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1 .\" $NetBSD: tail.1,v 1.15 2014/06/15 11:37:56 wiz Exp $
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33 .\" @(#)tail.1 8.1 (Berkeley) 6/6/93
34 .\"
35 .Dd June 15, 2014
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 there are no file arguments and the standard
96 input is a pipe or a FIFO.
97 .It Fl F
98 The
99 .Fl F
100 option is the same as the
101 .Fl f
102 option, except that every five seconds
103 .Nm
104 will check to see if the file named on the command line has been
105 shortened or moved (it is considered moved if the inode or device
106 number changes) and, if so, it will close
107 the current file, open the filename given, print out the entire
108 contents, and continue to wait for more data to be appended.
109 This option is used to follow log files though rotation by
110 .Xr newsyslog 8
111 or similar programs.
112 .It Fl n Ar number
113 The location is
114 .Ar number
115 lines.
116 .It Fl r
117 The
118 .Fl r
119 option causes the input to be displayed in reverse order, by line.
120 Additionally, this option changes the meaning of the
121 .Fl b ,
122 .Fl c
123 and
124 .Fl n
125 options.
126 When the
127 .Fl r
128 option is specified, these options specify the number of bytes, lines
129 or 512-byte blocks to display, instead of the bytes, lines or blocks
130 from the beginning or end of the input from which to begin the display.
131 The default for the
132 .Fl r
133 option is to display all of the input.
134 .El
135 .Pp
136 If more than a single file is specified, each file is preceded by a
137 header consisting of the string
138 .Dq ==\*[Gt] XXX \*[Le]=
139 where
140 .Dq XXX
141 is the name of the file.
142 .Pp
143 The
144 .Nm
145 utility exits 0 on success, and \*[Gt]0 if an error occurs.
146 .Sh SEE ALSO
147 .Xr cat 1 ,
148 .Xr head 1 ,
149 .Xr sed 1
150 .Sh STANDARDS
151 The
152 .Nm
153 utility is expected to be a superset of the
154 .St -p1003.2-92
155 specification.
156 In particular, the
157 .Fl b ,
158 .Fl r
159 and
160 .Fl F
161 options are extensions to that standard.
162 .Pp
163 The historic command line syntax of
164 .Nm
165 is supported by this implementation.
166 The only difference between this implementation and historic versions
167 of
168 .Nm ,
169 once the command line syntax translation has been done, is that the
170 .Fl b ,
171 .Fl c
172 and
173 .Fl n
174 options modify the
175 .Fl r
176 option, i.e., ``-r -c 4'' displays the last 4 characters of the last line
177 of the input, while the historic tail (using the historic syntax ``-4cr'')
178 would ignore the
179 .Fl c
180 option and display the last 4 lines of the input.
181 .Sh HISTORY
182 A
183 .Nm
184 command appeared in
185 .At v7 .
186 .Sh BUGS
187 When using the
188 .Fl F
189 option,
190 .Nm
191 will not detect a file truncation if, between the truncation
192 and the next check of the file size, data written to the file make
193 it larger than the last known file size.