Man Linux: Main Page and Category List

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
       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 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
       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, 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 .