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       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 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 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:

                                 ┌─────┬────────────────────────────────────────┐
                                 │SignCopying 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 variables that are unset or
                 null.  (See  the  Base  Definitions  volume  of   POSIX.1‐2008,   Section   8.2,
                 Internationalization   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
                 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 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. 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  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  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 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.

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 Electrical 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.kernel.org/doc/man-pages/reporting_bugs.html .