file -- determine file type
file [-bciknNsvzL] [-f namefile] [-F separator] [-m magicfiles] file ...
file -C [-m magicfile]
This manual page documents version 3.41 of the file utility which tests
each argument in an attempt to classify it. There are three sets of
tests, performed in this order: file system tests, magic number tests,
and language tests. The first test that succeeds causes the file type to
be printed.
The type printed will usually contain one of the words ``text'' (the file
contains only printing characters and a few common control characters and
is probably safe to read on an ASCII terminal), ``executable'' (the file
contains the result of compiling a program in a form understandable to
some UNIX kernel or another), or ``data'' meaning anything else (data is
usually `binary' or non-printable). Exceptions are well-known file formats
(core files, tar archives) that are known to contain binary data.
When modifying the file /usr/share/misc/magic or the program itself,
preserve these keywords. People depend on knowing that all the readable
files in a directory have the word ``text'' printed. Don't do as Berkeley
did and change ``shell commands text'' to ``shell script''. Note
that the file /usr/share/misc/magic is built mechanically from a large
number of small files in the subdirectory Magdir in the source distribution
of this program.
The file system tests are based on examining the return from a stat(2)
system call. The program checks to see if the file is empty, or if it's
some sort of special file. Any known file types appropriate to the system
you are running on (sockets, symbolic links, or named pipes (FIFOs)
on those systems that implement them) are intuited if they are defined in
the system header file <sys/stat.h>.
The magic number tests are used to check for files with data in particular
fixed formats. The canonical example of this is a binary executable
(compiled program) a.out file, whose format is defined in <a.out.h> and
possibly <exec.h> in the standard include directory. These files have a
`magic number' stored in a particular place near the beginning of the
file that tells the UNIX operating system that the file is a binary executable,
and which of several types thereof. The concept of `magic
number' has been applied by extension to data files. Any file with some
invariant identifier at a small fixed offset into the file can usually be
described in this way. The information identifying these files is read
from the compiled magic file /usr/share/misc/magic.mgc, or
/usr/share/misc/magic if the compile file does not exist.
If a file does not match any of the entries in the magic file, it is
examined to see if it seems to be a text file. ASCII, ISO-8859-x, nonISO
8-bit extended-ASCII character sets (such as those used on Macintosh
and IBM PC systems), UTF-8-encoded Unicode, UTF-16-encoded Unicode, and
EBCDIC character sets can be distinguished by the different ranges and
sequences of bytes that constitute printable text in each set. If a file
passes any of these tests, its character set is reported. ASCII,
ISO-8859-x, UTF-8, and extended-ASCII files are identified as ``text''
because they will be mostly readable on nearly any terminal; UTF-16 and
EBCDIC are only ``character data'' because, while they contain text, it
is text that will require translation before it can be read. In addition,
file will attempt to determine other characteristics of text-type
files. If the lines of a file are terminated by CR, CRLF, or NEL,
instead of the UNIX-standard LF, this will be reported. Files that contain
embedded escape sequences or overstriking will also be identified.
Once file has determined the character set used in a text-type file, it
will attempt to determine in what language the file is written. The language
tests look for particular strings (cf names.h) that can appear anywhere
in the first few blocks of a file. For example, the keyword .br
indicates that the file is most likely a troff(1) input file, just as the
keyword struct indicates a C program. These tests are less reliable than
the previous two groups, so they are performed last. The language test
routines also test for some miscellany (such as tar(1) archives).
Any file that cannot be identified as having been written in any of the
character sets listed above is simply said to be ``data''.
-b Do not prepend filenames to output lines (brief mode).
-c Cause a checking printout of the parsed form of the magic file.
This is usually used in conjunction with -m to debug a new magic
file before installing it.
-C Write a magic.mgc output file that contains a pre-parsed version
of file.
-f namefile
Read the names of the files to be examined from namefile (one per
line) before the argument list. Either namefile or at least one
filename argument must be present; to test the standard input,
use ``-'' as a filename argument.
-F separator
Use the specified separator character instead of `:'.
