.\" Copyright (c) 2001-2003 The Open Group, All Rights Reserved 
.TH "TAIL" 1P 2003 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\" tail 
.SH 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.
.SH NAME
tail \- copy the last part of a file
.SH SYNOPSIS
.LP
\fBtail\fP \fB[\fP\fB-f\fP\fB][\fP \fB-c\fP \fInumber\fP\fB| -n\fP
\fInumber\fP\fB][\fP\fIfile\fP\fB]\fP
.SH DESCRIPTION
.LP
The \fItail\fP utility shall copy its input file to the standard output
beginning at a designated place.
.LP
Copying shall begin at the point in the file indicated by the \fB-c\fP
\fInumber\fP or \fB-n\fP \fInumber\fP options. The
option-argument \fInumber\fP shall be counted in units of lines or
bytes, according to the options \fB-n\fP and \fB-c\fP. Both
line and byte counts start from 1.
.LP
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.
.SH OPTIONS
.LP
The \fItail\fP utility shall conform to the Base Definitions volume
of IEEE\ Std\ 1003.1-2001, Section 12.2, Utility Syntax Guidelines.
.LP
The following options shall be supported:
.TP 7
\fB-c\ \fP \fInumber\fP
The application shall ensure that the \fInumber\fP option-argument
is a decimal integer whose sign affects the location in the
file, measured in bytes, to begin the copying: 
.TS C
center; l l.
\fBSign\fP	\fBCopying Starts\fP
+	Relative to the beginning of the file.
-	Relative to the end of the file.
\fInone\fP	Relative to the end of the file.
.TE
.LP
The origin for counting shall be 1; that is, \fB-c\fP +1 represents
the first byte of the file, \fB-c\fP -1 the last.
.TP 7
\fB-f\fP
If the input file is a regular file or if the \fIfile\fP 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 \fIfile\fP
operand is specified and standard input is a pipe, the \fB-f\fP option
shall be ignored. If the input file is not a FIFO, pipe, or
regular file, it is unspecified whether or not the \fB-f\fP option
shall be ignored.
.TP 7
\fB-n\ \fP \fInumber\fP
This option shall be equivalent to \fB-c\fP \fInumber\fP, except the
starting location in the file shall be measured in lines
instead of bytes. The origin for counting shall be 1; that is, \fB-n\fP
+1 represents the first line of the file, \fB-n\fP -1 the
last.
.sp
.LP
If neither \fB-c\fP nor \fB-n\fP is specified, \fB-n\fP 10 shall be
assumed.
.SH OPERANDS
.LP
The following operand shall be supported:
.TP 7
\fIfile\fP
A pathname of an input file. If no \fIfile\fP operands are specified,
the standard input shall be used.
.sp
.SH STDIN
.LP
The standard input shall be used only if no \fIfile\fP operands are
specified. See the INPUT FILES section.
.SH INPUT FILES
.LP
If the \fB-c\fP option is specified, the input file can contain arbitrary
data; otherwise, the input file shall be a text
file.
.SH ENVIRONMENT VARIABLES
.LP
The following environment variables shall affect the execution of
\fItail\fP:
.TP 7
\fILANG\fP
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.)
.TP 7
\fILC_ALL\fP
If set to a non-empty string value, override the values of all the
other internationalization variables.
.TP 7
\fILC_CTYPE\fP
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).
.TP 7
\fILC_MESSAGES\fP
Determine the locale that should be used to affect the format and
contents of diagnostic messages written to standard
error.
.TP 7
\fINLSPATH\fP
Determine the location of message catalogs for the processing of \fILC_MESSAGES
\&.\fP 
.sp
.SH ASYNCHRONOUS EVENTS
.LP
Default.
.SH STDOUT
.LP
The designated portion of the input file shall be written to standard
output.
.SH STDERR
.LP
The standard error shall be used only for diagnostic messages.
.SH OUTPUT FILES
.LP
None.
.SH EXTENDED DESCRIPTION
.LP
None.
.SH EXIT STATUS
.LP
The following exit values shall be returned:
.TP 7
\ 0
Successful completion.
.TP 7
>0
An error occurred.
.sp
.SH CONSEQUENCES OF ERRORS
.LP
Default.
.LP
\fIThe following sections are informative.\fP
.SH APPLICATION USAGE
.LP
The \fB-c\fP 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.
.LP
Although the input file to \fItail\fP 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, \fItail\fP need not read all of
the data from devices that only perform block transfers.
.SH EXAMPLES
.LP
The \fB-f\fP option can be used to monitor the growth of a file that
is being written by some other process. For example, the
command:
.sp
.RS
.nf

\fBtail -f fred
\fP
.fi
.RE
.LP
prints the last ten lines of the file \fBfred\fP, followed by any
lines that are appended to \fBfred\fP between the time
\fItail\fP is initiated and killed. As another example, the command:
.sp
.RS
.nf

\fBtail -f -c 15 fred
\fP
.fi
.RE
.LP
prints the last 15 bytes of the file \fBfred\fP, followed by any bytes
that are appended to \fBfred\fP between the time
\fItail\fP is initiated and killed.
.SH RATIONALE
.LP
This version of \fItail\fP was created to allow conformance to the
Utility Syntax Guidelines. The historical \fB-b\fP option
was omitted because of the general non-portability of block-sized
units of text. The \fB-c\fP 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 \fB-b\fP to avoid confusion with the
historical \fB-b\fP.
.LP
The origin of counting both lines and bytes is 1, matching all widespread
historical implementations.
.LP
The restriction on the internal buffer is a compromise between the
historical System V implementation of 4096 bytes and the BSD
32768 bytes.
.LP
The \fB-f\fP 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.
.LP
Historical documentation indicates that \fItail\fP ignores the \fB-f\fP
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 \fB-f\fP flag
if a FIFO was named as the \fIfile\fP operand. Since the
\fB-f\fP option is not useful on pipes and all historical implementations
ignore \fB-f\fP if no \fIfile\fP operand is specified
and standard input is a pipe, this volume of IEEE\ Std\ 1003.1-2001
requires this behavior. However, since the \fB-f\fP
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 \fB-f\fP option shall not be ignored. Although historical
behavior does not ignore the \fB-f\fP option for other
file types, this is unspecified so that implementations are allowed
to ignore the \fB-f\fP option if it is known that the file
cannot be extended.
.LP
This was changed to the current form based on comments noting that
\fB-c\fP was almost never used without specifying a number
and that there was no need to specify \fB-l\fP if \fB-n\fP \fInumber\fP
was given.
.SH FUTURE DIRECTIONS
.LP
None.
.SH SEE ALSO
.LP
\fIhead\fP
.SH 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 .