TAIL(P) POSIX Programmer's Manual TAIL(P)NAMEtail - copy the last part of a file
SYNOPSIStail [-f][ -c number| -n number][file]
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 buf‐
fer, 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 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 spec‐
ifies 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 start‐
ing 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 operands are speci‐
fied. See the INPUT FILES section.
INPUT FILES
If the -c option is specified, the input file can contain 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 internationalization variables
that are unset or null. (See the Base Definitions volume of
IEEE Std 1003.1-2001, Section 8.2, Internationalization Vari‐
ables 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
Default.
STDOUT
The designated portion of the input file shall be written 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 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
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 pos‐
sible; it was not named -b to avoid confusion with the historical -b.
The origin of counting both lines and bytes is 1, matching all wide‐
spread historical implementations.
The restriction on the internal buffer is a compromise between the his‐
torical 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 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 histori‐
cal 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 Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 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 .
IEEE/The Open Group 2003 TAIL(P)
