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]
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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 IEEE Std 1003.1-2001, Section 12.2, Utility Syntax Guidelines.
33 The following options shall be supported:
35 -c number
37 The application shall ensure that the number option-argument is
38 a decimal integer whose sign affects the location in the file,
39 measured in bytes, to begin the copying:
40 Sign Copying Starts
42 + Relative to the beginning of the file.
43 - Relative to the end of the file.
44 none Relative to the end of the file.
45 The origin for counting shall be 1; that is, -c +1 represents the first
47 byte of the file, -c -1 the last.
48 -f If the input file is a regular file or if the file operand spec‐
50 ifies a FIFO, do not terminate after the last line of the input
51 file has been copied, but read and copy further bytes from the
52 input file when they become available. If no file operand is
53 specified and standard input is a pipe, the -f option shall be
54 ignored. If the input file is not a FIFO, pipe, or regular file,
55 it is unspecified whether or not the -f option shall be ignored.
56 -n number
58 This option shall be equivalent to -c number, except the start‐
59 ing location in the file shall be measured in lines instead of
60 bytes. The origin for counting shall be 1; that is, -n +1 repre‐
61 sents the first line of the file, -n -1 the last.
62 If neither -c nor -n is specified, -n 10 shall be assumed.
65
67 The following operand shall be supported:
68 file A pathname of an input file. If no file operands are specified,
70 the standard input shall be used.
71
74 The standard input shall be used only if no file operands are speci‐
75 fied. See the INPUT FILES section.
76
78 If the -c option is specified, the input file can contain arbitrary
79 data; otherwise, the input file shall be a text file.
80
82 The following environment variables shall affect the execution of tail:
83 LANG Provide a default value for the internationalization variables
85 that are unset or null. (See the Base Definitions volume of
86 IEEE Std 1003.1-2001, Section 8.2, Internationalization Vari‐
87 ables for the precedence of internationalization variables used
88 to determine the values of locale categories.)
89 LC_ALL If set to a non-empty string value, override the values of all
91 the other internationalization variables.
92 LC_CTYPE
94 Determine the locale for the interpretation of sequences of
95 bytes of text data as characters (for example, single-byte as
96 opposed to multi-byte characters in arguments and input files).
97 LC_MESSAGES
99 Determine the locale that should be used to affect the format
100 and contents of diagnostic messages written to standard error.
101 NLSPATH
103 Determine the location of message catalogs for the processing of
104 LC_MESSAGES .
105
108 Default.
109
111 The designated portion of the input file shall be written to standard
112 output.
113
115 The standard error shall be used only for diagnostic messages.
116
118 None.
119
121 None.
122
124 The following exit values shall be returned:
125 0 Successful completion.
127 >0 An error occurred.
129
132 Default.
133 The following sections are informative.
135
137 The -c option should be used with caution when the input is a text file
138 containing multi-byte characters; it may produce output that does not
139 start on a character boundary.
140 Although the input file to tail can be any type, the results might not
142 be what would be expected on some character special device files or on
143 file types not described by the System Interfaces volume of
144 IEEE Std 1003.1-2001. Since this volume of IEEE Std 1003.1-2001 does
145 not specify the block size used when doing input, tail need not read
146 all of the data from devices that only perform block transfers.
147
149 The -f option can be used to monitor the growth of a file that is being
150 written by some other process. For example, the command:
151 tail -f fred
154 prints the last ten lines of the file fred, followed by any lines that
156 are appended to fred between the time tail is initiated and killed. As
157 another example, the command:
158 tail -f -c 15 fred
161 prints the last 15 bytes of the file fred, followed by any bytes that
163 are appended to fred between the time tail is initiated and killed.
164
166 This version of tail was created to allow conformance to the Utility
167 Syntax Guidelines. The historical -b option was omitted because of the
168 general non-portability of block-sized units of text. The -c option
169 historically meant "characters", but this volume of
170 IEEE Std 1003.1-2001 indicates that it means "bytes". This was selected
171 to allow reasonable implementations when multi-byte characters are pos‐
172 sible; it was not named -b to avoid confusion with the historical -b.
173 The origin of counting both lines and bytes is 1, matching all wide‐
175 spread historical implementations.
176 The restriction on the internal buffer is a compromise between the his‐
178 torical System V implementation of 4096 bytes and the BSD 32768 bytes.
179 The -f option has been implemented as a loop that sleeps for 1 second
181 and copies any bytes that are available. This is sufficient, but if
182 more efficient methods of determining when new data are available are
183 developed, implementations are encouraged to use them.
184 Historical documentation indicates that tail ignores the -f option if
186 the input file is a pipe (pipe and FIFO on systems that support FIFOs).
187 On BSD-based systems, this has been true; on System V-based systems,
188 this was true when input was taken from standard input, but it did not
189 ignore the -f flag if a FIFO was named as the file operand. Since the
190 -f option is not useful on pipes and all historical implementations
191 ignore -f if no file operand is specified and standard input is a pipe,
192 this volume of IEEE Std 1003.1-2001 requires this behavior. However,
193 since the -f option is useful on a FIFO, this volume of
194 IEEE Std 1003.1-2001 also requires that if standard input is a FIFO or
195 a FIFO is named, the -f option shall not be ignored. Although histori‐
196 cal behavior does not ignore the -f option for other file types, this
197 is unspecified so that implementations are allowed to ignore the -f
198 option if it is known that the file cannot be extended.
199 This was changed to the current form based on comments noting that -c
201 was almost never used without specifying a number and that there was no
202 need to specify -l if -n number was given.
203
205 None.
206
208 head
209
211 Portions of this text are reprinted and reproduced in electronic form
212 from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
213 -- Portable Operating System Interface (POSIX), The Open Group Base
214 Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
215 Electrical and Electronics Engineers, Inc and The Open Group. In the
216 event of any discrepancy between this version and the original IEEE and
217 The Open Group Standard, the original IEEE and The Open Group Standard
218 is the referee document. The original Standard can be obtained online
219 at http://www.opengroup.org/unix/online.html .
220IEEE/The Open Group 2003 TAIL(1P)