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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 dif-
       fer (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-argu-
       ment 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 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 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, Internationalization Variables for  the  prece-
              dence 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  exam-
              ple, 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 writ-
              ten 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 char-
       acter  special  device  files  or  on  file  types  not  described  by  the   System   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  histori-
       cally  meant  "characters",  but  this volume of IEEE Std 1003.1-2001 indicates that it means "bytes". This was
       selected to allow reasonable 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 operand 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, Stan-
       dard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base  Specifica-
       tions  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 Stan-
       dard,  the  original  IEEE  and  The  Open Group Standard is the referee document. The original Standard can be
       obtained online at http://www.opengroup.org/unix/online.html .



IEEE/The Open Group                  2003                             TAIL(1P)