freebsd-dev/usr.bin/file/file.1

.\" $FreeBSD$
.Dd December 8, 2000
.Dt FILE 1 "Copyright but distributable"
.Os
.Sh NAME
.Nm file
.Nd determine file type
.Sh SYNOPSIS
.Nm
.Op Fl bciknsvzL
.Op Fl f Ar namefile
.Op Fl m Ar magicfiles
.Ar
.Sh DESCRIPTION
This manual page documents version 3.33 of the
.Nm
command.
.Nm 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
.Em first
test that succeeds causes the file type to be printed.
.Pp
The type printed will usually contain one of the words
.Em text
(the file contains only printing characters and a few
common control characters and is probably safe to read on
an
.Tn ASCII
terminal),
.Em executable
(the file contains the result of compiling a program
in a form understandable to some
.Ux
kernel or another), or
.Em data
meaning anything else (data is usually
.Sq 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
.Pa /usr/share/misc/magic
or the program itself,
.Em "preserve these keywords" .
People depend on knowing that all the readable files in a
directory have the word
.Dq text
printed.
Don't do as Berkeley did and change
.Dq shell commands text
to
.Dq shell script .
Note that the file
.Pa /usr/share/misc/magic
is built mechanically from a large number of
small files in the subdirectory
.Pa Magdir
in the source distribution of this program.
.Pp
The filesystem tests are based on examining the return
from a
.Xr 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
.Aq Pa 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)
.Pa a.out
file, whose format is defined in
.Pa a.out.h
and possibly
.Pa exec.h
in the standard include directory.
These files have a
.Sq magic number
stored in a particular place near the beginning of the file
that tells the
.Ux
operating system that the file is a binary executable,
and which of several types thereof.
The concept of
.Sq 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 magic file
.Pa /usr/share/misc/magic .
.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.
.Tn ASCII ,
.Tn ISO-8859-x ,
non-ISO 8-bit extended-ASCII character
sets (such as those used on Macintosh and IBM PC systems),
.Tn UTF-8-encoded Unicode ,
.Tn UTF-16-encoded Unicode ,
and
.Tn 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.
.Tn ASCII ,
.Tn ISO-8859-x ,
.Tn UTF-8 ,
and extended-ASCII files are identified as
.Dq text
because they will be mostly readable on nearly any terminal;
.Tn UTF-16
and
.Tn EBCDIC
are only
.Dq 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.
.Pp
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
.Pa names.h )
that can appear anywhere in the first few blocks of a file.
For example, the keyword
.Em \&.br
indicates that the file is most likely a
.Xr 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
.Xr 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
.Dq data .
.Sh OPTIONS
.Bl -tag -width indent
.It Fl b
Do not prepend filenames to output lines (brief mode).
.It Fl c
Cause a checking printout of the parsed form of the magic file.
This is usually used in conjunction with 
.Fl m
to debug a new magic file before installing it.
.It Fl f Ar namefile
Read the names of the files to be examined from 
.Ar namefile
(one per line) 
before the argument list.
Either 
.Ar namefile
or at least one filename argument must be present;
to test the standard input, use
.Dq Ar -
as a filename argument.
.It Fl i
Causes the file command to output mime type strings rather than the
more traditional human readable ones.
Thus it may say
.Dq text/plain; charset=us-ascii
rather than
.Dq 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
.Dq Pa magic
file.
(See
.Sx FILES
section, below).
.It Fl k
Don't stop at the first match, keep going.
.It Fl m Ar list
Specify an alternate
.Ar list
of files containing magic numbers.
This can be a single file, or a colon-separated list of files.
.It Fl 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.
.It Fl s
Normally, file only attempts to read and determine
the type of argument files which
.Xr stat 2
reports are ordinary files.
This prevents problems, because reading special files
may have peculiar consequences.
Specifying the
.Fl 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
.Xr stat 2
since on some systems it reports a zero size for raw
disk partitions.
.It Fl v
Print the version of the program and exit.
.It Fl z
Try to look inside compressed files.
.It Fl L
Cause symlinks to be followed, as the like-named option in
.Xr ls 1 .
