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FILE(1P) POSIX Programmer's Manual FILE(1P)

PROLOG
This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (con-
sult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on
Linux.

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 iden-
tified. If file is a symbolic link, by default the link shall be resolved and file shall test the type of file refer-
enced 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 IEEE Std 1003.1-2001, 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 clas-
sify 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 clas-
sify 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-sen-
sitive 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
Not 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 IEEE Std 1003.1-2001, Section 8.2, Internationalization Variables for the precedence of international-
ization 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 stan-
dard 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 identi-
fied by a position-sensitive test specified by a -M or -m option. Each space shown in the strings shall be exactly one
<space>.

Table: File Utility Output Strings

If file is: <type> shall contain the Notes
string:
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 4,6
ar archive library (see ar) archive 4,6
Extended cpio format (see pax) cpio archive 4,6
Extended tar format (see ustar in pax) tar archive 4,6
Shell script commands text 5,6
C-language source c program text 5,6
FORTRAN source fortran program text 5,6
Regular file whose type cannot be deter- data
mined

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 for-
mat 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 <blank>-separated fields:

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 num-
ber; 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 c, d,
f, s, and u, specifying character, signed decimal, floating point, 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, f, and u can be followed by an optional unsigned decimal integer that specifies the
number of bytes represented by the type. The type specification character f can be followed by an optional F, D, or L,
indicating that the value is of type float, double, or long double, respectively. 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, respec-
tively.

The default number of bytes represented by the type specifiers d, f, and u shall correspond to their respective C-lan-
guage 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 num-
ber 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.

For the type specifier f, the default number of bytes shall correspond to the number of bytes in the basic double preci-
sion floating-point data type of the underlying implementation. 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 float, double, and long dou-
ble. These numbers can also be specified by an application as the characters F, D, and L, respectively.

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 IEEE Std 1003.1-2001, Table 5-1,
Escape Sequences and Associated Actions ( '\\', '\a', '\b', '\f', '\n', '\r', '\t', '\v' ). 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 con-
sist of a backslash followed by the longest sequence of one, two, or three octal-digit characters (01234567). If
the size of a byte on the system is greater than 9 bits, the valid escape sequence used to represent a byte is
implementation-defined.

By default, any value that is not a string shall be interpreted as a signed decimal number. Any such value, with a lead-
ing 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 exe-
cutable 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 -Fq 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, char-
acter 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 util-
ities 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 IEEE Std 1003.1-2001 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-sen-
sitive 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 pro-
gram 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 con-
structs 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-sen-
sitive 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 IEEE Std 1003.1-2001.)

The historical -c option was omitted as not particularly useful to users or portable shell scripts. In addition, a rea-
sonable 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.

FUTURE DIRECTIONS
None.