1TAIL(1P) POSIX Programmer's Manual TAIL(1P)
2
6 This manual page is part of the POSIX Programmer's Manual. The Linux
7 implementation of this interface may differ (consult the corresponding
8 Linux manual page for details of Linux behavior), or the interface may
9 not be implemented on Linux.
10
13 tail — copy the last part of a file
14
16 tail [−f] [−c number|−n number] [file]
17
19 The tail utility shall copy its input file to the standard output
20 beginning at a designated place.
21 Copying shall begin at the point in the file indicated by the −c number
23 or −n number options. The option-argument number shall be counted in
24 units of lines or bytes, according to the options −n and −c. Both line
25 and byte counts start from 1.
26 Tails relative to the end of the file may be saved in an internal buf‐
28 fer, and thus may be limited in length. Such a buffer, if any, shall be
29 no smaller than {LINE_MAX}*10 bytes.
30
32 The tail utility shall conform to the Base Definitions volume of
33 POSIX.1‐2008, Section 12.2, Utility Syntax Guidelines, except that '+'
34 may be recognized as an option delimiter as well as '−'.
35 The following options shall be supported:
37 −c number The application shall ensure that the number option-argument
39 is a decimal integer, optionally including a sign. The sign
40 shall affect the location in the file, measured in bytes, to
41 begin the copying:
42 ┌─────┬────────────────────────────────────────┐
44 │Sign │ Copying Starts │
45 ├─────┼────────────────────────────────────────┤
46 │ + │ Relative to the beginning of the file. │
47 │ − │ Relative to the end of the file. │
48 │none │ Relative to the end of the file. │
49 └─────┴────────────────────────────────────────┘
50 The application shall ensure that if the sign of the number
51 option-argument is '+', the number option-argument is a non-
52 zero decimal integer.
53 The origin for counting shall be 1; that is, −c +1 represents
55 the first byte of the file, −c −1 the last.
56 −f If the input file is a regular file or if the file operand
58 specifies a FIFO, do not terminate after the last line of the
59 input file has been copied, but read and copy further bytes
60 from the input file when they become available. If no file
61 operand is specified and standard input is a pipe or FIFO,
62 the −f option shall be ignored. If the input file is not a
63 FIFO, pipe, or regular file, it is unspecified whether or not
64 the −f option shall be ignored.
65 −n number This option shall be equivalent to −c number, except the
67 starting location in the file shall be measured in lines
68 instead of bytes. The origin for counting shall be 1; that
69 is, −n +1 represents the first line of the file, −n −1 the
70 last.
71 If neither −c nor −n is specified, −n 10 shall be assumed.
73
75 The following operand shall be supported:
76 file A pathname of an input file. If no file operand is specified,
78 the standard input shall be used.
79
81 The standard input shall be used if no file operand is specified, and
82 shall be used if the file operand is '−' and the implementation treats
83 the '−' as meaning standard input. Otherwise, the standard input shall
84 not be used. See the INPUT FILES section.
85
87 If the −c option is specified, the input file can contain arbitrary
88 data; otherwise, the input file shall be a text file.
89
91 The following environment variables shall affect the execution of tail:
92 LANG Provide a default value for the internationalization vari‐
94 ables that are unset or null. (See the Base Definitions vol‐
95 ume of POSIX.1‐2008, Section 8.2, Internationalization Vari‐
96 ables for the precedence of internationalization variables
97 used to determine the values of locale categories.)
98 LC_ALL If set to a non-empty string value, override the values of
100 all the other internationalization variables.
101 LC_CTYPE Determine the locale for the interpretation of sequences of
103 bytes of text data as characters (for example, single-byte as
104 opposed to multi-byte characters in arguments and input
105 files).
106 LC_MESSAGES
108 Determine the locale that should be used to affect the format
109 and contents of diagnostic messages written to standard
110 error.
111 NLSPATH Determine the location of message catalogs for the processing
113 of LC_MESSAGES.
114
116 Default.
117
119 The designated portion of the input file shall be written to standard
120 output.
121
123 The standard error shall be used only for diagnostic messages.
124
126 None.
127
129 None.
130
132 The following exit values shall be returned:
133 0 Successful completion.
135 >0 An error occurred.
