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1 .\" $NetBSD: tail.1,v 1.12 2003/02/25 10:35:56 wiz Exp $
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33 .\" @(#)tail.1 8.1 (Berkeley) 6/6/93
34 .\"
35 .Dd June 6, 1993
36 .Dt TAIL 1
37 .Os
38 .Sh NAME
39 .Nm tail
40 .Nd display the last part of a file
41 .Sh SYNOPSIS
42 .Nm
43 .Oo
44 .Fl f |
45 .Fl F |
46 .Fl r
47 .Oc
48 .Oo
49 .Fl b Ar number |
50 .Fl c Ar number |
51 .Fl n Ar number
52 .Oc
53 .Op Ar file ...
54 .Sh DESCRIPTION
55 The
56 .Nm
57 utility displays the contents of
58 .Ar file
59 or, by default, its standard input, to the standard output.
60 .Pp
61 The display begins at a byte, line or 512-byte block location in the
62 input.
63 Numbers having a leading plus (``+'') sign are relative to the beginning
64 of the input, for example,
65 .Dq -c +2
66 starts the display at the second
67 byte of the input.
68 Numbers having a leading minus (``-'') sign or no explicit sign are
69 relative to the end of the input, for example,
70 .Dq -n 2
71 displays the last two lines of the input.
72 The default starting location is
73 .Dq -n 10 ,
74 or the last 10 lines of the input.
75 .Pp
76 The options are as follows:
77 .Bl -tag -width Ds
78 .It Fl b Ar number
79 The location is
80 .Ar number
81 512-byte blocks.
82 .It Fl c Ar number
83 The location is
84 .Ar number
85 bytes.
86 .It Fl f
87 The
88 .Fl f
89 option causes
90 .Nm
91 to not stop when end of file is reached, but rather to wait for additional
92 data to be appended to the input.
93 The
94 .Fl f
95 option is ignored if the standard input is a pipe, but not if it is a FIFO.
96 .It Fl F
97 The
98 .Fl F
99 option is the same as the
100 .Fl f
101 option, except that every five seconds
102 .Nm
103 will check to see if the file named on the command line has been
104 shortened or moved (it is considered moved if the inode or device
105 number changes) and, if so, it will close
106 the current file, open the filename given, print out the entire
107 contents, and continue to wait for more data to be appended.
108 This option is used to follow log files though rotation by
109 .Xr newsyslog 8
110 or similar programs.
111 .It Fl n Ar number
112 The location is
113 .Ar number
114 lines.
115 .It Fl r
116 The
117 .Fl r
118 option causes the input to be displayed in reverse order, by line.
119 Additionally, this option changes the meaning of the
120 .Fl b ,
121 .Fl c
122 and
123 .Fl n
124 options.
125 When the
126 .Fl r
127 option is specified, these options specify the number of bytes, lines
128 or 512-byte blocks to display, instead of the bytes, lines or blocks
129 from the beginning or end of the input from which to begin the display.
130 The default for the
131 .Fl r
132 option is to display all of the input.
133 .El
134 .Pp
135 If more than a single file is specified, each file is preceded by a
136 header consisting of the string
137 .Dq ==\*[Gt] XXX \*[Le]=
138 where
139 .Dq XXX
140 is the name of the file.
141 .Pp
142 The
143 .Nm
144 utility exits 0 on success, and \*[Gt]0 if an error occurs.
145 .Sh SEE ALSO
146 .Xr cat 1 ,
147 .Xr head 1 ,
148 .Xr sed 1
149 .Sh STANDARDS
150 The
151 .Nm
152 utility is expected to be a superset of the
153 .St -p1003.2-92
154 specification.
155 In particular, the
156 .Fl b ,
157 .Fl r
158 and
159 .Fl F
160 options are extensions to that standard.
161 .Pp
162 The historic command line syntax of
163 .Nm
164 is supported by this implementation.
165 The only difference between this implementation and historic versions
166 of
167 .Nm ,
168 once the command line syntax translation has been done, is that the
169 .Fl b ,
170 .Fl c
171 and
172 .Fl n
173 options modify the
174 .Fl r
175 option, i.e. ``-r -c 4'' displays the last 4 characters of the last line
176 of the input, while the historic tail (using the historic syntax ``-4cr'')
177 would ignore the
178 .Fl c
179 option and display the last 4 lines of the input.
180 .Sh HISTORY
181 A
182 .Nm
183 command appeared in
184 .At v7 .
185 .Sh BUGS
186 When using the
187 .Fl F
188 option,
189 .Nm
190 will not detect a file truncation if, between the truncation
191 and the next check of the file size, data written to the file make
192 it larger than the last known file size.