1TAIL(P) POSIX Programmer's Manual TAIL(P)
2
6 tail - copy the last part of a file
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9 tail [-f][ -c number| -n number][file]
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12 The tail utility shall copy its input file to the standard output
13 beginning at a designated place.
14 Copying shall begin at the point in the file indicated by the -c number
16 or -n number options. The option-argument number shall be counted in
17 units of lines or bytes, according to the options -n and -c. Both line
18 and byte counts start from 1.
19 Tails relative to the end of the file may be saved in an internal buf‐
21 fer, and thus may be limited in length. Such a buffer, if any, shall be
22 no smaller than {LINE_MAX}*10 bytes.
23
25 The tail utility shall conform to the Base Definitions volume of
26 IEEE Std 1003.1-2001, Section 12.2, Utility Syntax Guidelines.
27 The following options shall be supported:
29 -c number
31 The application shall ensure that the number option-argument is
32 a decimal integer whose sign affects the location in the file,
33 measured in bytes, to begin the copying:
34 Sign Copying Starts
36 + Relative to the beginning of the file.
37 - Relative to the end of the file.
38 none Relative to the end of the file.
39 The origin for counting shall be 1; that is, -c +1 represents the first
41 byte of the file, -c -1 the last.
42 -f If the input file is a regular file or if the file operand spec‐
44 ifies a FIFO, do not terminate after the last line of the input
45 file has been copied, but read and copy further bytes from the
46 input file when they become available. If no file operand is
47 specified and standard input is a pipe, the -f option shall be
48 ignored. If the input file is not a FIFO, pipe, or regular file,
49 it is unspecified whether or not the -f option shall be ignored.
50 -n number
52 This option shall be equivalent to -c number, except the start‐
53 ing location in the file shall be measured in lines instead of
54 bytes. The origin for counting shall be 1; that is, -n +1 repre‐
55 sents the first line of the file, -n -1 the last.
56 If neither -c nor -n is specified, -n 10 shall be assumed.
59
61 The following operand shall be supported:
62 file A pathname of an input file. If no file operands are specified,
64 the standard input shall be used.
65
68 The standard input shall be used only if no file operands are speci‐
69 fied. See the INPUT FILES section.
70
72 If the -c option is specified, the input file can contain arbitrary
73 data; otherwise, the input file shall be a text file.
74
76 The following environment variables shall affect the execution of tail:
77 LANG Provide a default value for the internationalization variables
79 that are unset or null. (See the Base Definitions volume of
80 IEEE Std 1003.1-2001, Section 8.2, Internationalization Vari‐
81 ables for the precedence of internationalization variables used
82 to determine the values of locale categories.)
83 LC_ALL If set to a non-empty string value, override the values of all
85 the other internationalization variables.
86 LC_CTYPE
88 Determine the locale for the interpretation of sequences of
89 bytes of text data as characters (for example, single-byte as
90 opposed to multi-byte characters in arguments and input files).
91 LC_MESSAGES
93 Determine the locale that should be used to affect the format
94 and contents of diagnostic messages written to standard error.
95 NLSPATH
97 Determine the location of message catalogs for the processing of
98 LC_MESSAGES .
99
102 Default.
103
105 The designated portion of the input file shall be written to standard
106 output.
107
109 The standard error shall be used only for diagnostic messages.
110
112 None.
113
115 None.
116
118 The following exit values shall be returned:
119 0 Successful completion.
121 >0 An error occurred.
123
126 Default.
127 The following sections are informative.
129
131 The -c option should be used with caution when the input is a text file
132 containing multi-byte characters; it may produce output that does not
133 start on a character boundary.
134 Although the input file to tail can be any type, the results might not
136 be what would be expected on some character special device files or on
137 file types not described by the System Interfaces volume of
138 IEEE Std 1003.1-2001. Since this volume of IEEE Std 1003.1-2001 does
139 not specify the block size used when doing input, tail need not read
140 all of the data from devices that only perform block transfers.
141
143 The -f option can be used to monitor the growth of a file that is being
144 written by some other process. For example, the command:
145 tail -f fred
148 prints the last ten lines of the file fred, followed by any lines that
150 are appended to fred between the time tail is initiated and killed. As
151 another example, the command:
152 tail -f -c 15 fred
155 prints the last 15 bytes of the file fred, followed by any bytes that
157 are appended to fred between the time tail is initiated and killed.
158
160 This version of tail was created to allow conformance to the Utility
161 Syntax Guidelines. The historical -b option was omitted because of the
162 general non-portability of block-sized units of text. The -c option
163 historically meant "characters", but this volume of
164 IEEE Std 1003.1-2001 indicates that it means "bytes". This was selected
165 to allow reasonable implementations when multi-byte characters are pos‐
166 sible; it was not named -b to avoid confusion with the historical -b.
167 The origin of counting both lines and bytes is 1, matching all wide‐
169 spread historical implementations.
170 The restriction on the internal buffer is a compromise between the his‐
172 torical System V implementation of 4096 bytes and the BSD 32768 bytes.
173 The -f option has been implemented as a loop that sleeps for 1 second
175 and copies any bytes that are available. This is sufficient, but if
176 more efficient methods of determining when new data are available are
177 developed, implementations are encouraged to use them.
178 Historical documentation indicates that tail ignores the -f option if
180 the input file is a pipe (pipe and FIFO on systems that support FIFOs).
181 On BSD-based systems, this has been true; on System V-based systems,
182 this was true when input was taken from standard input, but it did not
183 ignore the -f flag if a FIFO was named as the file operand. Since the
184 -f option is not useful on pipes and all historical implementations
185 ignore -f if no file operand is specified and standard input is a pipe,
186 this volume of IEEE Std 1003.1-2001 requires this behavior. However,
187 since the -f option is useful on a FIFO, this volume of
188 IEEE Std 1003.1-2001 also requires that if standard input is a FIFO or
189 a FIFO is named, the -f option shall not be ignored. Although histori‐
190 cal behavior does not ignore the -f option for other file types, this
191 is unspecified so that implementations are allowed to ignore the -f
192 option if it is known that the file cannot be extended.
193 This was changed to the current form based on comments noting that -c
195 was almost never used without specifying a number and that there was no
196 need to specify -l if -n number was given.
197
199 None.
200
202 head
203
205 Portions of this text are reprinted and reproduced in electronic form
206 from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
207 -- Portable Operating System Interface (POSIX), The Open Group Base
208 Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
209 Electrical and Electronics Engineers, Inc and The Open Group. In the
210 event of any discrepancy between this version and the original IEEE and
211 The Open Group Standard, the original IEEE and The Open Group Standard
212 is the referee document. The original Standard can be obtained online
213 at http://www.opengroup.org/unix/online.html .
214IEEE/The Open Group 2003 TAIL(P)