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 correspond-
ing 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 pro-
cessed, 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 char-
acter 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
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-sensi-
tive 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
-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
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).
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.
In the POSIX locale, the following format shall be used to identify each operand, file specified:
", <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 |
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 speci-
fied 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
", <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 deter-
mined, this shall not be considered an error that affects the exit status.
The standard error shall be used only for diagnostic messages.
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, specify-
ing 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
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
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 inter-
preted 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, respec-
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', '', 'f', '
', ' ', 'v'). In addition, the escape sequence
' ' (the <backslash> character followed by a <space> character) shall be recognized to represent a <space> charac-
ter. 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 speci-
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.
CONSEQUENCES OF ERRORS
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 cer-
tainty. 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
Determine whether an argument is a binary executable file:
file -- "$1" | grep -q ':.*executable' &&
printf "%s is executable.
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 speci-
fied 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
In addition, three options have been added to historical practice. The -d flag has been added to permit users to cause their tests to fol-
low 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-compati-
ble 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 implementa-
tion 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 37 36 Packed data
0 string 377 37 Compacted data
0 string 37235 Compressed data
>2 byte&0x80 >0 Block compressed
>2 byte&0x1f x %d bits
0 string 32 01 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>
__.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
Earlier versions of this standard allowed for implementations with bytes other than eight bits, but this has been modified in this version.
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
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2013 Edition, Standard for Information Technol-
ogy -- 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 Stan-
dard 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 .
/The Open Group 2013 FILE(1P)