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
12 tail — copy the last part of a file
13
15 tail [-f] [-c number|-n number] [file]
16
18 The tail utility shall copy its input file to the standard output
19 beginning at a designated place.
20 Copying shall begin at the point in the file indicated by the -c number
22 or -n number options. The option-argument number shall be counted in
23 units of lines or bytes, according to the options -n and -c. Both line
24 and byte counts start from 1.
25 Tails relative to the end of the file may be saved in an internal buf‐
27 fer, and thus may be limited in length. Such a buffer, if any, shall be
28 no smaller than {LINE_MAX}*10 bytes.
29
31 The tail utility shall conform to the Base Definitions volume of
32 POSIX.1‐2017, Section 12.2, Utility Syntax Guidelines, except that '+'
33 may be recognized as an option delimiter as well as '-'.
34 The following options shall be supported:
36 -c number The application shall ensure that the number option-argument
38 is a decimal integer, optionally including a sign. The sign
39 shall affect the location in the file, measured in bytes, to
40 begin the copying:
41 ┌─────┬────────────────────────────────────────┐
43 │Sign │ Copying Starts │
44 ├─────┼────────────────────────────────────────┤
45 │ + │ Relative to the beginning of the file. │
46 │ - │ Relative to the end of the file. │
47 │none │ Relative to the end of the file. │
48 └─────┴────────────────────────────────────────┘
49 The application shall ensure that if the sign of the number
50 option-argument is '+', the number option-argument is a non-
51 zero decimal integer.
52 The origin for counting shall be 1; that is, -c +1 represents
54 the first byte of the file, -c -1 the last.
55 -f If the input file is a regular file or if the file operand
57 specifies a FIFO, do not terminate after the last line of the
58 input file has been copied, but read and copy further bytes
59 from the input file when they become available. If no file
60 operand is specified and standard input is a pipe or FIFO,
61 the -f option shall be ignored. If the input file is not a
62 FIFO, pipe, or regular file, it is unspecified whether or not
63 the -f option shall be ignored.
64 -n number This option shall be equivalent to -c number, except the
66 starting location in the file shall be measured in lines
67 instead of bytes. The origin for counting shall be 1; that
68 is, -n +1 represents the first line of the file, -n -1 the
69 last.
70 If neither -c nor -n is specified, -n 10 shall be assumed.
72
74 The following operand shall be supported:
75 file A pathname of an input file. If no file operand is specified,
77 the standard input shall be used.
78
80 The standard input shall be used if no file operand is specified, and
81 shall be used if the file operand is '-' and the implementation treats
82 the '-' as meaning standard input. Otherwise, the standard input shall
83 not be used. See the INPUT FILES section.
84
86 If the -c option is specified, the input file can contain arbitrary
87 data; otherwise, the input file shall be a text file.
88
90 The following environment variables shall affect the execution of tail:
91 LANG Provide a default value for the internationalization vari‐
93 ables that are unset or null. (See the Base Definitions vol‐
94 ume of POSIX.1‐2017, Section 8.2, Internationalization Vari‐
95 ables for the precedence of internationalization variables
96 used to determine the values of locale categories.)
97 LC_ALL If set to a non-empty string value, override the values of
99 all the other internationalization variables.
100 LC_CTYPE Determine the locale for the interpretation of sequences of
102 bytes of text data as characters (for example, single-byte as
103 opposed to multi-byte characters in arguments and input
104 files).
105 LC_MESSAGES
107 Determine the locale that should be used to affect the format
108 and contents of diagnostic messages written to standard
109 error.
110 NLSPATH Determine the location of message catalogs for the processing
112 of LC_MESSAGES.
113
115 Default.
116
118 The designated portion of the input file shall be written to standard
119 output.
120
122 The standard error shall be used only for diagnostic messages.
123
125 None.
126
128 None.
129
131 The following exit values shall be returned:
132 0 Successful completion.
134 >0 An error occurred.
136
138 Default.
139 The following sections are informative.
141
143 The -c option should be used with caution when the input is a text file
144 containing multi-byte characters; it may produce output that does not
145 start on a character boundary.
146 Although the input file to tail can be any type, the results might not
148 be what would be expected on some character special device files or on
149 file types not described by the System Interfaces volume of
150 POSIX.1‐2017. Since this volume of POSIX.1‐2017 does not specify the
151 block size used when doing input, tail need not read all of the data
152 from devices that only perform block transfers.
