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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)
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.
tail — copy the last part of a file
tail [−f] [−c number|−n number] [file]
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.
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.
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.
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.
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
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.
Default.
The designated portion of the input file shall be written to standard
output.
The standard error shall be used only for diagnostic messages.
None.
None.
The following exit values shall be returned:
0 Successful completion.
>0 An error occurred.
Default.
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.1‐2008. 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.
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.
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.
None.
head(1p)
The Base Definitions volume of POSIX.1‐2008, Chapter 8, Environment
Variables, Section 12.2, Utility Syntax Guidelines
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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