freebsd-skq/usr.bin/tail/tail.1

.\" Copyright (c) 1980, 1990, 1991, 1993
.\"	The Regents of the University of California.  All rights reserved.
.\"
.\" This code is derived from software contributed to Berkeley by
.\" the Institute of Electrical and Electronics Engineers, Inc.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\"    notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\"    notice, this list of conditions and the following disclaimer in the
.\"    documentation and/or other materials provided with the distribution.
.\" 3. Neither the name of the University nor the names of its contributors
.\"    may be used to endorse or promote products derived from this software
.\"    without specific prior written permission.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
.\" ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
.\" SUCH DAMAGE.
.\"
.\"	@(#)tail.1	8.1 (Berkeley) 6/6/93
.\" $FreeBSD$
.\"
.Dd April 10, 2018
.Dt TAIL 1
.Os
.Sh NAME
.Nm tail
.Nd display the last part of a file
.Sh SYNOPSIS
.Nm
.Op Fl F | f | r
.Op Fl q
.Oo
.Fl b Ar number | Fl c Ar number | Fl n Ar number
.Oc
.Op Ar
.Sh DESCRIPTION
The
.Nm
utility displays the contents of
.Ar file
or, by default, its standard input, to the standard output.
.Pp
The display begins at a byte, line or 512-byte block location in the
input.
Numbers having a leading plus
.Pq Ql +
sign are relative to the beginning
of the input, for example,
.Dq Li "-c +2"
starts the display at the second
byte of the input.
Numbers having a leading minus
.Pq Ql -
sign or no explicit sign are
relative to the end of the input, for example,
.Dq Li "-n 2"
displays the last two lines of the input.
The default starting location is
.Dq Li "-n 10" ,
or the last 10 lines of the input.
.Pp
The options are as follows:
.Bl -tag -width indent
.It Fl b Ar number , Fl -blocks Ns = Ns Ar number
The location is
.Ar number
512-byte blocks.
.It Fl c Ar number , Fl -bytes Ns = Ns Ar number
The location is
.Ar number
bytes.
.It Fl f
The
.Fl f
option causes
.Nm
to not stop when end of file is reached, but rather to wait for additional
data to be appended to the input.
The
.Fl f
option is ignored if the standard input is a pipe, but not if it is a FIFO.
.It Fl F
The
.Fl F
option implies the
.Fl f
option, but
.Nm
will also check to see if the file being followed has been renamed or rotated.
The file is closed and reopened when
.Nm
detects that the filename being read from has a new inode number.
.Pp
If the file being followed does not (yet) exist or if it is removed, tail
will keep looking and will display the file from the beginning if and when
it is created.
.Pp
The
.Fl F
option is the same as the
.Fl f
option if reading from standard input rather than a file.
.It Fl n Ar number , Fl -lines Ns = Ns Ar number
The location is
.Ar number
lines.
.It Fl q
Suppresses printing of headers when multiple files are being examined.
.It Fl r
The
.Fl r
option causes the input to be displayed in reverse order, by line.
Additionally, this option changes the meaning of the
.Fl b , c
and
.Fl n
options.
When the
.Fl r
option is specified, these options specify the number of bytes, lines
or 512-byte blocks to display, instead of the bytes, lines or blocks
from the beginning or end of the input from which to begin the display.
The default for the
.Fl r
option is to display all of the input.
.El
.Pp
If more than a single file is specified, each file is preceded by a
header consisting of the string
.Dq Li "==> " Ns Ar XXX Ns Li " <=="
where
.Ar XXX
is the name of the file unless
.Fl q
flag is specified.
.Sh EXIT STATUS
.Ex -std
.Sh EXAMPLES
To display the last 500 lines of the file
.Ar foo :
.Pp
.Dl $ tail -n 500 foo
.Pp
Keep
.Pa /var/log/messages
open, displaying to the standard output anything appended to the file:
.Pp
.Dl $ tail -f /var/log/messages
.Sh SEE ALSO
.Xr cat 1 ,
.Xr head 1 ,
.Xr sed 1
.Sh STANDARDS
The
.Nm
utility is expected to be a superset of the
.St -p1003.2-92
specification.
In particular, the
.Fl F ,
.Fl b
and
.Fl r
options are extensions to that standard.
.Pp
The historic command line syntax of
.Nm
is supported by this implementation.
The only difference between this implementation and historic versions
of
.Nm ,
once the command line syntax translation has been done, is that the
.Fl b ,
.Fl c
and
.Fl n
options modify the
.Fl r
option, i.e.,
.Dq Li "-r -c 4"
displays the last 4 characters of the last line
of the input, while the historic tail (using the historic syntax
.Dq Li -4cr )
would ignore the
.Fl c
option and display the last 4 lines of the input.
.Sh HISTORY
A
.Nm
command appeared in PWB UNIX.