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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 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
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 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 represents 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 specified. 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 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
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 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.
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 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 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 .