FILE(1)

NAME
file - determine file type

SYNOPSIS
file [ -c ] [ -f namefile ] [ -m magicfile ] file ...

DESCRIPTION
File tests each argument in an attempt to classify it. There are three sets of tests, performed in this order: filesystem 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 ASCII 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 /etc/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 one computer vendor did - change ``shell commands text'' to ``shell script''.

The filesystem 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 and symbolic links on 4.2BSD, named pipes (FIFOs) on System V) 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 in these files is read from the magic file /etc/magic .

If an argument appears to be an ASCII file, file attempts to guess its language. 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 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 archives) and determine whether an unknown file should be labelled as `ascii text' or `data'.

Use -m file to specify an alternate file of magic numbers.

The -c option causes 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.

The -f namefile option specifies that the names of the files to be examined are to be read (one per line) from namefile 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.

FILES
/etc/magic - default list of magic numbers

SEE ALSO
Magic(FILES) - description of magic file format.
Strings(1), od(1) - tools for examining non-textfiles.

STANDARDS CONFORMANCE
This program is believed to exceed the System V Interface Definition 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)

The Sun Microsystems implementation of System V compatibility includes a file(1) command that has some extentions. My version differs from Sun's only in minor ways. The significant one is the `&' operator, which Sun's program expects as, for example,
>16    long&0x7fffffff    >0    not stripped
would be entered in my version as
>16    long    &0x7fffffff    not stripped
which is a little less general; it simply tests (location 16)&0x7ffffff and returns its truth value as a C expression.

MAGIC DIRECTORY
The magic file entries have been collected from various sources, mainly USENET, and contributed by various authors. Ian Darwin (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 /etc/magic.orig).

HISTORY
There has been a file command in every UNIX since at least Research Version 6 (man page dated January, 1975). 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 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. The program has undergone continued evolution since.

NOTICE
Copyright (c) Ian F. Darwin, 1986 and 1987. Written by Ian F. Darwin, UUCP address {utzoo | ihnp4}!darwin!ian, Internet address ian@sq.com, postal address: P.O. Box 603, Station F, Toronto, Ontario, CANADA M4Y 2L8.

Strtok.c and getopt.c written by and copyright by Henry Spencer, utzoo!henry.

This software is not subject to any license of the American Telephone and Telegraph Company or of the Regents of the University of California.

Permission is granted to anyone to use this software for any purpose on any computer system, and to alter it and redistribute it freely, subject to the following restrictions:

1. The author is not responsible for the consequences of use of this software, no matter how awful, even if they arise from flaws in it.

2. The origin of this software must not be misrepresented, either by explicit claim or by omission. Since few users ever read sources, credits must appear in the documentation.

3. Altered versions must be plainly marked as such, and must not be misrepresented as being the original software. Since few users ever read sources, credits must appear in the documentation.

4. This notice may not be removed or altered.

A few support files (getopt, strtok) distributed with this package are by Henry Spencer and are subject to the same terms as above.

A few simple support files (strtol, strchr) distributed with this package are in the public domain; they are so marked.

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 restrictions.

BUGS
There must be a way to automate the construction of the Magic file from all the glop in magdir. What is it?

File uses several algorithms that favor speed over accuracy, thus it can be misled about the contents of ASCII files.

The support for ASCII files (primarily for programming languages) is simplistic, inefficient and requires recompilation to update.

Should there be an ``else'' clause to follow a series of continuation lines?

Is it worthwhile to implement recursive file inspection, so that compressed files, uuencoded, etc., can say ``compressed ascii text'' or ``compressed executable'' or ``compressed tar archive" or whatever?

The magic file and keywords should have regular expression support.

It might be advisable to allow upper-case letters in keywords for e.g., troff 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.

The program should malloc the magic file structures, rather than using a fixed-size array as at present.

The magic file should be compiled into binary (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. But then there would have to be yet another magic number for the magic.out file.

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.

Perhaps the program should automatically try all tests with byte-swapping done, to avoid having to figure out the byte-swapped values when constructing the magic file. Of course this will run more slowly, so it should probably be an option (-a?).

This manual page, and particularly this section, is too long.