454 lines
14 KiB
Groff
454 lines
14 KiB
Groff
.TH FILE __CSECTION__ "Copyright but distributable"
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.\" $Id: file.man,v 1.42 2002/07/03 18:26:37 christos Exp $
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.SH NAME
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file
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\- determine file type
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.SH SYNOPSIS
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.B file
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[
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.B \-bciknsvzL
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]
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[
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.B \-f
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.I namefile
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]
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[
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.B \-m
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.I magicfiles
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]
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.I file
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\*[Am]...
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.br
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.B file
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.B -C
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[
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.B \-m
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magicfile ]
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.SH DESCRIPTION
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This manual page documents version __VERSION__ of the
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.B file
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command.
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.PP
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.B File
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tests each argument in an attempt to classify it.
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There are three sets of tests, performed in this order:
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filesystem tests, magic number tests, and language tests.
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The
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.I first
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test that succeeds causes the file type to be printed.
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.PP
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The type printed will usually contain one of the words
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.B text
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(the file contains only
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printing characters and a few common control
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characters and is probably safe to read on an
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.SM ASCII
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terminal),
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.B executable
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(the file contains the result of compiling a program
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in a form understandable to some \s-1UNIX\s0 kernel or another),
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or
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.B data
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meaning anything else (data is usually `binary' or non-printable).
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Exceptions are well-known file formats (core files, tar archives)
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that are known to contain binary data.
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When modifying the file
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.I __MAGIC__
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or the program itself,
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.B "preserve these keywords" .
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People depend on knowing that all the readable files in a directory
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have the word ``text'' printed.
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Don't do as Berkeley did and change ``shell commands text''
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to ``shell script''.
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Note that the file
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.I __MAGIC__
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is built mechanically from a large number of small files in
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the subdirectory
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.I Magdir
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in the source distribution of this program.
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.PP
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The filesystem tests are based on examining the return from a
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.BR stat (2)
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system call.
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The program checks to see if the file is empty,
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or if it's some sort of special file.
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Any known file types appropriate to the system you are running on
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(sockets, symbolic links, or named pipes (FIFOs) on those systems that
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implement them)
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are intuited if they are defined in
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the system header file
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.IR \*[Lt]sys/stat.h\*[Gt] .
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.PP
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The magic number tests are used to check for files with data in
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particular fixed formats.
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The canonical example of this is a binary executable (compiled program)
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.I a.out
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file, whose format is defined in
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.I a.out.h
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and possibly
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.I exec.h
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in the standard include directory.
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These files have a `magic number' stored in a particular place
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near the beginning of the file that tells the \s-1UNIX\s0 operating system
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that the file is a binary executable, and which of several types thereof.
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The concept of `magic number' has been applied by extension to data files.
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Any file with some invariant identifier at a small fixed
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offset into the file can usually be described in this way.
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The information identifying these files is read from the compiled
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magic file
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.I __MAGIC__.mgc ,
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or
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.I __MAGIC__
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if the compile file does not exist.
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.PP
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If a file does not match any of the entries in the magic file,
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it is examined to see if it seems to be a text file.
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ASCII, ISO-8859-x, non-ISO 8-bit extended-ASCII character sets
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(such as those used on Macintosh and IBM PC systems),
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UTF-8-encoded Unicode, UTF-16-encoded Unicode, and EBCDIC
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character sets can be distinguished by the different
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ranges and sequences of bytes that constitute printable text
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in each set.
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If a file passes any of these tests, its character set is reported.
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ASCII, ISO-8859-x, UTF-8, and extended-ASCII files are identified
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as ``text'' because they will be mostly readable on nearly any terminal;
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UTF-16 and EBCDIC are only ``character data'' because, while
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they contain text, it is text that will require translation
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before it can be read.
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In addition,
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.B file
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will attempt to determine other characteristics of text-type files.
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If the lines of a file are terminated by CR, CRLF, or NEL, instead
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of the Unix-standard LF, this will be reported.
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Files that contain embedded escape sequences or overstriking
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will also be identified.
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.PP
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Once
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.B file
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has determined the character set used in a text-type file,
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it will
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attempt to determine in what language the file is written.
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The language tests look for particular strings (cf
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.IR names.h )
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that can appear anywhere in the first few blocks of a file.
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For example, the keyword
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.B .br
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indicates that the file is most likely a
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.BR troff (1)
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input file, just as the keyword
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.B struct
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indicates a C program.
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These tests are less reliable than the previous
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two groups, so they are performed last.
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The language test routines also test for some miscellany
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(such as
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.BR tar (1)
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archives).
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.PP
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Any file that cannot be identified as having been written
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in any of the character sets listed above is simply said to be ``data''.
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.SH OPTIONS
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.TP 8
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.B \-b
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Do not prepend filenames to output lines (brief mode).
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.TP 8
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.B \-c
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Cause a checking printout of the parsed form of the magic file.
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This is usually used in conjunction with
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.B \-m
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to debug a new magic file before installing it.
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.TP 8
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.B \-C
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Write a magic.mgc output file that contains a pre-parsed version of
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file.
