OpenBSD-4.6/usr.bin/tail/tail.1

.\"	$OpenBSD: tail.1,v 1.17 2009/02/08 17:15:10 jmc Exp $
.\"	$NetBSD: tail.1,v 1.4 1994/11/23 07:42:13 jtc Exp $
.\"
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.\"	@(#)tail.1	8.1 (Berkeley) 6/6/93
.\"
.Dd $Mdocdate: February 8 2009 $
.Dt TAIL 1
.Os
.Sh NAME
.Nm tail
.Nd display the last part of a file
.Sh SYNOPSIS
.Nm tail
.Op Fl f | r
.Oo
.Fl b Ar number |
.Fl c Ar number |
.Fl n Ar number |
.Fl Ns 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,
.Ic -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,
.Ic -n 2
displays the last two lines of the input.
The default starting location is
.Ic -n 10 ,
or the last 10 lines of the input.
.Pp
The options are as follows:
.Bl -tag -width Ds
.It Fl b Ar number
The location is
.Ar number
512-byte blocks.
.It Fl c Ar number
The location is
.Ar number
bytes.
.It Fl f
Do not stop when end-of-file is reached; instead, wait for additional
data to be appended to the input.
If the file is replaced (i.e., the inode number changes),
.Nm
will reopen the file and continue.
If the file is truncated,
.Nm
will reset its position to the beginning.
This makes
.Nm
more useful for watching log files that may get rotated.
The
.Fl f
option is ignored if the standard input is a pipe, but not if it is a FIFO.
.It Fl n Ar number \*(Ba Fl Ns Ar number
The location is
.Ar number
lines.
.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 ,
.Fl 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 ==> XXX <==
where
.Dq XXX
is the name of the file.
.Pp
.Ex -std tail
.Sh EXAMPLES
To display the last 500 lines of the file
.Ar foo :
.Pp
.Dl $ tail -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 compliant with the
.St -p1003.1-2008
specification.
.Pp
The flags
.Op Fl br
are extensions to that specification.
.Pp
The historic command line syntax of
.Nm
is supported by this implementation.
The only difference between this implementation and historic versions
of
.Nm tail ,
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.,
.Ic -r -c 4
displays the last 4 characters of the last line
of the input, while the historic tail (using the historic syntax
.Ic -4cr )
would ignore the
.Fl c
option and display the last 4 lines of the input.
.Sh HISTORY
A
.Nm
command appeared in
.At v7 .