mirror of https://github.com/mkerrisk/man-pages
250 lines
8.4 KiB
Groff
250 lines
8.4 KiB
Groff
.\" Copyright (c) 2001-2003 The Open Group, All Rights Reserved
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.TH "TAIL" P 2003 "IEEE/The Open Group" "POSIX Programmer's Manual"
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.\" tail
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.SH NAME
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tail \- copy the last part of a file
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.SH SYNOPSIS
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.LP
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\fBtail\fP \fB[\fP\fB-f\fP\fB][\fP \fB-c\fP \fInumber\fP\fB| -n\fP
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\fInumber\fP\fB][\fP\fIfile\fP\fB]\fP
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.SH DESCRIPTION
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.LP
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The \fItail\fP utility shall copy its input file to the standard output
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beginning at a designated place.
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.LP
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Copying shall begin at the point in the file indicated by the \fB-c\fP
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\fInumber\fP or \fB-n\fP \fInumber\fP options. The
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option-argument \fInumber\fP shall be counted in units of lines or
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bytes, according to the options \fB-n\fP and \fB-c\fP. Both
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line and byte counts start from 1.
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.LP
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Tails relative to the end of the file may be saved in an internal
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buffer, and thus may be limited in length. Such a buffer, if
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any, shall be no smaller than {LINE_MAX}*10 bytes.
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.SH OPTIONS
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.LP
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The \fItail\fP utility shall conform to the Base Definitions volume
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of IEEE\ Std\ 1003.1-2001, Section 12.2, Utility Syntax Guidelines.
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.LP
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The following options shall be supported:
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.TP 7
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\fB-c\ \fP \fInumber\fP
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The application shall ensure that the \fInumber\fP option-argument
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is a decimal integer whose sign affects the location in the
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file, measured in bytes, to begin the copying:
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.TS C
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center; l l.
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\fBSign\fP \fBCopying Starts\fP
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+ Relative to the beginning of the file.
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- Relative to the end of the file.
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\fInone\fP Relative to the end of the file.
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.TE
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.LP
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The origin for counting shall be 1; that is, \fB-c\fP +1 represents
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the first byte of the file, \fB-c\fP -1 the last.
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.TP 7
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\fB-f\fP
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If the input file is a regular file or if the \fIfile\fP operand specifies
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a FIFO, do not terminate after the last line of the
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input file has been copied, but read and copy further bytes from the
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input file when they become available. If no \fIfile\fP
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operand is specified and standard input is a pipe, the \fB-f\fP option
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shall be ignored. If the input file is not a FIFO, pipe, or
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regular file, it is unspecified whether or not the \fB-f\fP option
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shall be ignored.
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.TP 7
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\fB-n\ \fP \fInumber\fP
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This option shall be equivalent to \fB-c\fP \fInumber\fP, except the
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starting location in the file shall be measured in lines
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instead of bytes. The origin for counting shall be 1; that is, \fB-n\fP
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+1 represents the first line of the file, \fB-n\fP -1 the
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last.
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.sp
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.LP
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If neither \fB-c\fP nor \fB-n\fP is specified, \fB-n\fP 10 shall be
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assumed.
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.SH OPERANDS
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.LP
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The following operand shall be supported:
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.TP 7
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\fIfile\fP
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A pathname of an input file. If no \fIfile\fP operands are specified,
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the standard input shall be used.
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.sp
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.SH STDIN
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.LP
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The standard input shall be used only if no \fIfile\fP operands are
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specified. See the INPUT FILES section.
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.SH INPUT FILES
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.LP
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If the \fB-c\fP option is specified, the input file can contain arbitrary
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data; otherwise, the input file shall be a text
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file.
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.SH ENVIRONMENT VARIABLES
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.LP
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The following environment variables shall affect the execution of
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\fItail\fP:
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.TP 7
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\fILANG\fP
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Provide a default value for the internationalization variables that
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are unset or null. (See the Base Definitions volume of
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IEEE\ Std\ 1003.1-2001, Section 8.2, Internationalization Variables
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for
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the precedence of internationalization variables used to determine
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the values of locale categories.)
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.TP 7
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\fILC_ALL\fP
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If set to a non-empty string value, override the values of all the
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other internationalization variables.
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.TP 7
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\fILC_CTYPE\fP
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Determine the locale for the interpretation of sequences of bytes
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of text data as characters (for example, single-byte as
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opposed to multi-byte characters in arguments and input files).
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.TP 7
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\fILC_MESSAGES\fP
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Determine the locale that should be used to affect the format and
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contents of diagnostic messages written to standard
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error.
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.TP 7
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\fINLSPATH\fP
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Determine the location of message catalogs for the processing of \fILC_MESSAGES
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\&.\fP
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.sp
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.SH ASYNCHRONOUS EVENTS
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.LP
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Default.
