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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.
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 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.
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.
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.
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 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.
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 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.
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
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.
FUTURE DIRECTIONS
None.
SEE ALSO
head
The Base Definitions volume of POSIX.1‐2017, Chapter 8, Environment Variables, Section
12.2, Utility Syntax Guidelines
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std
1003.1-2017, Standard for Information Technology -- Portable Operating System Interface
(POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 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 .
Any typographical or formatting errors that appear in this page are most likely to have
been introduced during the conversion of the source files to man page format. To report
such errors, see https://www.kernel.org/doc/man-pages/reporting_bugs.html .