1FILE(1P) POSIX Programmer's Manual FILE(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 file — determine file type
13
15 file [-dh] [-M file] [-m file] file...
16 file -i [-h] file...
18
20 The file utility shall perform a series of tests in sequence on each
21 specified file in an attempt to classify it:
22 1. If file does not exist, cannot be read, or its file status could
24 not be determined, the output shall indicate that the file was pro‐
25 cessed, but that its type could not be determined.
26 2. If the file is not a regular file, its file type shall be identi‐
28 fied. The file types directory, FIFO, socket, block special, and
29 character special shall be identified as such. Other implementa‐
30 tion-defined file types may also be identified. If file is a sym‐
31 bolic link, by default the link shall be resolved and file shall
32 test the type of file referenced by the symbolic link. (See the -h
33 and -i options below.)
34 3. If the length of file is zero, it shall be identified as an empty
36 file.
37 4. The file utility shall examine an initial segment of file and shall
39 make a guess at identifying its contents based on position-sensi‐
40 tive tests. (The answer is not guaranteed to be correct; see the
41 -d, -M, and -m options below.)
42 5. The file utility shall examine file and make a guess at identifying
44 its contents based on context-sensitive default system tests. (The
45 answer is not guaranteed to be correct.)
46 6. The file shall be identified as a data file.
48 If file does not exist, cannot be read, or its file status could not be
50 determined, the output shall indicate that the file was processed, but
51 that its type could not be determined.
52 If file is a symbolic link, by default the link shall be resolved and
54 file shall test the type of file referenced by the symbolic link.
55
57 The file utility shall conform to the Base Definitions volume of
58 POSIX.1‐2017, Section 12.2, Utility Syntax Guidelines, except that the
59 order of the -m, -d, and -M options shall be significant.
60 The following options shall be supported by the implementation:
62 -d Apply any position-sensitive default system tests and con‐
64 text-sensitive default system tests to the file. This is the
65 default if no -M or -m option is specified.
66 -h When a symbolic link is encountered, identify the file as a
68 symbolic link. If -h is not specified and file is a symbolic
69 link that refers to a nonexistent file, file shall identify
70 the file as a symbolic link, as if -h had been specified.
71 -i If a file is a regular file, do not attempt to classify the
73 type of the file further, but identify the file as specified
74 in the STDOUT section.
75 -M file Specify the name of a file containing position-sensitive
77 tests that shall be applied to a file in order to classify it
78 (see the EXTENDED DESCRIPTION). No position-sensitive default
79 system tests nor context-sensitive default system tests shall
80 be applied unless the -d option is also specified.
81 -m file Specify the name of a file containing position-sensitive
83 tests that shall be applied to a file in order to classify it
84 (see the EXTENDED DESCRIPTION).
85 If the -m option is specified without specifying the -d option or the
87 -M option, position-sensitive default system tests shall be applied
88 after the position-sensitive tests specified by the -m option. If the
89 -M option is specified with the -d option, the -m option, or both, or
90 the -m option is specified with the -d option, the concatenation of the
91 position-sensitive tests specified by these options shall be applied in
92 the order specified by the appearance of these options. If a -M or -m
93 file option-argument is -, the results are unspecified.
94
96 The following operand shall be supported:
97 file A pathname of a file to be tested.
99
101 The standard input shall be used if a file operand is '-' and the
102 implementation treats the '-' as meaning standard input. Otherwise,
103 the standard input shall not be used.
104
106 The file can be any file type.
107
109 The following environment variables shall affect the execution of file:
110 LANG Provide a default value for the internationalization vari‐
112 ables that are unset or null. (See the Base Definitions vol‐
113 ume of POSIX.1‐2017, Section 8.2, Internationalization Vari‐
114 ables for the precedence of internationalization variables
115 used to determine the values of locale categories.)
116 LC_ALL If set to a non-empty string value, override the values of
118 all the other internationalization variables.
