file(1posix)
NAME
file — determine file type
SYNOPSIS
file [−dh] [−M file] [−m file] file... file −i [−h] file...
DESCRIPTION
The file utility shall perform a series of tests in sequence on each
specified file in an attempt to classify it:
1. If file does not exist, cannot be read, or its file status could
not be determined, the output shall indicate that the file was
processed, but that its type could not be determined.
2. If the file is not a regular file, its file type shall be
identified. The file types directory, FIFO, socket, block special,
and character special shall be identified as such. Other
implementation-defined file types may also be identified. If file
is a symbolic link, by default the link shall be resolved and file
shall test the type of file referenced by the symbolic link. (See
the −h and −i options below.)
3. If the length of file is zero, it shall be identified as an empty
file.
4. The file utility shall examine an initial segment of file and shall
make a guess at identifying its contents based on position-
sensitive tests. (The answer is not guaranteed to be correct; see
the −d, −M, and −m options below.)
5. The file utility shall examine file and make a guess at identifying
its contents based on context-sensitive default system tests. (The
answer is not guaranteed to be correct.)
6. The file shall be identified as a data file.
If file does not exist, cannot be read, or its file status could not be
determined, the output shall indicate that the file was processed, but
that its type could not be determined.
If file is a symbolic link, by default the link shall be resolved and
file shall test the type of file referenced by the symbolic link.
OPTIONS
The file utility shall conform to the Base Definitions volume of
POSIX.1‐2008, Section 12.2, Utility Syntax Guidelines, except that the
order of the −m, −d, and −M options shall be significant.
The following options shall be supported by the implementation:
−d Apply any position-sensitive default system tests and
context-sensitive default system tests to the file. This is
the default if no −M or −m option is specified.
−h When a symbolic link is encountered, identify the file as a
symbolic link. If −h is not specified and file is a symbolic
link that refers to a nonexistent file, file shall identify
the file as a symbolic link, as if −h had been specified.
−i If a file is a regular file, do not attempt to classify the
type of the file further, but identify the file as specified
in the STDOUT section.
−M file Specify the name of a file containing position-sensitive
tests that shall be applied to a file in order to classify it
(see the EXTENDED DESCRIPTION). No position-sensitive default
system tests nor context-sensitive default system tests shall
be applied unless the −d option is also specified.
−m file Specify the name of a file containing position-sensitive
tests that shall be applied to a file in order to classify it
(see the EXTENDED DESCRIPTION).
If the −m option is specified without specifying the −d option or the
−M option, position-sensitive default system tests shall be applied
after the position-sensitive tests specified by the −m option. If the
−M option is specified with the −d option, the −m option, or both, or
the −m option is specified with the −d option, the concatenation of the
position-sensitive tests specified by these options shall be applied in
the order specified by the appearance of these options. If a −M or −m
file option-argument is −, the results are unspecified.
OPERANDS
The following operand shall be supported: file A pathname of a file to be tested.
STDIN
The standard input shall be used if a file operand is '−' and the implementation treats the '−' as meaning standard input. Otherwise, the standard input shall not be used.
INPUT FILES
The file can be any file type.
ENVIRONMENT VARIABLES
The following environment variables shall affect the execution of file:
LANG Provide a default value for the internationalization
variables that are unset or null. (See the Base Definitions
volume of POSIX.1‐2008, Section 8.2, Internationalization
Variables for the precedence of internationalization
variables used to determine the values of locale categories.)
LC_ALL If set to a non-empty string value, override the values of
all the other internationalization variables.
LC_CTYPE Determine the locale for the interpretation of sequences of
bytes of text data as characters (for example, single-byte as
opposed to multi-byte characters in arguments and input
files).
LC_MESSAGES
Determine the locale that should be used to affect the format
and contents of diagnostic messages written to standard error
and informative messages written to standard output.
NLSPATH Determine the location of message catalogs for the processing
of LC_MESSAGES.
ASYNCHRONOUS EVENTS
Default.
STDOUT
In the POSIX locale, the following format shall be used to identify
each operand, file specified:
"%s: %s\n", <file>, <type>
The values for <type> are unspecified, except that in the POSIX locale,
if file is identified as one of the types listed in the following
table, <type> shall contain (but is not limited to) the corresponding
string, unless the file is identified by a position-sensitive test
specified by a −M or −m option. Each <space> shown in the strings shall
be exactly one <space>.
