TAIL(1)                    POSIX Programmer's Manual                   TAIL(1)



PROLOG
       This manual page is part of the POSIX Programmer's Manual.  The OS
       implementation of this interface may differ (consult the corresponding
       OS manual page for details of OS behavior), or the interface may not be
       implemented on OS.

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.1-2017, 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:

                        +-----+----------------------------------------+
                        |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 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.1-2017, 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.1-2017. Since this volume of POSIX.1-2017 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.

       When using tail to process pathnames, and the -c option is not
       specified, it is recommended that LC_ALL, or at least LC_CTYPE and
       LC_COLLATE, are set to POSIX or C in the environment, since pathnames
       can contain byte sequences that do not form valid characters in some
       locales, in which case the utility's behavior would be undefined. In
       the POSIX locale each byte is a valid single-byte character, and
       therefore this problem is avoided.

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.1-2017
       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.1-2017 requires this behavior. However, since the
       -f option is useful on a FIFO, this volume of POSIX.1-2017 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.1-2017, 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 1003.1-2017, Standard for Information Technology --
       Portable Operating System Interface (POSIX), The Open Group Base
       Specifications Issue 7, 2018 Edition, Copyright (C) 2018 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 .

       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 .



IEEE/The Open Group                  2017                              TAIL(1)
