551 lines
16 KiB
Groff
551 lines
16 KiB
Groff
.\" $File: file.man,v 1.73 2008/02/19 17:58:00 rrt Exp $
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.Dd February 19, 2008
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.Dt FILE __CSECTION__
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.Os
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.Sh NAME
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.Nm file
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.Nd determine file type
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.Sh SYNOPSIS
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.Nm
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.Op Fl bchikLnNprsvz
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.Op Fl -mime-type
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.Op Fl -mime-encoding
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.Op Fl f Ar namefile
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.Op Fl F Ar separator
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.Op Fl m Ar magicfiles
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.Ar file
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.Nm
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.Fl C
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.Op Fl m Ar magicfile
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.Nm
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.Op Fl -help
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.Sh DESCRIPTION
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This manual page documents version __VERSION__ of the
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.Nm
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command.
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.Pp
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.Nm
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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 tests, and language tests.
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The
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.Em 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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.Em 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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.Dv ASCII
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terminal),
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.Em executable
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(the file contains the result of compiling a program
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in a form understandable to some
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.Dv UNIX
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kernel or another),
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or
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.Em data
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meaning anything else (data is usually
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.Sq binary
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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 magic files or the program itself, make sure to
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.Em "preserve these keywords" .
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Users depend on knowing that all the readable files in a directory
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have the word
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.Dq text
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printed.
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Don't do as Berkeley did and change
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.Dq shell commands text
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to
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.Dq shell script .
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.Pp
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The filesystem tests are based on examining the return from a
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.Xr 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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.In sys/stat.h .
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.Pp
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The magic 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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.Dv a.out
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file, whose format is defined in
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.In elf.h ,
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.In a.out.h
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and possibly
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.In exec.h
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in the standard include directory.
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These files have a
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.Sq "magic number"
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stored in a particular place
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near the beginning of the file that tells the
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.Dv UNIX 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 a
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.Sq "magic"
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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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.Pa __MAGIC__.mgc ,
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or the files in the directory
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.Pa __MAGIC__
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if the compiled file does not exist. In addition, if
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.Pa $HOME/.magic.mgc
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or
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.Pa $HOME/.magic
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exists, it will be used in preference to the system magic files.
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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
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.Dq text
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because they will be mostly readable on nearly any terminal;
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UTF-16 and EBCDIC are only
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.Dq character data
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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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.Nm
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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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.Nm
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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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.In 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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.Em .br
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indicates that the file is most likely a
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.Xr troff 1
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input file, just as the keyword
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.Em 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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.Xr 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
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.Dq data .
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.Sh OPTIONS
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.Bl -tag -width indent
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.It Fl b , -brief
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Do not prepend filenames to output lines (brief mode).
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.It Fl c , -checking-printout
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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 the
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.Fl m
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flag to debug a new magic file before installing it.
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.It Fl C , -compile
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Write a
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.Pa magic.mgc
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output file that contains a pre-parsed version of the magic file or directory.
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.It Fl e , -exclude Ar testname
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Exclude the test named in
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.Ar testname
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from the list of tests made to determine the file type. Valid test names
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are:
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.Bl -tag -width
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.It apptype
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Check for
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.Dv EMX
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application type (only on EMX).
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.It ascii
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Check for various types of ascii files.
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.It compress
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Don't look for, or inside compressed files.
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.It elf
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Don't print elf details.
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.It fortran
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Don't look for fortran sequences inside ascii files.
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.It soft
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Don't consult magic files.
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.It tar
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Don't examine tar files.
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.It token
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Don't look for known tokens inside ascii files.
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.It troff
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Don't look for troff sequences inside ascii files.
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.El
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.It Fl f , -files-from Ar namefile
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Read the names of the files to be examined from
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.Ar 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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.Ar namefile
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or at least one filename argument must be present;
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to test the standard input, use
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.Sq -
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as a filename argument.
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.It Fl F , -separator Ar separator
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Use the specified string as the separator between the filename and the
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file result returned. Defaults to
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.Sq \&: .