Table 4-9: File Utility Output Strings
│ If file is: <type> shall contain the string: Notes│ │
Nonexistent cannot open │ │
│ │ │ │
│Block special │ block special │ 1 │
│Character special │ character special │ 1 │
│Directory │ directory │ 1 │
│FIFO │ fifo │ 1 │
│Socket │ socket │ 1 │
│Symbolic link │ symbolic link to │ 1 │
│Regular file │ regular file │ 1,2 │
│Empty regular file │ empty │ 3 │
│Regular file that cannot be read │ cannot open │ 3 │
│ │ │ │
│Executable binary │ executable │ 3,4,6 │
│ar archive library (see ar) │ archive │ 3,4,6 │
│Extended cpio format (see pax) │ cpio archive │ 3,4,6 │
│Extended tar format (see ustar in pax) │ tar archive │ 3,4,6 │
│ │ │ │
│Shell script │ commands text │ 3,5,6 │
│C-language source │ c program text │ 3,5,6 │
│FORTRAN source │ fortran program text │ 3,5,6 │
│ │ │ │
│Regular file whose type cannot be determined │ data │ 3 │
└─────────────────────────────────────────────┴──────────────────────────────────┴───────┘
Notes:
1. This is a file type test.
2. This test is applied only if the −i option is specified.
3. This test is applied only if the −i option is not
specified.
4. This is a position-sensitive default system test.
5. This is a context-sensitive default system test.
6. Position-sensitive default system tests and context-
sensitive default system tests are not applied if the −M
option is specified unless the −d option is also
specified.
In the POSIX locale, if file is identified as a symbolic link (see the
−h option), the following alternative output format shall be used:
"%s: %s %s\n", <file>, <type>, <contents of link>"
If the file named by the file operand does not exist, cannot be read,
or the type of the file named by the file operand cannot be determined,
this shall not be considered an error that affects the exit status.
STDERR
The standard error shall be used only for diagnostic messages.
OUTPUT FILES
None.
EXTENDED DESCRIPTION
A file specified as an option-argument to the −m or −M options shall
contain one position-sensitive test per line, which shall be applied to
the file. If the test succeeds, the message field of the line shall be
printed and no further tests shall be applied, with the exception that
tests on immediately following lines beginning with a single '>'
character shall be applied.
Each line shall be composed of the following four <tab>-separated
fields. (Implementations may allow any combination of one or more
white-space characters other than <newline> to act as field
separators.)
offset An unsigned number (optionally preceded by a single '>'
character) specifying the offset, in bytes, of the value in
the file that is to be compared against the value field of
the line. If the file is shorter than the specified offset,
the test shall fail.
If the offset begins with the character '>', the test
contained in the line shall not be applied to the file unless
the test on the last line for which the offset did not begin
with a '>' was successful. By default, the offset shall be
interpreted as an unsigned decimal number. With a leading 0x
or 0X, the offset shall be interpreted as a hexadecimal
number; otherwise, with a leading 0, the offset shall be
interpreted as an octal number.
type The type of the value in the file to be tested. The type
shall consist of the type specification characters d, s, and
u, specifying signed decimal, string, and unsigned decimal,
respectively.
The type string shall be interpreted as the bytes from the
file starting at the specified offset and including the same
number of bytes specified by the value field. If insufficient
bytes remain in the file past the offset to match the value
field, the test shall fail.
The type specification characters d and u can be followed by
an optional unsigned decimal integer that specifies the
number of bytes represented by the type. The type
specification characters d and u can be followed by an
optional C, S, I, or L, indicating that the value is of type
char, short, int, or long, respectively.
The default number of bytes represented by the type
specifiers d, f, and u shall correspond to their respective
C-language types as follows. If the system claims conformance
to the C-Language Development Utilities option, those
specifiers shall correspond to the default sizes used in the
c99 utility. Otherwise, the default sizes shall be
implementation-defined.