-i Causes the file command to output mime type strings rather than
the more traditional human readable ones. Thus it may say
``text/plain; charset=us-ascii'' rather than ``ASCII text''. In
order for this option to work, file changes the way it handles
files recognised by the command itself (such as many of the text
file types, directories etc), and makes use of an alternative
magic file. (See FILES section, below).
-k Don't stop at the first match, keep going.
-m list
Specify an alternate list of files containing magic numbers.
This can be a single file, or a colon-separated list of files.
-n Force stdout to be flushed after checking each file. This is
only useful if checking a list of files. It is intended to be
used by programs that want filetype output from a pipe.
-N Don't pad output to align filenames nicely.
-v Print the version of the program and exit.
-z Try to look inside compressed files.
-L option causes symlinks to be followed, as the like-named option
in ls(1). (on systems that support symbolic links).
-s Normally, file only attempts to read and determine the type of
argument files which stat(2) reports are ordinary files. This
prevents problems, because reading special files may have peculiar
consequences. Specifying the -s option causes file to also
read argument files which are block or character special files.
This is useful for determining the file system types of the data
in raw disk partitions, which are block special files. This
option also causes file to disregard the file size as reported by
stat(2) since on some systems it reports a zero size for raw disk
partitions.
/usr/share/misc/magic.mgc default compiled list of magic numbers
/usr/share/misc/magic default list of magic numbers
/usr/share/misc/magic.mime default list of magic numbers, used to output
mime types when the -i option is specified.
The environment variable MAGIC can be used to set the default magic number
files.
hexdump(1), od(1), strings(1), magic(5)
STANDARDS CONFORMANCE [Toc] [Back] This program is believed to exceed the System V Interface Definition,
Fourth Edition (``SVID4'') of FILE(CMD), as near as one can determine
from the vague language contained therein. Its behaviour is mostly compatible
with the System V program of the same name. This version knows
more magic, however, so it will produce different (albeit more accurate)
output in many cases.
The one significant difference between this version and System V is that
this version treats any white space as a delimiter, so that spaces in
pattern strings must be escaped. For example,
>10 string language impress (imPRESS data)
in an existing magic file would have to be changed to
>10 string language\ impress (imPRESS data)
In addition, in this version, if a pattern string contains a backslash,
it must be escaped. For example
0 string \begindata Andrew Toolkit document
in an existing magic file would have to be changed to
0 string \\begindata Andrew Toolkit document
SunOS releases 3.2 and later from Sun Microsystems include a file(1) command
derived from the System V one, but with some extensions. My version
differs from Sun's only in minor ways. It includes the extension of the
`&' operator, used as, for example,
>16 long&0x7fffffff >0 not stripped
The magic file entries have been collected from various sources, mainly
USENET, and contributed by various authors. Christos Zoulas (address
below) will collect additional or corrected magic file entries. A consolidation
of magic file entries will be distributed periodically.
The order of entries in the magic file is significant. Depending on what
system you are using, the order that they are put together may be incorrect.
If your old file command uses a magic file, keep the old magic
file around for comparison purposes (rename it to
/usr/share/misc/magic.orig).
$ file file.c file /dev/{wd0a,hda}
file.c: C program text
file: ELF 32-bit LSB executable, Intel 80386, version 1 (SYSV),
dynamically linked (uses shared libs), stripped
/dev/wd0a: block special (0/0)
/dev/hda: block special (3/0)
$ file -s /dev/wd0{b,d}
/dev/wd0b: data
/dev/wd0d: x86 boot sector
$ file -s /dev/hda{,1,2,3,4,5,6,7,8,9,10} # Linux
/dev/hda: x86 boot sector
/dev/hda1: Linux/i386 ext2 filesystem
/dev/hda2: x86 boot sector
/dev/hda3: x86 boot sector, extended partition table
/dev/hda4: Linux/i386 ext2 filesystem
/dev/hda5: Linux/i386 swap file
/dev/hda6: Linux/i386 swap file
/dev/hda7: Linux/i386 swap file
/dev/hda8: Linux/i386 swap file
/dev/hda9: empty
/dev/hda10: empty
$ file -s /dev/rwd0e # BSD
/dev/rwd0e:
Unix Fast File system (little-endian),
last mounted on /usr,
last written at Mon Feb 10 13:22:40 2003,
clean flag 2,
number of blocks 28754208,
number of data blocks 27812712,
number of cylinder groups 3566,
block size 8192,
fragment size 1024,
minimum percentage of free blocks 5,
rotational delay 0ms,
disk rotational speed 60rps,
TIME optimization
$ file -i file.c file /dev/{wd0a,hda}
file.c: text/x-c
file: application/x-executable, dynamically linked (uses shared libs),
not stripped
/dev/hda: application/x-not-regular-file
/dev/wd0a: application/x-not-regular-file
There has been a file command in every UNIX since at least Research Version
4 (man page dated November, 1973). The System V version introduced
one significant major change: the external list of magic number types.