119 LC_CTYPE Determine the locale for the interpretation of sequences of
121 bytes of text data as characters (for example, single-byte as
122 opposed to multi-byte characters in arguments and input
123 files).
124 LC_MESSAGES
126 Determine the locale that should be used to affect the format
127 and contents of diagnostic messages written to standard error
128 and informative messages written to standard output.
129 NLSPATH Determine the location of message catalogs for the processing
131 of LC_MESSAGES.
132
134 Default.
135
137 In the POSIX locale, the following format shall be used to identify
138 each operand, file specified:
139 "%s: %s\n", <file>, <type>
142 The values for <type> are unspecified, except that in the POSIX locale,
144 if file is identified as one of the types listed in the following ta‐
145 ble, <type> shall contain (but is not limited to) the corresponding
146 string, unless the file is identified by a position-sensitive test
147 specified by a -M or -m option. Each <space> shown in the strings shall
148 be exactly one <space>.
149 Table 4-9: File Utility Output Strings
151───────┬─────────────────────────────────────────────┬──────────────────────────────────┬─ │
153 │ If file is: <type│> shall contain the string: Notes│ │
154───────┼─────────────────────────────────────────────┼──────────────────────────────────┼─ │
155 Nonexi│stent canno│t open │ │
156 │ │ │ │
157 │Block special │ block special │ 1 │
158 │Character special │ character special │ 1 │
159 │Directory │ directory │ 1 │
160 │FIFO │ fifo │ 1 │
161 │Socket │ socket │ 1 │
162 │Symbolic link │ symbolic link to │ 1 │
163 │Regular file │ regular file │ 1,2 │
164 │Empty regular file │ empty │ 3 │
165 │Regular file that cannot be read │ cannot open │ 3 │
166 │ │ │ │
167 │Executable binary │ executable │ 3,4,6 │
168 │ar archive library (see ar) │ archive │ 3,4,6 │
169 │Extended cpio format (see pax) │ cpio archive │ 3,4,6 │
170 │Extended tar format (see ustar in pax) │ tar archive │ 3,4,6 │
171 │ │ │ │
172 │Shell script │ commands text │ 3,5,6 │
173 │C-language source │ c program text │ 3,5,6 │
174 │FORTRAN source │ fortran program text │ 3,5,6 │
175 │ │ │ │
176 │Regular file whose type cannot be determined │ data │ 3 │
177 └─────────────────────────────────────────────┴──────────────────────────────────┴───────┘
178 Notes:
179 1. This is a file type test.
181 2. This test is applied only if the -i option is specified.
183 3. This test is applied only if the -i option is not speci‐
185 fied.
186 4. This is a position-sensitive default system test.
188 5. This is a context-sensitive default system test.
190 6. Position-sensitive default system tests and context-sen‐
192 sitive default system tests are not applied if the -M
193 option is specified unless the -d option is also speci‐
194 fied.
195 In the POSIX locale, if file is identified as a symbolic link (see the
197 -h option), the following alternative output format shall be used:
198 "%s: %s %s\n", <file>, <type>, <contents of link>"
201 If the file named by the file operand does not exist, cannot be read,
203 or the type of the file named by the file operand cannot be determined,
204 this shall not be considered an error that affects the exit status.
205
207 The standard error shall be used only for diagnostic messages.
208
210 None.
211
213 A file specified as an option-argument to the -m or -M options shall
214 contain one position-sensitive test per line, which shall be applied to
215 the file. If the test succeeds, the message field of the line shall be
216 printed and no further tests shall be applied, with the exception that
217 tests on immediately following lines beginning with a single '>' char‐
218 acter shall be applied.
219 Each line shall be composed of the following four <tab>-separated
221 fields. (Implementations may allow any combination of one or more
222 white-space characters other than <newline> to act as field separa‐
223 tors.)
224 offset An unsigned number (optionally preceded by a single '>' char‐
226 acter) specifying the offset, in bytes, of the value in the
227 file that is to be compared against the value field of the
228 line. If the file is shorter than the specified offset, the
229 test shall fail.