For the type specifier characters d and u, the default number
of bytes shall correspond to the size of a basic integer type
of the implementation. For these specifier characters, the
implementation shall support values of the optional number of
bytes to be converted corresponding to the number of bytes in
the C-language types char, short, int, or long. These
numbers can also be specified by an application as the
characters C, S, I, and L, respectively. The byte order used
when interpreting numeric values is implementation-defined,
but shall correspond to the order in which a constant of the
corresponding type is stored in memory on the system.
All type specifiers, except for s, can be followed by a mask
specifier of the form &number. The mask value shall be AND'ed
with the value of the input file before the comparison with
the value field of the line is made. By default, the mask
shall be interpreted as an unsigned decimal number. With a
leading 0x or 0X, the mask shall be interpreted as an
unsigned hexadecimal number; otherwise, with a leading 0, the
mask shall be interpreted as an unsigned octal number.
The strings byte, short, long, and string shall also be
supported as type fields, being interpreted as dC, dS, dL,
and s, respectively.
value The value to be compared with the value from the file.
If the specifier from the type field is s or string, then
interpret the value as a string. Otherwise, interpret it as a
number. If the value is a string, then the test shall succeed
only when a string value exactly matches the bytes from the
file.
If the value is a string, it can contain the following
sequences:
\character The <backslash>-escape sequences as specified in
the Base Definitions volume of POSIX.1‐2008,
Table 5-1, Escape Sequences and Associated
Actions ('\\', '
', ' ', '\f', '\n', '\r',
'\t', '\v'). In addition, the escape sequence
'\ ' (the <backslash> character followed by a
<space> character) shall be recognized to
represent a <space> character. The results of
using any other character, other than an octal
digit, following the <backslash> are unspecified.
\octal Octal sequences that can be used to represent
characters with specific coded values. An octal
sequence shall consist of a <backslash> followed
by the longest sequence of one, two, or three
octal-digit characters (01234567).
By default, any value that is not a string shall be
interpreted as a signed decimal number. Any such value, with
a leading 0x or 0X, shall be interpreted as an unsigned
hexadecimal number; otherwise, with a leading zero, the value
shall be interpreted as an unsigned octal number.
If the value is not a string, it can be preceded by a
character indicating the comparison to be performed.
Permissible characters and the comparisons they specify are
as follows:
= The test shall succeed if the value from the file
equals the value field.
< The test shall succeed if the value from the file is
less than the value field.
> The test shall succeed if the value from the file is
greater than the value field.
& The test shall succeed if all of the set bits in the
value field are set in the value from the file.
^ The test shall succeed if at least one of the set bits
in the value field is not set in the value from the
file.
x The test shall succeed if the file is large enough to
contain a value of the type specified starting at the
offset specified.
message The message to be printed if the test succeeds. The message
shall be interpreted using the notation for the printf
formatting specification; see printf. If the value field was
a string, then the value from the file shall be the argument
for the printf formatting specification; otherwise, the value
from the file shall be the argument.
EXIT STATUS
The following exit values shall be returned:
0 Successful completion.
>0 An error occurred.
CONSEQUENCES OF ERRORS
Default. The following sections are informative.
APPLICATION USAGE
The file utility can only be required to guess at many of the file types because only exhaustive testing can determine some types with certainty. For example, binary data on some implementations might match the initial segment of an executable or a tar archive. Note that the table indicates that the output contains the stated string. Systems may add text before or after the string. For executables, as an example, the machine architecture and various facts about how the file was link-edited may be included. Note also that on systems that recognize shell script files starting with "#!" as executable files, these may be identified as executable binary files rather than as shell scripts.
EXAMPLES
Determine whether an argument is a binary executable file:
file −− "$1" | grep −q ':.*executable' &&
printf "%s is executable.\n$1"
RATIONALE
The −f option was omitted because the same effect can (and should) be
obtained using the xargs utility.
Historical versions of the file utility attempt to identify the
following types of files: symbolic link, directory, character special,
block special, socket, tar archive, cpio archive, SCCS archive, archive
library, empty, compress output, pack output, binary data, C source,
FORTRAN source, assembler source, nroff/troff/eqn/tbl source troff
output, shell script, C shell script, English text, ASCII text, various
executables, APL workspace, compiled terminfo entries, and CURSES
screen images. Only those types that are reasonably well specified in
POSIX or are directly related to POSIX utilities are listed in the
table.
