file/doc/file.man

.TH FILE __CSECTION__ "March 2006" "Debian/GNU Linux" "Copyrighted but distributable"
.\" $Id: file.man,v 1.59 2006/11/17 16:11:10 christos Exp $
.SH NAME
file
\- determine file type
.SH SYNOPSIS
.B file
[
.B \-bchikLnNprsvz
]
[
.B \-f
.I namefile
]
[
.B \-F
.I separator
]
[
.B \-m 
.I magicfiles
]
.I file
\&...
.br
.B file
.B -C
[
.B \-m 
magicfile ]
.SH DESCRIPTION
This manual page documents version __VERSION__ of the
.B file
command.
.PP
.B 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
.I first
test that succeeds causes the file type to be printed.
.PP
The type printed will usually contain one of the words
.B text
(the file contains only
printing characters and a few common control
characters and is probably safe to read on an
.SM ASCII
terminal),
.B executable
(the file contains the result of compiling a program
in a form understandable to some \s-1UNIX\s0 kernel or another),
or
.B 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
.IR /etc/magic ,
.BR "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
.I __MAGIC__
is built mechanically from a large number of small files in
the subdirectory
.I Magdir
in the source distribution of this program.
.PP
The filesystem tests are based on examining the return from a
.BR 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
.IR <sys/stat.h>  .
.PP
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)
.I a.out
file, whose format is defined in 
.I a.out.h
and possibly
.I 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 \s-1UNIX\s0 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
.I /etc/magic
and the compiled
magic file
.I __MAGIC__.mgc ,
or 
.I __MAGIC__
if the compile file does not exist. In addition
.B file
will look in
.I $HOME/.magic.mgc ,
or
.I $HOME/.magic
for magic entries.
.PP
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,
.B 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.
.PP
Once
.B 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
.IR names.h )
that can appear anywhere in the first few blocks of a file.
For example, the keyword
.B .br
indicates that the file is most likely a
.BR troff (1)
input file, just as the keyword 
.B 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 
.BR tar (1)
archives).
.PP
Any file that cannot be identified as having been written
in any of the character sets listed above is simply said to be ``data''.
.SH OPTIONS
.TP 8
.B "\-b, \-\-brief"
Do not prepend filenames to output lines (brief mode).
.TP 8
.B "\-c, \-\-checking\-printout"
Cause a checking printout of the parsed form of the magic file.
This is usually used in conjunction with 
.B \-m
to debug a new magic file before installing it.
.TP 8
.B "\-C, \-\-compile"
Write a magic.mgc output file that contains a pre-parsed version of
file.
.TP 8
.BI "\-f, \-\-files\-from" " namefile"
Read the names of the files to be examined from 
.I namefile
(one per line) 
before the argument list.
Either 
.I namefile
or at least one filename argument must be present;
to test the standard input, use ``\-'' as a filename argument.
.TP 8
.BI "\-F, \-\-separator" " separator"
Use the specified string as the separator between the filename and the
file result returned. Defaults to ``:''.
.TP 8
.B "\-h, \-\-no-dereference"
option causes symlinks not to be followed
(on systems that support symbolic links). This is the default if the
environment variable
.I POSIXLY_CORRECT
is not defined.
.TP 8
.B "\-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 recognized 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).
.TP 8
.B "\-k, \-\-keep\-going"
Don't stop at the first match, keep going. Subsequent matches will be
prepended by ``\\012\- ''. (If you want a newline, see ``\-r'' option.)
.TP 8
.B "\-L, \-\-dereference"
option causes symlinks to be followed, as the like-named option in
.BR ls (1)
(on systems that support symbolic links).
This is the default if the environment variable
.I POSIXLY_CORRECT
is defined.
.TP 8
.BI "\-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.
If a compiled magic file is found alongside, it will be used instead.
With the \-i or \-\-mime option, the program adds ".mime" to each file name.
.TP 8
.B "\-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.
.TP 8
.B "\-N, \-\-no\-pad"
Don't pad filenames so that they align in the output.
.TP 8
.B "\-p, \-\-preserve\-date"
On systems that support
.BR utime (2)
or
.BR utimes(2),
attempt to preserve the access time of files analyzed, to pretend that
.BR file (2)
never read them.
.TP 8
.B "\-r, \-\-raw"
Don't translate unprintable characters to \eooo.
Normally
.B file
translates unprintable characters to their octal representation.
.TP 8
.B "\-s, \-\-special\-files"
Normally,
.B file
only attempts to read and determine the type of argument files which
.BR stat (2)
reports are ordinary files.
This prevents problems, because reading special files may have peculiar
consequences.
Specifying the
.BR \-s
option causes
.B 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
.B file
to disregard the file size as reported by
.BR stat (2)
since on some systems it reports a zero size for raw disk partitions.
.TP 8
.B "\-v, \-\-version"
Print the version of the program and exit.
.TP 8
.B "\-z, \-\-uncompress"
Try to look inside compressed files.
.B "\-0, \-\-print0"
Output a null character ('\0') after the end of the filename. Nice to
.BR cut (1)
the output. This does not affect the separator which is still printed.
.TP 8
.B "\-\-help"
Print a help message and exit.
.SH FILES
.TP
.I __MAGIC__.mgc
Default compiled list of magic numbers
.TP
.I __MAGIC__
Default list of magic numbers
.TP
.I __MAGIC__.mime.mgc
Default compiled list of magic numbers, used to output mime types when
the -i option is specified.
.TP
.I __MAGIC__.mime
Default list of magic numbers, used to output mime types when the -i option
is specified.