230 If the offset begins with the character '>', the test con‐
232 tained in the line shall not be applied to the file unless
233 the test on the last line for which the offset did not begin
234 with a '>' was successful. By default, the offset shall be
235 interpreted as an unsigned decimal number. With a leading 0x
236 or 0X, the offset shall be interpreted as a hexadecimal num‐
237 ber; otherwise, with a leading 0, the offset shall be inter‐
238 preted as an octal number.
239 type The type of the value in the file to be tested. The type
241 shall consist of the type specification characters d, s, and
242 u, specifying signed decimal, string, and unsigned decimal,
243 respectively.
244 The type string shall be interpreted as the bytes from the
246 file starting at the specified offset and including the same
247 number of bytes specified by the value field. If insufficient
248 bytes remain in the file past the offset to match the value
249 field, the test shall fail.
250 The type specification characters d and u can be followed by
252 an optional unsigned decimal integer that specifies the num‐
253 ber of bytes represented by the type. The type specification
254 characters d and u can be followed by an optional C, S, I, or
255 L, indicating that the value is of type char, short, int, or
256 long, respectively.
257 The default number of bytes represented by the type speci‐
259 fiers d, f, and u shall correspond to their respective C-lan‐
260 guage types as follows. If the system claims conformance to
261 the C-Language Development Utilities option, those specifiers
262 shall correspond to the default sizes used in the c99 util‐
263 ity. Otherwise, the default sizes shall be implementation-
264 defined.
265 For the type specifier characters d and u, the default number
267 of bytes shall correspond to the size of a basic integer type
268 of the implementation. For these specifier characters, the
269 implementation shall support values of the optional number of
270 bytes to be converted corresponding to the number of bytes in
271 the C-language types char, short, int, or long. These num‐
272 bers can also be specified by an application as the charac‐
273 ters C, S, I, and L, respectively. The byte order used when
274 interpreting numeric values is implementation-defined, but
275 shall correspond to the order in which a constant of the cor‐
276 responding type is stored in memory on the system.
277 All type specifiers, except for s, can be followed by a mask
279 specifier of the form &number. The mask value shall be AND'ed
280 with the value of the input file before the comparison with
281 the value field of the line is made. By default, the mask
282 shall be interpreted as an unsigned decimal number. With a
283 leading 0x or 0X, the mask shall be interpreted as an
284 unsigned hexadecimal number; otherwise, with a leading 0, the
285 mask shall be interpreted as an unsigned octal number.
286 The strings byte, short, long, and string shall also be sup‐
288 ported as type fields, being interpreted as dC, dS, dL, and
289 s, respectively.
290 value The value to be compared with the value from the file.
292 If the specifier from the type field is s or string, then
294 interpret the value as a string. Otherwise, interpret it as a
295 number. If the value is a string, then the test shall succeed
296 only when a string value exactly matches the bytes from the
297 file.
298 If the value is a string, it can contain the following
300 sequences:
301 \character The <backslash>-escape sequences as specified in
303 the Base Definitions volume of POSIX.1‐2017, Ta‐
304 ble 5-1, Escape Sequences and Associated Actions
305 ('\\', '\a', '\b', '\f', '\n', '\r', '\t', '\v').
306 In addition, the escape sequence '\ ' (the <back‐
307 slash> character followed by a <space> character)
308 shall be recognized to represent a <space> char‐
309 acter. The results of using any other character,
310 other than an octal digit, following the <back‐
311 slash> are unspecified.
312 \octal Octal sequences that can be used to represent
314 characters with specific coded values. An octal
315 sequence shall consist of a <backslash> followed
316 by the longest sequence of one, two, or three
317 octal-digit characters (01234567).
318 By default, any value that is not a string shall be inter‐
320 preted as a signed decimal number. Any such value, with a
321 leading 0x or 0X, shall be interpreted as an unsigned hexa‐
322 decimal number; otherwise, with a leading zero, the value
323 shall be interpreted as an unsigned octal number.