Historical systems have used a ``magic file'' named /etc/magic to help
identify file types. Because it is generally useful for users and
scripts to be able to identify special file types, the −m flag and a
portable format for user-created magic files has been specified. No
requirement is made that an implementation of file use this method of
identifying files, only that users be permitted to add their own
classifying tests.
In addition, three options have been added to historical practice. The
−d flag has been added to permit users to cause their tests to follow
any default system tests. The −i flag has been added to permit users to
test portably for regular files in shell scripts. The −M flag has been
added to permit users to ignore any default system tests.
The POSIX.1‐2008 description of default system tests and the
interaction between the −d, −M, and −m options did not clearly indicate
that there were two types of ``default system tests''. The ``position-
sensitive tests'' determine file types by looking for certain string or
binary values at specific offsets in the file being examined. These
position-sensitive tests were implemented in historical systems using
the magic file described above. Some of these tests are now built into
the file utility itself on some implementations so the output can
provide more detail than can be provided by magic files. For example, a
magic file can easily identify a core file on most implementations, but
cannot name the program file that dropped the core. A magic file could
produce output such as:
/home/dwc/core: ELF 32-bit MSB core file SPARC Version 1
but by building the test into the file utility, you could get output
such as:
/home/dwc/core: ELF 32-bit MSB core file SPARC Version 1, from 'testprog'
These extended built-in tests are still to be treated as position-
sensitive default system tests even if they are not listed in
/etc/magic or any other magic file.
The context-sensitive default system tests were always built into the
file utility. These tests looked for language constructs in text files
trying to identify shell scripts, C, FORTRAN, and other computer
language source files, and even plain text files. With the addition of
the −m and −M options the distinction between position-sensitive and
context-sensitive default system tests became important because the
order of testing is important. The context-sensitive system default
tests should never be applied before any position-sensitive tests even
if the −d option is specified before a −m option or −M option due to
the high probability that the context-sensitive system default tests
will incorrectly identify arbitrary text files as text files before
position-sensitive tests specified by the −m or −M option would be
applied to give a more accurate identification.
Leaving the meaning of −M − and −m − unspecified allows an existing
prototype of these options to continue to work in a backwards-
compatible manner. (In that implementation, −M − was roughly equivalent
to −d in POSIX.1‐2008.)
The historical −c option was omitted as not particularly useful to
users or portable shell scripts. In addition, a reasonable
implementation of the file utility would report any errors found each
time the magic file is read.
The historical format of the magic file was the same as that specified
by the Rationale in the ISO POSIX‐2:1993 standard for the offset,
value, and message fields; however, it used less precise type fields
than the format specified by the current normative text. The new type
field values are a superset of the historical ones.
The following is an example magic file:
0 short 070707 cpio archive
0 short 0143561 Byte-swapped cpio archive
0 string 070707 ASCII cpio archive
0 long 0177555 Very old archive
0 short 0177545 Old archive
0 short 017437 Old packed data
0 string \037\036 Packed data
0 string \377\037 Compacted data
0 string \037\235 Compressed data
>2 byte&0x80 >0 Block compressed
>2 byte&0x1f x %d bits
0 string \032\001 Compiled Terminfo Entry
0 short 0433 Curses screen image
0 short 0434 Curses screen image
0 string <ar> System V Release 1 archive
0 string !<arch>\n__.SYMDEF Archive random library
0 string !<arch> Archive
0 string ARF_BEGARF PHIGS clear text archive
0 long 0x137A2950 Scalable OpenFont binary
0 long 0x137A2951 Encrypted scalable OpenFont binary
The use of a basic integer data type is intended to allow the
implementation to choose a word size commonly used by applications on
that architecture.
Earlier versions of this standard allowed for implementations with
bytes other than eight bits, but this has been modified in this
version.
FUTURE DIRECTIONS
None.
SEE ALSO
ar, ls, pax, printf The Base Definitions volume of POSIX.1‐2008, Table 5-1, Escape Sequences and Associated Actions, Chapter 8, Environment Variables, Section 12.2, Utility Syntax Guidelines
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, Copyright (C) 2013 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at http://www.unix.org/online.html . Any typographical or formatting errors that appear in this page are most likely to have been introduced during the conversion of the source files to man page format. To report such errors, see https://www.kernel.org/doc/man-pages/reporting_bugs.html .
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