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.TP 8
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.BI \-f " namefile"
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Read the names of the files to be examined from
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.I namefile
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(one per line)
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before the argument list.
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Either
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.I namefile
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or at least one filename argument must be present;
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to test the standard input, use ``\-'' as a filename argument.
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.TP 8
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.B \-i
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Causes the file command to output mime type strings rather than the more
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traditional human readable ones. Thus it may say
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``text/plain; charset=us-ascii''
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rather
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than ``ASCII text''. In order for this option to work, file changes the way
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it handles files recognised by the command itself (such as many of the
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text file types, directories etc), and makes use of an alternative
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``magic'' file.
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(See ``FILES'' section, below).
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.TP 8
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.B \-k
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Don't stop at the first match, keep going.
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.TP 8
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.BI \-m " list"
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Specify an alternate list of files containing magic numbers.
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This can be a single file, or a colon-separated list of files.
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.TP 8
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.B \-n
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Force stdout to be flushed after checking each file. This is only useful if
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checking a list of files. It is intended to be used by programs that want
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filetype output from a pipe.
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.TP 8
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.B \-v
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Print the version of the program and exit.
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.TP 8
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.B \-z
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Try to look inside compressed files.
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.TP 8
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.B \-L
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option causes symlinks to be followed, as the like-named option in
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.BR ls (1).
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(on systems that support symbolic links).
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.TP 8
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.B \-s
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Normally,
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.B file
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only attempts to read and determine the type of argument files which
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.BR stat (2)
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reports are ordinary files.
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This prevents problems, because reading special files may have peculiar
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consequences.
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Specifying the
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.BR \-s
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option causes
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.B file
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to also read argument files which are block or character special files.
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This is useful for determining the filesystem types of the data in raw
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disk partitions, which are block special files.
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This option also causes
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.B file
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to disregard the file size as reported by
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.BR stat (2)
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since on some systems it reports a zero size for raw disk partitions.
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.SH FILES
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.I __MAGIC__.mgc
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\- defaults compiled list of magic numbers
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.PP
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.I __MAGIC__
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\- default list of magic numbers
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.PP
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.I __MAGIC__.mime
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\- default list of magic numbers, used to output mime types when the -i option
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is specified.
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.SH ENVIRONMENT
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The environment variable
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.B MAGIC
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can be used to set the default magic number files.
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.SH SEE ALSO
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.BR magic (__FSECTION__)
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\- description of magic file format.
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.br
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.BR strings (1), " od" (1), " hexdump(1)"
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\- tools for examining non-textfiles.
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.SH STANDARDS CONFORMANCE
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This program is believed to exceed the System V Interface Definition
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of FILE(CMD), as near as one can determine from the vague language
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contained therein.
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Its behaviour is mostly compatible with the System V program of the same name.
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This version knows more magic, however, so it will produce
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different (albeit more accurate) output in many cases.
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.PP
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The one significant difference
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between this version and System V
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is that this version treats any white space
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as a delimiter, so that spaces in pattern strings must be escaped.
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For example,
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.br
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\*[Gt]10 string language impress\ (imPRESS data)
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.br
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in an existing magic file would have to be changed to
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.br
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\*[Gt]10 string language\e impress (imPRESS data)
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.br
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In addition, in this version, if a pattern string contains a backslash,
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it must be escaped. For example
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.br
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0 string \ebegindata Andrew Toolkit document
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.br
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in an existing magic file would have to be changed to
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.br
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0 string \e\ebegindata Andrew Toolkit document
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.br
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.PP
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SunOS releases 3.2 and later from Sun Microsystems include a
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.BR file (1)
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command derived from the System V one, but with some extensions.
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My version differs from Sun's only in minor ways.
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It includes the extension of the `\*[Am]' operator, used as,
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for example,
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.br
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\*[Gt]16 long\*[Am]0x7fffffff \*[Gt]0 not stripped
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.SH MAGIC DIRECTORY
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The magic file entries have been collected from various sources,
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mainly USENET, and contributed by various authors.
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Christos Zoulas (address below) will collect additional
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or corrected magic file entries.
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A consolidation of magic file entries
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will be distributed periodically.
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.PP
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The order of entries in the magic file is significant.
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Depending on what system you are using, the order that
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they are put together may be incorrect.
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If your old
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.B file
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command uses a magic file,
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keep the old magic file around for comparison purposes
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(rename it to
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.IR __MAGIC__.orig ).
