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PROLOG | NAME | SYNOPSIS | DESCRIPTION | OPTIONS | OPERANDS | STDIN | INPUT FILES | ENVIRONMENT VARIABLES | ASYNCHRONOUS EVENTS | STDOUT | STDERR | OUTPUT FILES | EXTENDED DESCRIPTION | EXIT STATUS | CONSEQUENCES OF ERRORS | APPLICATION USAGE | EXAMPLES | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
FILE(1P) POSIX Programmer's Manual FILE(1P)
This manual page is part of the POSIX Programmer's Manual. The Linux
implementation of this interface may differ (consult the
corresponding Linux manual page for details of Linux behavior), or
the interface may not be implemented on Linux.
file — determine file type
file [−dh] [−M file] [−m file] file...
file −i [−h] file...
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.
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.
The following operand shall be supported:
file A pathname of a file to be tested.
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.
The file can be any file type.
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.
Default.
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│ │
───────┼─────────────────────────────────────────────┼──────────────────────────────────┼─ │
Nonexi│stent canno│t 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.
The standard error shall be used only for diagnostic messages.
None.
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 ('\\', '\a', '\b', '\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.
The following exit values shall be returned:
0 Successful completion.
>0 An error occurred.
Default.
The following sections are informative.
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.
Determine whether an argument is a binary executable file:
file −− "$1" | grep −q ':.*executable' &&
printf "%s is executable.\n$1"
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.
None.
ar(1p), ls(1p), pax(1p), printf(1p)
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
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 .
IEEE/The Open Group 2013 FILE(1P)