.SH ENVIRONMENT
The environment variable
.B MAGIC
can be used to set the default magic number file name.
If that variable is set, then
.B file
will not attempt to open
.BR $HOME/.magic .
.B file
adds ".mime" and/or ".mgc" to the value of this variable as appropriate.
However,
.B file
has to exist in order for
.BR file .mime
to be considered.
The environment variable
.B POSIXLY_CORRECT
controls (on systems that support symbolic links), if
.B file
will attempt to follow symlinks or not. If set, then
.B file
follows symlink, otherwise it does not. This is also controlled
by the
.B L
and
.B h
options.
.SH SEE ALSO
.BR magic (__FSECTION__)
\- description of magic file format.
.br
.BR strings (1), " od" (1), " hexdump(1)"
\- tools for examining non-textfiles.
.SH 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 behavior 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. 
.\" URL: http://www.opengroup.org/onlinepubs/009695399/utilities/file.html
.PP
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,
.br
>10	string	language impress\ 	(imPRESS data)
.br
in an existing magic file would have to be changed to
.br
>10	string	language\e impress	(imPRESS data)
.br
In addition, in this version, if a pattern string contains a backslash,
it must be escaped.
For example
.br
0	string		\ebegindata	Andrew Toolkit document
.br
in an existing magic file would have to be changed to
.br
0	string		\e\ebegindata	Andrew Toolkit document
.br
.PP
SunOS releases 3.2 and later from Sun Microsystems include a
.BR 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,
.br
>16	long&0x7fffffff	>0		not stripped
.SH MAGIC DIRECTORY
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.
.PP
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.
.SH EXAMPLES
.nf
$ 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}
/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/{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

.fi
.SH HISTORY
There has been a 
.B file
command in every \s-1UNIX\s0 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.
.PP
This program, based on the System V version,
was written by Ian Darwin <ian@darwinsys.com>
without looking at anybody else's source code.
.PP
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.
.PP
Guy Harris, guy@netapp.com, made many changes from 1993 to the present.
.PP
Primary development and maintenance from 1990 to the present by
Christos Zoulas (christos@astron.com).
.PP
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.
.PP
Altered by Eric Fischer (enf@pobox.com), July, 2000,
to identify character codes and attempt to identify the languages
of non-ASCII files.
.PP
The list of contributors to the "Magdir" directory (source for the
.I __MAGIC__
file) is too long to include here.
You know who you are; thank you.
.SH LEGAL NOTICE
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.
.PP
The files
.I tar.h
and
.I is_tar.c
were written by John Gilmore from his public-domain
.B tar
program, and are not covered by the above license.
.SH BUGS
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,
.BR ndbm (3)
or, better yet, fixed-length
.SM 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.
.PP
.B File
uses several algorithms that favor speed over accuracy,
thus it can be misled about the contents of
text
files.
.PP
The support for
text
files (primarily for programming languages)
is simplistic, inefficient and requires recompilation to update.
.PP
There should be an ``else'' clause to follow a series of continuation lines.
.PP
The magic file and keywords should have regular expression support.
Their use of
.SM "ASCII TAB"
as a field delimiter is ugly and makes
it hard to edit the files, but is entrenched.
.PP
It might be advisable to allow upper-case letters in keywords
for e.g.,
.BR troff (1)
commands vs man page macros.
Regular expression support would make this easy.
.PP
The program doesn't grok \s-2FORTRAN\s0.
It should be able to figure \s-2FORTRAN\s0 by seeing some keywords which 
appear indented at the start of line.
Regular expression support would make this easy.
.PP
The list of keywords in 
.I ascmagic
probably belongs in the Magic file.
This could be done by using some keyword like `*' for the offset value.
.PP
Another optimization 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?
.PP
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.  
.PP
This program is slower than some vendors' file commands.
The new support for multiple character codes makes it even slower.
.PP
This manual page, and particularly this section, is too long.
.SH RETURN CODE
.B file
almost always returns 0. It returns a different if it cannot open a file.
.SH AVAILABILITY
You can obtain the original author's latest version by anonymous FTP
on
.B ftp.astron.com
in the directory
.I /pub/file/file-X.YZ.tar.gz
.PP
This
.B Debian
version adds a number of new magix entries. It can be
obtained from every site carrying a
.B Debian
distribution (ftp.debian.org and mirrors).