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.SH EXAMPLES
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.nf
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$ file file.c file /dev/{wd0a,hda}
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file.c: C program text
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file: ELF 32-bit LSB executable, Intel 80386, version 1 (SYSV),
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dynamically linked (uses shared libs), stripped
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/dev/wd0a: block special (0/0)
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/dev/hda: block special (3/0)
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$ file -s /dev/wd0{b,d}
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/dev/wd0b: data
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/dev/wd0d: x86 boot sector
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$ file -s /dev/hda{,1,2,3,4,5,6,7,8,9,10}
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/dev/hda: x86 boot sector
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/dev/hda1: Linux/i386 ext2 filesystem
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/dev/hda2: x86 boot sector
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/dev/hda3: x86 boot sector, extended partition table
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/dev/hda4: Linux/i386 ext2 filesystem
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/dev/hda5: Linux/i386 swap file
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/dev/hda6: Linux/i386 swap file
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/dev/hda7: Linux/i386 swap file
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/dev/hda8: Linux/i386 swap file
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/dev/hda9: empty
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/dev/hda10: empty
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$ file -i file.c file /dev/{wd0a,hda}
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file.c: text/x-c
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file: application/x-executable, dynamically linked (uses shared libs),
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not stripped
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/dev/hda: application/x-not-regular-file
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/dev/wd0a: application/x-not-regular-file
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.fi
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.SH HISTORY
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There has been a
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.B file
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command in every \s-1UNIX\s0 since at least Research Version 4
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(man page dated November, 1973).
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The System V version introduced one significant major change:
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the external list of magic number types.
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This slowed the program down slightly but made it a lot more flexible.
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.PP
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This program, based on the System V version,
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was written by Ian Darwin \*[Lt]ian@darwinsys.com\*[Gt]
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without looking at anybody else's source code.
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.PP
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John Gilmore revised the code extensively, making it better than
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the first version.
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Geoff Collyer found several inadequacies
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and provided some magic file entries.
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Contributions by the `\*[Am]' operator by Rob McMahon, cudcv@warwick.ac.uk, 1989.
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.PP
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Guy Harris, guy@netapp.com, made many changes from 1993 to the present.
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.PP
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Primary development and maintenance from 1990 to the present by
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Christos Zoulas (christos@astron.com).
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.PP
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Altered by Chris Lowth, chris@lowth.com, 2000:
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Handle the ``-i'' option to output mime type strings and using an alternative
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magic file and internal logic.
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.PP
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Altered by Eric Fischer (enf@pobox.com), July, 2000,
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to identify character codes and attempt to identify the languages
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of non-ASCII files.
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.PP
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The list of contributors to the "Magdir" directory (source for the
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/etc/magic
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file) is too long to include here. You know who you are; thank you.
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.SH LEGAL NOTICE
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Copyright (c) Ian F. Darwin, Toronto, Canada, 1986-1999.
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Covered by the standard Berkeley Software Distribution copyright; see the file
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LEGAL.NOTICE in the source distribution.
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.PP
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The files
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.I tar.h
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and
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.I is_tar.c
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were written by John Gilmore from his public-domain
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.B tar
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program, and are not covered by the above license.
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.SH BUGS
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There must be a better way to automate the construction of the Magic
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file from all the glop in Magdir. What is it?
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Better yet, the magic file should be compiled into binary (say,
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.BR ndbm (3)
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or, better yet, fixed-length
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.SM ASCII
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strings for use in heterogenous network environments) for faster startup.
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Then the program would run as fast as the Version 7 program of the same name,
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with the flexibility of the System V version.
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.PP
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.B File
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uses several algorithms that favor speed over accuracy,
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thus it can be misled about the contents of
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text
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files.
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.PP
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The support for
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text
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files (primarily for programming languages)
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is simplistic, inefficient and requires recompilation to update.
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.PP
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There should be an ``else'' clause to follow a series of continuation lines.
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.PP
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The magic file and keywords should have regular expression support.
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Their use of
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.SM "ASCII TAB"
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as a field delimiter is ugly and makes
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it hard to edit the files, but is entrenched.
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.PP
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It might be advisable to allow upper-case letters in keywords
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for e.g.,
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.BR troff (1)
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commands vs man page macros.
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Regular expression support would make this easy.
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.PP
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The program doesn't grok \s-2FORTRAN\s0.
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It should be able to figure \s-2FORTRAN\s0 by seeing some keywords which
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appear indented at the start of line.
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Regular expression support would make this easy.
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.PP
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The list of keywords in
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.I ascmagic
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probably belongs in the Magic file.
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This could be done by using some keyword like `*' for the offset value.
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.PP
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Another optimisation would be to sort
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the magic file so that we can just run down all the
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tests for the first byte, first word, first long, etc, once we
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have fetched it. Complain about conflicts in the magic file entries.
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Make a rule that the magic entries sort based on file offset rather
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than position within the magic file?
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.PP
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The program should provide a way to give an estimate
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of ``how good'' a guess is.
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We end up removing guesses (e.g. ``From '' as first 5 chars of file) because
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they are not as good as other guesses (e.g. ``Newsgroups:'' versus
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``Return-Path:''). Still, if the others don't pan out, it should be
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possible to use the first guess.
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.PP
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This program is slower than some vendors' file commands.
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The new support for multiple character codes makes it even slower.
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.PP
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This manual page, and particularly this section, is too long.
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.SH AVAILABILITY
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You can obtain the original author's latest version by anonymous FTP
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on
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.B ftp.astron.com
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in the directory
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.I /pub/file/file-X.YY.tar.gz
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