tail(P)                                                 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 stan-
       dard 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  rep-
       resents 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 repre-
              sents 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  oper-
       ands are specified. See the INPUT FILES section.

INPUT FILES
       If  the  -c option is specified, the input file can con-
       tain 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 internationaliza-
              tion  variables  that are unset or null. (See the
              Base Definitions volume of  IEEE Std 1003.1-2001,
              Section  8.2,  Internationalization Variables for
              the precedence of internationalization  variables
              used  to  determine  the  values  of locale cate-
              gories.)

       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 mes-
              sages 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  writ-
       ten 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 bound-
       ary.

       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
       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 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, match-
       ing 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 oper-
       and. 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,
       Standard for Information Technology -- Portable  Operat-
       ing System Interface (POSIX), The Open Group Base Speci-
       fications Issue 6, Copyright (C) 2001-2003 by the Insti-
       tute  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 original Standard can be
       obtained        online        at        http://www.open-
       group.org/unix/online.html .



POSIX                         2003                      tail(P)
