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 -cnumber or -nnumber 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
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:
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:
Relative to the beginning of the file.
Relative to the end of the file.
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.
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.
This option shall be equivalent to -cnumber,
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 environment variables shall affect the execution of tail:
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.)
If set to a non-empty string value, override the values of
all the other internationalization variables.
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).
Determine the locale that should be used to affect the
format and contents of diagnostic messages written to standard error.
Determine the location of message catalogs for the
processing of LC_MESSAGES.
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.
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
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 -nnumber was given.
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