file - Unix, Linux Command

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 modifying the file
/usr/share/file/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/file/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 the compiled
magic file
/usr/share/file/magic.mgc , or
/usr/share/file/magic if the compile file does not exist. In addition
file will look in
$HOME/.magic.mgc , or
$HOME/.magic for magic entries.


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’’.




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 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/file/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}
/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



$ 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




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
/usr/share/file/magic file) is too long to include here.
You know who you are; thank you.


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.


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.


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