137
139 Default.
140 The following sections are informative.
142
144 The −c option should be used with caution when the input is a text file
145 containing multi-byte characters; it may produce output that does not
146 start on a character boundary.
147 Although the input file to tail can be any type, the results might not
149 be what would be expected on some character special device files or on
150 file types not described by the System Interfaces volume of
151 POSIX.1‐2008. Since this volume of POSIX.1‐2008 does not specify the
152 block size used when doing input, tail need not read all of the data
153 from devices that only perform block transfers.
154
156 The −f option can be used to monitor the growth of a file that is being
157 written by some other process. For example, the command:
158 tail −f fred
160 prints the last ten lines of the file fred, followed by any lines that
162 are appended to fred between the time tail is initiated and killed. As
163 another example, the command:
164 tail −f −c 15 fred
166 prints the last 15 bytes of the file fred, followed by any bytes that
168 are appended to fred between the time tail is initiated and killed.
169
171 This version of tail was created to allow conformance to the Utility
172 Syntax Guidelines. The historical −b option was omitted because of the
173 general non-portability of block-sized units of text. The −c option
174 historically meant ``characters'', but this volume of POSIX.1‐2008
175 indicates that it means ``bytes''. This was selected to allow reason‐
176 able implementations when multi-byte characters are possible; it was
177 not named −b to avoid confusion with the historical −b.
178 The origin of counting both lines and bytes is 1, matching all wide‐
180 spread historical implementations. Hence tail −n +0 is not conforming
181 usage because it attempts to output line zero; but note that tail −n 0
182 does conform, and outputs nothing.
183 Earlier versions of this standard allowed the following forms in the
185 SYNOPSIS:
186 tail −[number][b|c|l][f] [file]
188 tail +[number][b|c|l][f] [file]
189 These forms are no longer specified by POSIX.1‐2008, but may be present
191 in some implementations.
192 The restriction on the internal buffer is a compromise between the his‐
194 torical System V implementation of 4096 bytes and the BSD 32768 bytes.
195 The −f option has been implemented as a loop that sleeps for 1 second
197 and copies any bytes that are available. This is sufficient, but if
198 more efficient methods of determining when new data are available are
199 developed, implementations are encouraged to use them.
200 Historical documentation indicates that tail ignores the −f option if
202 the input file is a pipe (pipe and FIFO on systems that support FIFOs).
203 On BSD-based systems, this has been true; on System V-based systems,
204 this was true when input was taken from standard input, but it did not
205 ignore the −f flag if a FIFO was named as the file operand. Since the
206 −f option is not useful on pipes and all historical implementations
207 ignore −f if no file operand is specified and standard input is a pipe,
208 this volume of POSIX.1‐2008 requires this behavior. However, since the
209 −f option is useful on a FIFO, this volume of POSIX.1‐2008 also
210 requires that if a FIFO is named, the −f option shall not be ignored.
211 Earlier versions of this standard did not state any requirement for the
212 case where no file operand is specified and standard input is a FIFO.
213 The standard has been updated to reflect current practice which is to
214 treat this case the same as a pipe on standard input. Although histor‐
215 ical behavior does not ignore the −f option for other file types, this
216 is unspecified so that implementations are allowed to ignore the −f
217 option if it is known that the file cannot be extended.
218
220 None.
221
223 head
224 The Base Definitions volume of POSIX.1‐2008, Chapter 8, Environment
226 Variables, Section 12.2, Utility Syntax Guidelines
227
229 Portions of this text are reprinted and reproduced in electronic form
230 from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
231 -- Portable Operating System Interface (POSIX), The Open Group Base
232 Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri‐
233 cal and Electronics Engineers, Inc and The Open Group. (This is
234 POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In the
235 event of any discrepancy between this version and the original IEEE and
236 The Open Group Standard, the original IEEE and The Open Group Standard
237 is the referee document. The original Standard can be obtained online
238 at http://www.unix.org/online.html .
239 Any typographical or formatting errors that appear in this page are
241 most likely to have been introduced during the conversion of the source
242 files to man page format. To report such errors, see https://www.ker‐
243 nel.org/doc/man-pages/reporting_bugs.html .
244IEEE/The Open Group 2013 TAIL(1P)