tail(1p) - phpMan

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
       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 spec-
              ifies  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  start-
              ing  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  operands  are  speci-
       fied. 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  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
       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 pos-
       sible; 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.
       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  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  histori-
       cal  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 original Standard can be  obtained  online
       at http://www.opengroup.org/unix/online.html .

IEEE/The Open Group                  2003                             TAIL(1P)