c0f2b64a89
There is an example in tail(1) manual page explaining how to use tail(1) to track the contents of /var/log/messages. The example uses the -f flag to follow the file. The problem with the -f flag is that it cannot handle the situation where /var/log/messages is rotated. Hence, use -F instead in the example. Reviewed by: asomers MFC after: 3 days Differential Revision: https://reviews.freebsd.org/D24157
203 lines
5.4 KiB
Groff
203 lines
5.4 KiB
Groff
.\" Copyright (c) 1980, 1990, 1991, 1993
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.\" The Regents of the University of California. All rights reserved.
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.\"
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.\" This code is derived from software contributed to Berkeley by
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.\" the Institute of Electrical and Electronics Engineers, Inc.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 3. Neither the name of the University nor the names of its contributors
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.\" may be used to endorse or promote products derived from this software
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.\" without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" @(#)tail.1 8.1 (Berkeley) 6/6/93
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.\" $FreeBSD$
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.\"
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.Dd March 22, 2020
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.Dt TAIL 1
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.Os
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.Sh NAME
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.Nm tail
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.Nd display the last part of a file
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.Sh SYNOPSIS
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.Nm
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.Op Fl F | f | r
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.Op Fl q
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.Oo
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.Fl b Ar number | Fl c Ar number | Fl n Ar number
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.Oc
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.Op Ar
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.Sh DESCRIPTION
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The
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.Nm
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utility displays the contents of
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.Ar file
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or, by default, its standard input, to the standard output.
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.Pp
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The display begins at a byte, line or 512-byte block location in the
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input.
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Numbers having a leading plus
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.Pq Ql +
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sign are relative to the beginning
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of the input, for example,
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.Dq Li "-c +2"
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starts the display at the second
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byte of the input.
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Numbers having a leading minus
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.Pq Ql -
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sign or no explicit sign are
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relative to the end of the input, for example,
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.Dq Li "-n 2"
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displays the last two lines of the input.
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The default starting location is
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.Dq Li "-n 10" ,
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or the last 10 lines of the input.
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.Pp
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The options are as follows:
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.Bl -tag -width indent
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.It Fl b Ar number , Fl -blocks Ns = Ns Ar number
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The location is
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.Ar number
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512-byte blocks.
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.It Fl c Ar number , Fl -bytes Ns = Ns Ar number
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The location is
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.Ar number
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bytes.
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.It Fl f
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The
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.Fl f
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option causes
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.Nm
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to not stop when end of file is reached, but rather to wait for additional
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data to be appended to the input.
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The
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.Fl f
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option is ignored if the standard input is a pipe, but not if it is a FIFO.
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.It Fl F
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The
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.Fl F
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option implies the
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.Fl f
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option, but
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.Nm
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will also check to see if the file being followed has been renamed or rotated.
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The file is closed and reopened when
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.Nm
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detects that the filename being read from has a new inode number.
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.Pp
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If the file being followed does not (yet) exist or if it is removed, tail
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will keep looking and will display the file from the beginning if and when
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it is created.
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.Pp
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The
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.Fl F
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option is the same as the
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.Fl f
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option if reading from standard input rather than a file.
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.It Fl n Ar number , Fl -lines Ns = Ns Ar number
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The location is
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.Ar number
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lines.
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.It Fl q
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Suppresses printing of headers when multiple files are being examined.
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.It Fl r
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The
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.Fl r
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option causes the input to be displayed in reverse order, by line.
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Additionally, this option changes the meaning of the
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.Fl b , c
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and
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.Fl n
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options.
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When the
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.Fl r
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option is specified, these options specify the number of bytes, lines
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or 512-byte blocks to display, instead of the bytes, lines or blocks
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from the beginning or end of the input from which to begin the display.
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The default for the
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.Fl r
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option is to display all of the input.
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.El
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.Pp
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If more than a single file is specified, each file is preceded by a
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header consisting of the string
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.Dq Li "==> " Ns Ar XXX Ns Li " <=="
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where
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.Ar XXX
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is the name of the file unless
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.Fl q
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flag is specified.
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.Sh EXIT STATUS
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.Ex -std
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.Sh EXAMPLES
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To display the last 500 lines of the file
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.Ar foo :
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.Pp
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.Dl $ tail -n 500 foo
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.Pp
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Keep
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.Pa /var/log/messages
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open, displaying to the standard output anything appended to the file:
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.Pp
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.Dl $ tail -F /var/log/messages
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.Sh SEE ALSO
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.Xr cat 1 ,
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.Xr head 1 ,
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.Xr sed 1
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.Sh STANDARDS
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The
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.Nm
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utility is expected to be a superset of the
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.St -p1003.2-92
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specification.
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In particular, the
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.Fl F ,
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.Fl b
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and
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.Fl r
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options are extensions to that standard.
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.Pp
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The historic command line syntax of
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.Nm
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is supported by this implementation.
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The only difference between this implementation and historic versions
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of
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.Nm ,
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once the command line syntax translation has been done, is that the
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.Fl b ,
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.Fl c
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and
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.Fl n
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options modify the
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.Fl r
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option, i.e.,
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.Dq Li "-r -c 4"
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displays the last 4 characters of the last line
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of the input, while the historic tail (using the historic syntax
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.Dq Li -4cr )
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would ignore the
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.Fl c
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option and display the last 4 lines of the input.
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.Sh HISTORY
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A
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.Nm
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command appeared in PWB UNIX.
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