скопировать последнюю часть файла (copy the last part of a file)
Пролог (Prolog)
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.
Имя (Name)
tail — copy the last part of a file
Синопсис (Synopsis)
tail [-f] [-c number|-n number] [file]
Описание (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.
Параметры (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.
Операнды (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.
Стандартный ввод (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.
Входные файлы (Input files)
If the -c option is specified, the input file can contain
arbitrary data; otherwise, the input file shall be a text file.
Переменные окружения (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.
Асинхронные события (Asynchronous events)
Default.
Стандартный вывод (Stdout)
The designated portion of the input file shall be written to
standard output.
Стандартный вывод сообщений (Stderr)
The standard error shall be used only for diagnostic messages.
Выходные файлы (Output files)
None.
Расширенное описание (Extended description)
None.
Статус выхода (Exit)
The following exit values shall be returned:
0 Successful completion.
>0 An error occurred.
Последствия ошибок (Consequences of errors)
Default.
The following sections are informative.
Использование в приложениях (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.
Примеры (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.
Обоснование (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.
Будущие направления (Future directions)
None.
Смотри также (See also)
head(1p)
The Base Definitions volume of POSIX.1‐2017, Chapter 8,
Environment Variables, Section 12.2, Utility Syntax Guidelines
