.

RATIONALE
       This version of tail was created to allow conformance to the  Utility  Syntax  Guidelines.
       The historical -b option was omitted because of the general non-portability of block-sized
       units of text.  The  -c	option	historically  meant  "characters",  but  this  volume  of
       IEEE Std 1003.1-2001  indicates	that it means "bytes". This was selected to allow reason-
       able implementations when multi-byte characters are possible; it was not named -b to avoid
       confusion with the historical -b.

       The  origin  of	counting  both	lines  and bytes is 1, matching all widespread historical
       implementations.

       The restriction on the internal buffer is a compromise between  the  historical	System	V
       implementation of 4096 bytes and the BSD 32768 bytes.

       The -f option has been implemented as a loop that sleeps for 1 second and copies any bytes
       that are available. This is sufficient, but if more efficient methods of determining  when
       new data are available are developed, implementations are encouraged to use them.

       Historical  documentation indicates that tail ignores the -f option if the input file is a
       pipe (pipe and FIFO on systems that support FIFOs). On BSD-based systems,  this	has  been
       true;  on  System V-based systems, this was true when input was taken from standard input,
       but it did not ignore the -f flag if a FIFO was named as the file operand.  Since  the  -f
       option  is not useful on pipes and all historical implementations ignore -f if no file op-
       erand is specified and standard input is  a  pipe,  this  volume  of  IEEE Std 1003.1-2001
       requires  this  behavior. However, since the -f option is useful on a FIFO, this volume of
       IEEE Std 1003.1-2001 also requires that if standard input is a FIFO or a  FIFO  is  named,
       the  -f	option	shall not be ignored. Although historical behavior does not ignore the -f
       option for other file types, this is unspecified so that implementations  are  allowed  to
       ignore the -f option if it is known that the file cannot be extended.

       This  was  changed  to  the current form based on comments noting that -c was almost never
       used without specifying a number and that there was no need to specify -l if -n number was
       given.

FUTURE DIRECTIONS
       None.

SEE ALSO
       head

COPYRIGHT
       Portions  of  this  text  are  reprinted  and  reproduced in electronic form from IEEE Std
       1003.1, 2003 Edition, Standard for Information Technology  --  Portable	Operating  System
       Interface  (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003 by
       the Institute of Electrical and Electronics Engineers, Inc and  The  Open  Group.  In  the
       event  of  any  discrepancy  between this version and the original IEEE and The Open Group
       Standard, the original IEEE and The Open Group Standard is the referee document. The orig-
       inal Standard can be obtained online at http://www.opengroup.org/unix/online.html .

IEEE/The Open Group			       2003					  TAIL(P)


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