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TAIL(1P)		   POSIX Programmer's Manual		      TAIL(1P)

PROLOG
       This  manual  page is part of the POSIX Programmer's Manual.  The Linux
       implementation of this interface may differ (consult the	 corresponding
       Linux  manual page for details of Linux behavior), or the interface may
       not be implemented on Linux.

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 designated 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  buf‐
       fer, 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
       POSIX.1‐2008,  Section 12.2, Utility Syntax Guidelines, except that '+'
       may be recognized as an option delimiter as well as '−'.

       The following options shall be supported:

       −c number The application shall ensure that the number  option-argument
		 is  a decimal integer, optionally including a sign.  The sign
		 shall affect the location in the file, measured in bytes,  to
		 begin the copying:

			┌─────┬────────────────────────────────────────┐
			│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  application  shall ensure that if the sign of the number
		 option-argument is '+', the number option-argument is a  non-
		 zero decimal integer.

		 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  or	 FIFO,
		 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 operand is specified,
		 the standard input shall be used.

STDIN
       The standard input shall be used if no file operand is  specified,  and
       shall  be used if the file operand is '−' and the implementation treats
       the '−' as meaning standard input.  Otherwise, the standard input shall
       not be used.  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	 vari‐
		 ables	that are unset or null. (See the Base Definitions vol‐
		 ume of POSIX.1‐2008, Section 8.2, Internationalization	 Vari‐
		 ables	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 internationalization 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  arguments  and input
		 files).

       LC_MESSAGES
		 Determine the locale that should be used to affect the format
		 and  contents	of  diagnostic	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  System   Interfaces   volume   of
       POSIX.1‐2008.  Since  this  volume of POSIX.1‐2008 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  POSIX.1‐2008
       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 wide‐
       spread historical implementations. Hence tail −n +0 is  not  conforming
       usage  because it attempts to output line zero; but note that tail −n 0
       does conform, and outputs nothing.

       Earlier versions of this standard allowed the following	forms  in  the
       SYNOPSIS:

	   tail −[number][b|c|l][f] [file]
	   tail +[number][b|c|l][f] [file]

       These forms are no longer specified by POSIX.1‐2008, but may be present
       in some implementations.

       The restriction on the internal buffer is a compromise between the his‐
       torical 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 operand is specified and standard input is a pipe,
       this  volume of POSIX.1‐2008 requires this behavior. However, since the
       −f option is useful  on	a  FIFO,  this	volume	of  POSIX.1‐2008  also
       requires	 that  if a FIFO is named, the −f option shall not be ignored.
       Earlier versions of this standard did not state any requirement for the
       case  where  no file operand is specified and standard input is a FIFO.
       The standard has been updated to reflect current practice which	is  to
       treat this case the same as a pipe on standard input.  Although histor‐
       ical 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.

FUTURE DIRECTIONS
       None.

SEE ALSO
       head

       The Base Definitions volume of  POSIX.1‐2008,  Chapter  8,  Environment
       Variables, Section 12.2, Utility Syntax Guidelines

COPYRIGHT
       Portions	 of  this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
       --  Portable  Operating	System	Interface (POSIX), The Open Group Base
       Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri‐
       cal  and	 Electronics  Engineers,  Inc  and  The	 Open Group.  (This is
       POSIX.1-2008 with the 2013 Technical Corrigendum	 1  applied.)  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 original Standard can be obtained online
       at http://www.unix.org/online.html .

       Any typographical or formatting errors that appear  in  this  page  are
       most likely to have been introduced during the conversion of the source
       files to man page format. To report such errors,	 see  https://www.ker‐
       nel.org/doc/man-pages/reporting_bugs.html .

IEEE/The Open Group		     2013			      TAIL(1P)
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