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tail(P) tail(P)
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 stan-
dard 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-argu-
ment 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 IEEE Std 1003.1-2001, Section 12.2, Utility
Syntax Guidelines.
The following options shall be supported:
-c number
The application shall ensure that the number
option-argument is a decimal integer whose sign
affects 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 origin for counting shall be 1; that is, -c +1 rep-
resents 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, 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 repre-
sents 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 operands
are specified, the standard input shall be used.
STDIN
The standard input shall be used only if no file oper-
ands are specified. See the INPUT FILES section.
INPUT FILES
If the -c option is specified, the input file can con-
tain 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 internationaliza-
tion variables that are unset or null. (See the
Base Definitions volume of IEEE Std 1003.1-2001,
Section 8.2, Internationalization Variables for
the precedence of internationalization variables
used to determine the values of locale cate-
gories.)
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 mes-
sages 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 writ-
ten 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 bound-
ary.
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
IEEE Std 1003.1-2001. Since this volume of
IEEE Std 1003.1-2001 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
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
IEEE Std 1003.1-2001 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, match-
ing all widespread historical 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 oper-
and. 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 IEEE Std 1003.1-2001 requires this behavior. However,
since the -f option is useful on a FIFO, this volume of
IEEE Std 1003.1-2001 also requires that if standard
input is a FIFO or a FIFO is named, the -f option shall
not be ignored. 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.
This was changed to the current form based on comments
noting that -c was almost never used without specifying
a number and that there was no need to specify -l if -n
number was given.
FUTURE DIRECTIONS
None.
SEE ALSO
head
COPYRIGHT
Portions of this text are reprinted and reproduced in
electronic form from IEEE Std 1003.1, 2003 Edition,
Standard for Information Technology -- Portable Operat-
ing System Interface (POSIX), The Open Group Base Speci-
fications Issue 6, Copyright (C) 2001-2003 by the Insti-
tute 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.open-
group.org/unix/online.html .
POSIX 2003 tail(P)
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