tail(1p) — Linux manual page


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

PROLOG         top

       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         top

       tail — copy the last part of a file

SYNOPSIS         top

       tail [-f] [-c number|-n number] [file]

DESCRIPTION         top

       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         top

       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         top

       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         top

       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         top

       If the -c option is specified, the input file can contain
       arbitrary data; otherwise, the input file shall be a text file.


       The following environment variables shall affect the execution of

       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).

                 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.



STDOUT         top

       The designated portion of the input file shall be written to
       standard output.

STDERR         top

       The standard error shall be used only for diagnostic messages.

OUTPUT FILES         top




EXIT STATUS         top

       The following exit values shall be returned:

        0    Successful completion.

       >0    An error occurred.



       The following sections are informative.


       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         top

       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

RATIONALE         top

       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.



SEE ALSO         top


       The Base Definitions volume of POSIX.1‐2017, Chapter 8,
       Environment Variables, Section 12.2, Utility Syntax Guidelines

COPYRIGHT         top

       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(1P)

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