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.SH STDOUT
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.LP
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The designated portion of the input file shall be written to standard
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output.
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.SH STDERR
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.LP
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The standard error shall be used only for diagnostic messages.
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.SH OUTPUT FILES
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.LP
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None.
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.SH EXTENDED DESCRIPTION
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.LP
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None.
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.SH EXIT STATUS
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.LP
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The following exit values shall be returned:
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.TP 7
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\ 0
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Successful completion.
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.TP 7
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>0
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An error occurred.
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.sp
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.SH CONSEQUENCES OF ERRORS
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.LP
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Default.
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.LP
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\fIThe following sections are informative.\fP
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.SH APPLICATION USAGE
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.LP
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The \fB-c\fP option should be used with caution when the input is
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a text file containing multi-byte characters; it may produce
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output that does not start on a character boundary.
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.LP
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Although the input file to \fItail\fP can be any type, the results
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might not be what would be expected on some character
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special device files or on file types not described by the System
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Interfaces volume of IEEE\ Std\ 1003.1-2001. Since this
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volume of IEEE\ Std\ 1003.1-2001 does not specify the block size used
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when doing input, \fItail\fP need not read all of
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the data from devices that only perform block transfers.
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.SH EXAMPLES
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.LP
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The \fB-f\fP option can be used to monitor the growth of a file that
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is being written by some other process. For example, the
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command:
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.sp
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.RS
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.nf
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\fBtail -f fred
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\fP
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.fi
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.RE
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.LP
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prints the last ten lines of the file \fBfred\fP, followed by any
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lines that are appended to \fBfred\fP between the time
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\fItail\fP is initiated and killed. As another example, the command:
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.sp
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.RS
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.nf
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\fBtail -f -c 15 fred
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\fP
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.fi
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.RE
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.LP
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prints the last 15 bytes of the file \fBfred\fP, followed by any bytes
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that are appended to \fBfred\fP between the time
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\fItail\fP is initiated and killed.
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.SH RATIONALE
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.LP
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This version of \fItail\fP was created to allow conformance to the
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Utility Syntax Guidelines. The historical \fB-b\fP option
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was omitted because of the general non-portability of block-sized
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units of text. The \fB-c\fP option historically meant
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"characters", but this volume of IEEE\ Std\ 1003.1-2001 indicates
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that it means "bytes". This was selected to allow
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reasonable implementations when multi-byte characters are possible;
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it was not named \fB-b\fP to avoid confusion with the
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historical \fB-b\fP.
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.LP
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The origin of counting both lines and bytes is 1, matching all widespread
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historical implementations.
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.LP
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The restriction on the internal buffer is a compromise between the
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historical System V implementation of 4096 bytes and the BSD
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32768 bytes.
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.LP
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The \fB-f\fP option has been implemented as a loop that sleeps for
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1 second and copies any bytes that are available. This is
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sufficient, but if more efficient methods of determining when new
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data are available are developed, implementations are encouraged
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to use them.
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.LP
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Historical documentation indicates that \fItail\fP ignores the \fB-f\fP
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option if the input file is a pipe (pipe and FIFO on
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systems that support FIFOs). On BSD-based systems, this has been true;
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on System V-based systems, this was true when input was
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taken from standard input, but it did not ignore the \fB-f\fP flag
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if a FIFO was named as the \fIfile\fP operand. Since the
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\fB-f\fP option is not useful on pipes and all historical implementations
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ignore \fB-f\fP if no \fIfile\fP operand is specified
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and standard input is a pipe, this volume of IEEE\ Std\ 1003.1-2001
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requires this behavior. However, since the \fB-f\fP
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option is useful on a FIFO, this volume of IEEE\ Std\ 1003.1-2001
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also requires that if standard input is a FIFO or a FIFO
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is named, the \fB-f\fP option shall not be ignored. Although historical
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behavior does not ignore the \fB-f\fP option for other
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file types, this is unspecified so that implementations are allowed
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to ignore the \fB-f\fP option if it is known that the file
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cannot be extended.
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.LP
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This was changed to the current form based on comments noting that
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\fB-c\fP was almost never used without specifying a number
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and that there was no need to specify \fB-l\fP if \fB-n\fP \fInumber\fP
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was given.
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.SH FUTURE DIRECTIONS
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.LP
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None.
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.SH SEE ALSO
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.LP
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\fIhead\fP
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.SH COPYRIGHT
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Portions of this text are reprinted and reproduced in electronic form
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from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
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-- Portable Operating System Interface (POSIX), The Open Group Base
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Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
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Electrical and Electronics Engineers, Inc and The Open Group. In the
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event of any discrepancy between this version and the original IEEE and
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The Open Group Standard, the original IEEE and The Open Group Standard
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is the referee document. The original Standard can be obtained online at
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http://www.opengroup.org/unix/online.html .
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