NetBSD-5.0.2/usr.bin/tail/tail.1

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.\"	$NetBSD: tail.1,v 1.13 2003/08/07 11:16:02 agc Exp $
.\"
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.\"	@(#)tail.1	8.1 (Berkeley) 6/6/93
.\"
.Dd June 6, 1993
.Dt TAIL 1
.Os
.Sh NAME
.Nm tail
.Nd display the last part of a file
.Sh SYNOPSIS
.Nm
.Oo
.Fl f |
.Fl F |
.Fl r
.Oc
.Oo
.Fl b Ar number |
.Fl c Ar number |
.Fl n Ar number
.Oc
.Op Ar file ...
.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 (``+'') sign are relative to the beginning
of the input, for example,
.Dq -c +2
starts the display at the second
byte of the input.
Numbers having a leading minus (``-'') sign or no explicit sign are
relative to the end of the input, for example,
.Dq -n 2
displays the last two lines of the input.
The default starting location is
.Dq -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
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 is the same as the
.Fl f
option, except that every five seconds
.Nm
will check to see if the file named on the command line has been
shortened or moved (it is considered moved if the inode or device
number changes) and, if so, it will close
the current file, open the filename given, print out the entire
contents, and continue to wait for more data to be appended.
This option is used to follow log files though rotation by
.Xr newsyslog 8
or similar programs.
.It Fl n 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 ==\*[Gt] XXX \*[Le]=
where
.Dq XXX
is the name of the file.
.Pp
The
.Nm
utility exits 0 on success, and \*[Gt]0 if an error occurs.
.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 b ,
.Fl r
and
.Fl F
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. ``-r -c 4'' displays the last 4 characters of the last line
of the input, while the historic tail (using the historic syntax ``-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 .
.Sh BUGS
When using the
.Fl F
option,
.Nm
will not detect a file truncation if, between the truncation
and the next check of the file size, data written to the file make
it larger than the last known file size.