153 When using tail to process pathnames, and the -c option is not speci‐
155 fied, it is recommended that LC_ALL, or at least LC_CTYPE and LC_COL‐
156 LATE, are set to POSIX or C in the environment, since pathnames can
157 contain byte sequences that do not form valid characters in some
158 locales, in which case the utility's behavior would be undefined. In
159 the POSIX locale each byte is a valid single-byte character, and there‐
160 fore this problem is avoided.
161
163 The -f option can be used to monitor the growth of a file that is being
164 written by some other process. For example, the command:
165 tail -f fred
168 prints the last ten lines of the file fred, followed by any lines that
170 are appended to fred between the time tail is initiated and killed. As
171 another example, the command:
172 tail -f -c 15 fred
175 prints the last 15 bytes of the file fred, followed by any bytes that
177 are appended to fred between the time tail is initiated and killed.
178
180 This version of tail was created to allow conformance to the Utility
181 Syntax Guidelines. The historical -b option was omitted because of the
182 general non-portability of block-sized units of text. The -c option
183 historically meant ``characters'', but this volume of POSIX.1‐2017
184 indicates that it means ``bytes''. This was selected to allow reason‐
185 able implementations when multi-byte characters are possible; it was
186 not named -b to avoid confusion with the historical -b.
187 The origin of counting both lines and bytes is 1, matching all wide‐
189 spread historical implementations. Hence tail -n +0 is not conforming
190 usage because it attempts to output line zero; but note that tail -n 0
191 does conform, and outputs nothing.
192 Earlier versions of this standard allowed the following forms in the
194 SYNOPSIS:
195 tail -[number][b|c|l][f] [file]
198 tail +[number][b|c|l][f] [file]
199 These forms are no longer specified by POSIX.1‐2008, but may be present
201 in some implementations.
202 The restriction on the internal buffer is a compromise between the his‐
204 torical System V implementation of 4096 bytes and the BSD 32768 bytes.
205 The -f option has been implemented as a loop that sleeps for 1 second
207 and copies any bytes that are available. This is sufficient, but if
208 more efficient methods of determining when new data are available are
209 developed, implementations are encouraged to use them.
210 Historical documentation indicates that tail ignores the -f option if
212 the input file is a pipe (pipe and FIFO on systems that support FIFOs).
213 On BSD-based systems, this has been true; on System V-based systems,
214 this was true when input was taken from standard input, but it did not
215 ignore the -f flag if a FIFO was named as the file operand. Since the
216 -f option is not useful on pipes and all historical implementations
217 ignore -f if no file operand is specified and standard input is a pipe,
218 this volume of POSIX.1‐2017 requires this behavior. However, since the
219 -f option is useful on a FIFO, this volume of POSIX.1‐2017 also
220 requires that if a FIFO is named, the -f option shall not be ignored.
221 Earlier versions of this standard did not state any requirement for the
222 case where no file operand is specified and standard input is a FIFO.
223 The standard has been updated to reflect current practice which is to
224 treat this case the same as a pipe on standard input. Although histor‐
225 ical behavior does not ignore the -f option for other file types, this
226 is unspecified so that implementations are allowed to ignore the -f
227 option if it is known that the file cannot be extended.
228
230 None.
231
233 head
234 The Base Definitions volume of POSIX.1‐2017, Chapter 8, Environment
236 Variables, Section 12.2, Utility Syntax Guidelines
237
239 Portions of this text are reprinted and reproduced in electronic form
240 from IEEE Std 1003.1-2017, Standard for Information Technology -- Por‐
241 table Operating System Interface (POSIX), The Open Group Base Specifi‐
242 cations Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of
243 Electrical and Electronics Engineers, Inc and The Open Group. In the
244 event of any discrepancy between this version and the original IEEE and
245 The Open Group Standard, the original IEEE and The Open Group Standard
246 is the referee document. The original Standard can be obtained online
247 at http://www.opengroup.org/unix/online.html .
248 Any typographical or formatting errors that appear in this page are
250 most likely to have been introduced during the conversion of the source
251 files to man page format. To report such errors, see https://www.ker‐
252 nel.org/doc/man-pages/reporting_bugs.html .
253IEEE/The Open Group 2017 TAIL(1P)