(on systems that support symbolic links).
.El
.Sh FILES
.Bl -tag -width /usr/share/misc/magic.mime -compact
.It Pa /usr/share/misc/magic
default list of magic numbers
.It Pa /usr/share/misc/magic.mime
default list of magic numbers, used to output mime types when the
.Fl i
option is specified.
.El
.Sh ENVIRONMENT
The environment variable
.Ev MAGIC
can be used to set the default magic number files.
.Sh SEE ALSO
.Xr od 1 ,
.Xr strings 1 ,
.Xr magic 5
.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 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. 
.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,
.Bd -literal -compact
>10	string	language impress	(imPRESS data)
.Ed
in an existing magic file would have to be changed to
.Bd -literal -compact
>10	string	language\e impress	(imPRESS data)
.Ed
.Pp
In addition, in this version, if a pattern string contains a backslash,
it must be escaped.  For example
.Bd -literal -compact
0	string		\ebegindata	Andrew Toolkit document
.Ed
in an existing magic file would have to be changed to
.Bd -literal -compact
0	string		\e\ebegindata	Andrew Toolkit document
.Ed
.Pp
SunOS releases 3.2 and later from Sun Microsystems include a
.Xr 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,
.Bd -literal -compact
>16	long&0x7fffffff	>0		not stripped
.Ed
.Sh MAGIC DIRECTORY
The magic file entries have been collected from various sources,
mainly USENET, and contributed by various authors.
.An 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.
If your old
.Nm
command uses a magic file,
keep the old magic file around for comparison purposes
(rename it to 
.Pa /usr/share/misc/magic.orig ) .
.Sh EXAMPLES
.Bd -literal
$ 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
.Ed
.Sh HISTORY
There has been a 
.Nm
command in every
.Ux
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.
.Pp
This program, based on the System V version,
was written by
.An Ian Darwin Aq ian@darwinsys.com
without looking at anybody else's source code.
.Pp
.An John Gilmore
revised the code extensively, making it better than
the first version.
.An Geoff Collyer
found several inadequacies
and provided some magic file entries.
Contributions by the
.Sq \&&
operator by
.An Rob McMahon Aq cudcv@warwick.ac.uk ,
1989.
.Pp
.An Guy Harris Aq guy@netapp.com ,
made many changes from 1993 to the present.
.Pp
Primary development and maintenance from 1990 to the
present by
.An Christos Zoulas Aq christos@astron.com .
.Pp
Altered by
.An Chris Lowth Aq chris@lowth.com ,
2000: Handle  the
.Fl i
option to output mime type strings and using an
alternative magic file and internal logic.
.Pp
Altered by
.An Eric Fischer Aq 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
.Pa Magdir
directory (source for the
.Pa /usr/share/misc/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
.Pa tar.h
and
.Pa is_tar.c
were written by
.An John Gilmore
from his public-domain tar program, and are not covered by
the above license.
.Sh BUGS
There must be a better way to automate the construction of
the
.Pa Magic
file from all the glop in
.Pa Magdir .
What is it?
Better yet, the magic file should be compiled into binary
(say,
.Xr ndbm 3
or, better yet, fixed-length
.Tn 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
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
.Dq else
clause to follow a series of continuation lines.
.Pp
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.
.Pp
It might be advisable to allow upper-case letters in
keywords for e.g.,
.Xr troff 1
commands vs man page macros.
Regular expression support would make this easy.
.Pp
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.
.Pp
The list of keywords in ascmagic probably belongs in the
.Pa Magic
file.
This could be done by using some keyword like
`*' for the offset value.
.Pp
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?
.Pp
The program should provide a way to give an estimate of
.Dq how good
a guess is.
We end up removing guesses (e.g.
.Dq From
as first 5 chars of file) because they are not
as good as other guesses (e.g.
.Dq Newsgroups:
versus
.Dq 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 AVAILABILITY
You can obtain the original author's latest version by
anonymous FTP on
.Pa ftp.astron.com
in the directory
.Pa /pub/file/file-X.YY.tar.gz