file (command)

"File (software)" redirects here. For the file management program bundled with iOS 11, see Files (software).

file
Example usage of file
Developer	AT&T Bell Laboratories
Initial release	1973 (1973) as part of Unix Research Version 4; 1986 (1986) open-source reimplementation
Stable release	5.46  / 27 November 2024 (27 November 2024)
Repository	github.com/file/file
Written in	C
Operating system	Unix, Unix-like, Plan 9, IBM i
Platform	Cross-platform
Type	File type detector
License	BSD license, CDDL Plan 9: MIT License
Website	darwinsys.com/file/

file is shell command for reporting the type of data contained in a file. It is commonly supported in Unix and Unix-like operating systems.

The Single UNIX Specification (SUS) requires the command to exhibit the following behavior with respect to the file specified via the command-line:

1. If the file cannot be read, or its Unix file type is undetermined, the command will report that the file was processed but its type was undetermined
2. The command must be able to determine the types directory, FIFO, socket, block special file, and character special file
3. A zero-length file is reported as such
4. An initial part of file is considered and the command is to use position-sensitive tests
5. The entire file is considered and the command is to use context-sensitive tests
6. Otherwise, the file is reported as a data file

Position-sensitive tests are normally implemented by matching various locations within the file against a textual database of magic numbers (see the Usage section). This differs from other simpler methods such as file extensions and schemes like MIME.

In the System V implementation, the Ian Darwin implementation, and the OpenBSD implementation, the command uses a database to drive the probing of the lead bytes. That database is stored as a file that is located in /etc/magic, /usr/share/file/magic or similar.

History

The file command originated in Unix Research Version 4 in 1973. System V brought a major update with several important changes, most notably moving the file type information into an external text file rather than compiling it into the binary itself.

Most major BSD and Linux distributions include a free, open-source implementation that was written from scratch by Ian Darwin in 1986–87. It keeps file type information in a text file with a format based on that of the System V version. It was expanded by Geoff Collyer in 1989 and since then has had input from many others, including Guy Harris, Chris Lowth and Eric Fischer. From late 1993 onward, its maintenance has been organized by Christos Zoulas. The OpenBSD system has its own subset implementation written from scratch, but still uses the Darwin/Zoulas collection of magic file formatted information.

The file command was ported to the IBM i operating system.

Usage

SUS mandates the following command-line options:

• -M file, prevents the default position-sensitive and context-sensitive tests in favor of the tests specified in a specially formatted file
• -m file, same as for -M, but with tests in addition to the default
• -d, selects default position-sensitive and context-sensitive tests; this is the default behavior unless -M or -m are specified
• -h, do not dereference symbolic links that point to an existing file or directory
• -L, dereference the symbolic link that points to an existing file or directory
• -i, do not classify the file further than to report as: nonexistent, a block special file, a character special file, a directory, a FIFO, a socket, a symbolic link, or a regular file; Linux and BSD systems behave differently with this option and instead output an Internet media type ("MIME type") identifying the recognized file format

Implementations may add extra options. Ian Darwin's implementation adds -s 'special files', -k 'keep-going' or -r 'raw' (examples below), among many others.

The command can be fooled various ways including by putting a magic number into the content even if the rest of the content does not match what the magic number indicates. The command is not fool-proof, and its report cannot be taken as completely trustworthy.

Examples

$ file file.c
file.c: C program text

$ file program
program: ELF 32-bit LSB executable, Intel 80386, version 1 (SYSV), dynamically linked
    (uses shared libs), stripped

$ file /dev/hda1
/dev/hda1: block special (0/0)

$ file -s /dev/hda1
/dev/hda1: Linux/i386 ext2 filesystem

Note that -s is a non-standard option available only on the Ian Darwin branch, which tells file to read device files and try to identify their contents rather than merely identifying them as device files. Normally file does not try to read device files since reading such a file can have undesirable side effects.

$ file -k -r libmagic-dev_5.35-4_armhf.deb    # (on Linux)
libmagic-dev_5.35-4_armhf.deb: Debian binary package (format 2.0)
- current ar archive
- data

Through Ian Darwin's non-standard option -k the program does not stop after the first hit found, but looks for other matching patterns. The -r option, which is available in some versions, causes the unprintable new line character to be displayed in its raw form rather than in its octal representation.

$ file compressed.gz
compressed.gz: gzip compressed data, deflated, original filename, `compressed', last
    modified: Thu Jan 26 14:08:23 2006, os: Unix

$ file -i compressed.gz    # (on Linux)
compressed.gz: application/x-gzip; charset=binary

$ file data.ppm
data.ppm: Netpbm PPM "rawbits" image data

$ file /bin/cat
/bin/cat: Mach-O universal binary with 2 architectures
/bin/cat (for architecture ppc7400):	Mach-O executable ppc
/bin/cat (for architecture i386):	Mach-O executable i386

$ file /usr/bin/vi
/usr/bin/vi: symbolic link to vim

Identifying symbolic links is not available on all platforms and will be dereferenced if -L is passed or POSIXLY_CORRECT is set.

Libmagic library

As of version 4.00 of the Ian Darwin/Christos Zoulas version of file, the functionality of file is incorporated into a libmagic library that is accessible via C (and C-compatible) linking; file is implemented using that library.

External links

• file: determine file type – Shell and Utilities Reference, The Single UNIX Specification, Version 5 from The Open Group

Manual pages

• file(1) – Linux User Manual – User Commands from Manned.org
• libmagic(3) – NetBSD Library Functions Manual
• libmagic(3) – Linux Programmer's Manual – Library Functions from Manned.org
• file(1) – OpenBSD General Commands Manual – a non-Ian Darwin implementation
• file(1) – Plan 9 Programmer's Manual, Volume 1 – a non-Ian Darwin, non-SUS implementation

Other

• Fine Free File Command – homepage for Ian Darwin's version of file used in major BSD and Linux distributions.
  • mailing list
  • releases
• binwalk, a firmware analysis tool that carves files based on libmagic signatures
• TrID, an alternative providing ranked answers (instead of just one) based on statistics.
• Magika, an ML-based tool, by Google Research