Manpages

FILE

NAME

SYNOPSIS

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 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 adding local definitions to /etc/magic, 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 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, 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 /etc/magic and 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, non-ISO 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''.  

OPTIONS

-b, --brief

Do not prepend filenames to output lines.

-c, --checking-printout

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, --compile

Write a magic.mgc output file that contains a pre-parsed version of file.

-f, --files-from 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.

-i, --mime

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, --keep-going

Don't stop at the first match, keep going.

-m, --magic-file list

Specify an alternate list of files containing magic numbers. This can be a single file, or a colon-separated list of files.

-n, --no-buffer

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.

-v

Print the version of the program and exit.

-z, --uncompress

Try to look inside compressed files.

-L, --dereference

This option causes symlinks to be followed, as the like-named option in ls(1). (on systems that support symbolic links).

-s, --special-files

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 filesystem 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.

--help

Print a help message and exit.

--version

Print version information and exit.

FILES

/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.

/etc/magic

Local additions to magic wisdom.

ENVIRONMENT

SEE ALSO

STANDARDS CONFORMANCE

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
 

MAGIC DIRECTORY

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.  

EXAMPLES

$ file file.c file /dev/hda
file.c:   C program text
file:     ELF 32-bit LSB executable, Intel 80386, version 1,
          dynamically linked, not stripped
/dev/hda: block special

$ file -s /dev/hda{,1,2,3,4,5,6,7,8,9,10}
/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 -i file.c file /dev/hda
file.c:      text/x-c
file:        application/x-executable, dynamically linked (uses shared libs), not stripped
/dev/hda:    application/x-not-regular-file

HISTORY

This program, based on the System V version, was written by Ian Darwin <ian@darwinsys.com> 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, cudcv@warwick.ac.uk, 1989.

Guy Harris, guy@netapp.com, made many changes from 1993 to the present.

Primary development and maintenance from 1990 to the present by Christos Zoulas (christos@astron.com).

Altered by Chris Lowth, chris@lowth.com, 2000: Handle the ``-i'' option to output mime type strings and using an alternative magic file and internal logic.

Altered by Eric Fischer (enf@pobox.com), 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 /etc/magic file) is too long to include here. You know who you are; thank you.  

LEGAL NOTICE

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.  

BUGS

File 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.  

AVAILABILITY

This Debian version adds long options and corrects some bugs. It can be obtained from every site carrying a Debian distribution (ftp.debian.org and mirrors).

Index

NAME

SYNOPSIS

DESCRIPTION

OPTIONS

FILES

ENVIRONMENT

SEE ALSO

STANDARDS CONFORMANCE

MAGIC DIRECTORY

EXAMPLES

HISTORY

LEGAL NOTICE

BUGS

AVAILABILITY