PROLOG | NAME | SYNOPSIS | DESCRIPTION | OPTIONS | OPERANDS | STDIN | INPUT FILES | ENVIRONMENT VARIABLES | ASYNCHRONOUS EVENTS | STDOUT | STDERR | OUTPUT FILES | EXTENDED DESCRIPTION | EXIT STATUS | CONSEQUENCES OF ERRORS | APPLICATION USAGE | EXAMPLES | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT

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.1‐2008, 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.

ENVIRONMENT VARIABLES         top

       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.1‐2008, 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         top

       Default.

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

       None.

EXTENDED DESCRIPTION         top

       None.

EXIT STATUS         top

       The following exit values shall be returned:

        0    Successful completion.

       >0    An error occurred.

CONSEQUENCES OF ERRORS         top

       Default.

       The following sections are informative.

APPLICATION USAGE         top

       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.1‐2008. Since this volume of POSIX.1‐2008 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         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 killed.

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.1‐2008 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.1‐2008 requires
       this behavior. However, since the −f option is useful on a FIFO, this
       volume of POSIX.1‐2008 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         top

       None.

SEE ALSO         top

       head(1p)

       The Base Definitions volume of POSIX.1‐2008, 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, 2013 Edition, Standard for Information
       Technology -- Portable Operating System Interface (POSIX), The Open
       Group Base Specifications Issue 7, Copyright (C) 2013 by the
       Institute of Electrical and Electronics Engineers, Inc and The Open
       Group.  (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
       applied.) 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.unix.org/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                 2013                            TAIL(1P)

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