324 If the value is not a string, it can be preceded by a charac‐
326 ter indicating the comparison to be performed. Permissible
327 characters and the comparisons they specify are as follows:
328 = The test shall succeed if the value from the file
330 equals the value field.
331 < The test shall succeed if the value from the file is
333 less than the value field.
334 > The test shall succeed if the value from the file is
336 greater than the value field.
337 & The test shall succeed if all of the set bits in the
339 value field are set in the value from the file.
340 ^ The test shall succeed if at least one of the set bits
342 in the value field is not set in the value from the
343 file.
344 x The test shall succeed if the file is large enough to
346 contain a value of the type specified starting at the
347 offset specified.
348 message The message to be printed if the test succeeds. The message
350 shall be interpreted using the notation for the printf for‐
351 matting specification; see printf. If the value field was a
352 string, then the value from the file shall be the argument
353 for the printf formatting specification; otherwise, the value
354 from the file shall be the argument.
355
357 The following exit values shall be returned:
358 0 Successful completion.
360 >0 An error occurred.
362
364 Default.
365 The following sections are informative.
367
369 The file utility can only be required to guess at many of the file
370 types because only exhaustive testing can determine some types with
371 certainty. For example, binary data on some implementations might match
372 the initial segment of an executable or a tar archive.
373 Note that the table indicates that the output contains the stated
375 string. Systems may add text before or after the string. For executa‐
376 bles, as an example, the machine architecture and various facts about
377 how the file was link-edited may be included. Note also that on systems
378 that recognize shell script files starting with "#!" as executable
379 files, these may be identified as executable binary files rather than
380 as shell scripts.
381
383 Determine whether an argument is a binary executable file:
384 file -- "$1" | grep -q ':.*executable' &&
387 printf "%s is executable.\n$1"
388
390 The -f option was omitted because the same effect can (and should) be
391 obtained using the xargs utility.
392 Historical versions of the file utility attempt to identify the follow‐
394 ing types of files: symbolic link, directory, character special, block
395 special, socket, tar archive, cpio archive, SCCS archive, archive
396 library, empty, compress output, pack output, binary data, C source,
397 FORTRAN source, assembler source, nroff/troff/eqn/tbl source troff out‐
398 put, shell script, C shell script, English text, ASCII text, various
399 executables, APL workspace, compiled terminfo entries, and CURSES
400 screen images. Only those types that are reasonably well specified in
401 POSIX or are directly related to POSIX utilities are listed in the ta‐
402 ble.
403 Historical systems have used a ``magic file'' named /etc/magic to help
405 identify file types. Because it is generally useful for users and
406 scripts to be able to identify special file types, the -m flag and a
407 portable format for user-created magic files has been specified. No
408 requirement is made that an implementation of file use this method of
409 identifying files, only that users be permitted to add their own clas‐
410 sifying tests.
411 In addition, three options have been added to historical practice. The
413 -d flag has been added to permit users to cause their tests to follow
414 any default system tests. The -i flag has been added to permit users to
415 test portably for regular files in shell scripts. The -M flag has been
416 added to permit users to ignore any default system tests.
417 The POSIX.1‐2008 description of default system tests and the interac‐
419 tion between the -d, -M, and -m options did not clearly indicate that
420 there were two types of ``default system tests''. The ``position-sensi‐
421 tive tests'' determine file types by looking for certain string or
422 binary values at specific offsets in the file being examined. These
423 position-sensitive tests were implemented in historical systems using
424 the magic file described above. Some of these tests are now built into
425 the file utility itself on some implementations so the output can pro‐
426 vide more detail than can be provided by magic files. For example, a
427 magic file can easily identify a core file on most implementations, but
428 cannot name the program file that dropped the core. A magic file could
429 produce output such as:
430 /home/dwc/core: ELF 32-bit MSB core file SPARC Version 1
433 but by building the test into the file utility, you could get output
435 such as:
436 /home/dwc/core: ELF 32-bit MSB core file SPARC Version 1, from 'testprog'
439 These extended built-in tests are still to be treated as position-sen‐
441 sitive default system tests even if they are not listed in /etc/magic
442 or any other magic file.