This slowed the program down slightly but made it a lot more flexible.
This program, based on the System V version, was written by Ian Darwin
<[email protected]> without looking at anybody else's source code.
John Gilmore revised the code extensively, making it better than the
first version. Geoff Collyer found several inadequacies and provided
some magic file entries. Contributions by the `&' operator by Rob
McMahon <[email protected]>, 1989.
Guy Harris <[email protected]>, made many changes from 1993 to the present.
Primary development and maintenance from 1990 to the present by Christos
Zoulas <[email protected]>.
Altered by Chris Lowth <[email protected]>, 2000: Handle the -i option to
output mime type strings and using an alternative magic file and internal
logic.
Altered by Eric Fischer <[email protected]>, July, 2000, to identify character
codes and attempt to identify the languages of non-ASCII files.
The list of contributors to the Magdir directory (source for the
/usr/share/misc/magic file) is too long to include here. You know who
you are; thank you.
Copyright (c) Ian F. Darwin, Toronto, Canada, 1986-1999. Covered by the
standard Berkeley Software Distribution copyright; see the file
LEGAL.NOTICE in the source distribution.
The files tar.h and is_tar.c were written by John Gilmore from his public-domain
tar program, and are not covered by the above license.
There must be a better way to automate the construction of the Magic file
from all the glop in Magdir. What is it? Better yet, the magic file
should be compiled into binary (say, ndbm(3) or, better yet, fixed-length
ASCII strings for use in heterogenous network environments) for faster
startup. Then the program would run as fast as the Version 7 program of
the same name, with the flexibility of the System V version.
The file utility uses several algorithms that favor speed over accuracy,
thus it can be misled about the contents of text files.
The support for text files (primarily for programming languages) is simplistic,
inefficient and requires recompilation to update.
There should be an else clause to follow a series of continuation lines.
The magic file and keywords should have regular expression support.
Their use of ASCII TAB as a field delimiter is ugly and makes it hard to
edit the files, but is entrenched.
It might be advisable to allow upper-case letters in keywords for e.g.,
troff(1) commands vs man page macros. Regular expression support would
make this easy.
The program doesn't grok FORTRAN. It should be able to figure FORTRAN by
seeing some keywords which appear indented at the start of line. Regular
expression support would make this easy.
The list of keywords in ascmagic probably belongs in the Magic file.
This could be done by using some keyword like `*' for the offset value.
Another optimisation would be to sort the magic file so that we can just
run down all the tests for the first byte, first word, first long, etc,
once we have fetched it. Complain about conflicts in the magic file
entries. Make a rule that the magic entries sort based on file offset
rather than position within the magic file?
The program should provide a way to give an estimate of ``how good'' a
guess is. We end up removing guesses (e.g. ``From '' as first 5 chars of
file) because they are not as good as other guesses (e.g. ``Newsgroups:''
versus ``Return-Path:''). Still, if the others don't pan out, it should
be possible to use the first guess.
This program is slower than some vendors' file commands. The new support
for multiple character codes makes it even slower.
This manual page, and particularly this section, is too long.
You can obtain the original author's latest version by anonymous FTP on
ftp.astron.com in the directory /pub/file/file-X.YZ.tar.gz
FreeBSD 5.2.1 February 27, 2003 FreeBSD 5.2.1 [ Back ] |