443 The context-sensitive default system tests were always built into the
445 file utility. These tests looked for language constructs in text files
446 trying to identify shell scripts, C, FORTRAN, and other computer lan‐
447 guage source files, and even plain text files. With the addition of the
448 -m and -M options the distinction between position-sensitive and con‐
449 text-sensitive default system tests became important because the order
450 of testing is important. The context-sensitive system default tests
451 should never be applied before any position-sensitive tests even if the
452 -d option is specified before a -m option or -M option due to the high
453 probability that the context-sensitive system default tests will incor‐
454 rectly identify arbitrary text files as text files before position-sen‐
455 sitive tests specified by the -m or -M option would be applied to give
456 a more accurate identification.
457 Leaving the meaning of -M - and -m - unspecified allows an existing
459 prototype of these options to continue to work in a backwards-compati‐
460 ble manner. (In that implementation, -M - was roughly equivalent to -d
461 in POSIX.1‐2008.)
462 The historical -c option was omitted as not particularly useful to
464 users or portable shell scripts. In addition, a reasonable implementa‐
465 tion of the file utility would report any errors found each time the
466 magic file is read.
467 The historical format of the magic file was the same as that specified
469 by the Rationale in the ISO POSIX‐2:1993 standard for the offset,
470 value, and message fields; however, it used less precise type fields
471 than the format specified by the current normative text. The new type
472 field values are a superset of the historical ones.
473 The following is an example magic file:
475 0 short 070707 cpio archive
478 0 short 0143561 Byte-swapped cpio archive
479 0 string 070707 ASCII cpio archive
480 0 long 0177555 Very old archive
481 0 short 0177545 Old archive
482 0 short 017437 Old packed data
483 0 string \037\036 Packed data
484 0 string \377\037 Compacted data
485 0 string \037\235 Compressed data
486 >2 byte&0x80 >0 Block compressed
487 >2 byte&0x1f x %d bits
488 0 string \032\001 Compiled Terminfo Entry
489 0 short 0433 Curses screen image
490 0 short 0434 Curses screen image
491 0 string <ar> System V Release 1 archive
492 0 string !<arch>\n__.SYMDEF Archive random library
493 0 string !<arch> Archive
494 0 string ARF_BEGARF PHIGS clear text archive
495 0 long 0x137A2950 Scalable OpenFont binary
496 0 long 0x137A2951 Encrypted scalable OpenFont binary
497 The use of a basic integer data type is intended to allow the implemen‐
499 tation to choose a word size commonly used by applications on that
500 architecture.
501 Earlier versions of this standard allowed for implementations with
503 bytes other than eight bits, but this has been modified in this ver‐
504 sion.
505
507 None.
508
510 ar, ls, pax, printf
511 The Base Definitions volume of POSIX.1‐2017, Table 5-1, Escape
513 Sequences and Associated Actions, Chapter 8, Environment Variables,
514 Section 12.2, Utility Syntax Guidelines
515
517 Portions of this text are reprinted and reproduced in electronic form
518 from IEEE Std 1003.1-2017, Standard for Information Technology -- Por‐
519 table Operating System Interface (POSIX), The Open Group Base Specifi‐
520 cations Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of
521 Electrical and Electronics Engineers, Inc and The Open Group. In the
522 event of any discrepancy between this version and the original IEEE and
523 The Open Group Standard, the original IEEE and The Open Group Standard
524 is the referee document. The original Standard can be obtained online
525 at http://www.opengroup.org/unix/online.html .
526 Any typographical or formatting errors that appear in this page are
528 most likely to have been introduced during the conversion of the source
529 files to man page format. To report such errors, see https://www.ker‐
530 nel.org/doc/man-pages/reporting_bugs.html .
531IEEE/The Open Group 2017 FILE(1P)