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.It Fl h , -no-dereference
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option causes symlinks not to be followed
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(on systems that support symbolic links). This is the default if the
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environment variable
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.Dv POSIXLY_CORRECT
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is not defined.
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.It Fl i , -mime
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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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.Dq text/plain charset=us-ascii
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rather than
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.Dq ASCII text .
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In order for this option to work, file changes the way
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it handles files recognized 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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.Dq magic
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file.
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(See
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.Dq FILES
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section, below).
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.It Fl -mime-type , -mime-encoding
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Like
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.Fl i ,
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but print only the specified element(s).
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.It Fl k , -keep-going
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Don't stop at the first match, keep going. Subsequent matches will be
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have the string
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.Dq "\[rs]012\- "
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prepended.
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(If you want a newline, see the
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.Dq "\-r"
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option.)
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.It Fl L , -dereference
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option causes symlinks to be followed, as the like-named option in
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.Xr ls 1
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(on systems that support symbolic links).
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This is the default if the environment variable
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.Dv POSIXLY_CORRECT
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is defined.
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.It Fl m , -magic-file Ar list
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Specify an alternate list of files and directories containing magic.
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This can be a single item, or a colon-separated list.
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If a compiled magic file is found alongside a file or directory, it will be used instead.
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.It Fl n , -no-buffer
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Force stdout to be flushed after checking each file.
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This is only useful if checking a list of files.
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It is intended to be used by programs that want filetype output from a pipe.
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.It Fl N , -no-pad
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Don't pad filenames so that they align in the output.
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.It Fl p , -preserve-date
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On systems that support
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.Xr utime 2
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or
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.Xr utimes 2 ,
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attempt to preserve the access time of files analyzed, to pretend that
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.Nm
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never read them.
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.It Fl r , -raw
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Don't translate unprintable characters to \eooo.
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Normally
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.Nm
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translates unprintable characters to their octal representation.
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.It Fl s , -special-files
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Normally,
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.Nm
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only attempts to read and determine the type of argument files which
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.Xr 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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.Fl s
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option causes
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.Nm
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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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.Nm
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to disregard the file size as reported by
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.Xr stat 2
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since on some systems it reports a zero size for raw disk partitions.
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.It Fl v , -version
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Print the version of the program and exit.
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.It Fl z , -uncompress
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Try to look inside compressed files.
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.It Fl 0 , -print0
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Output a null character
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.Sq \e0
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after the end of the filename. Nice to
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.Xr cut 1
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the output. This does not affect the separator which is still printed.
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.It Fl -help
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Print a help message and exit.
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.El
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.Sh FILES
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.Bl -tag -width __MAGIC__.mgc -compact
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.It Pa __MAGIC__.mgc
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Default compiled list of magic.
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.It Pa __MAGIC__
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Directory containing default magic files.
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.El
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.Sh ENVIRONMENT
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The environment variable
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.Dv MAGIC
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can be used to set the default magic file name.
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If that variable is set, then
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.Nm
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will not attempt to open
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.Pa $HOME/.magic .
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.Nm
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adds
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.Dq .mgc
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to the value of this variable as appropriate.
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The environment variable
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.Dv POSIXLY_CORRECT
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controls (on systems that support symbolic links), whether
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.Nm
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will attempt to follow symlinks or not. If set, then
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.Nm
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follows symlink, otherwise it does not. This is also controlled
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by the
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.Fl L
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and
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.Fl h
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options.
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.Sh SEE ALSO
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.Xr magic __FSECTION__ ,
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.Xr strings 1 ,
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.Xr od 1 ,
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.Xr hexdump 1,
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.Xr file 1posix
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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 behavior 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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.\" URL: http://www.opengroup.org/onlinepubs/009695399/utilities/file.html
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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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.Bd -literal -offset indent
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>10 string language impress\ (imPRESS data)
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.Ed
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.Pp
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in an existing magic file would have to be changed to
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.Bd -literal -offset indent
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>10 string language\e impress (imPRESS data)
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.Ed
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.Pp
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In addition, in this version, if a pattern string contains a backslash,
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it must be escaped.
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For example
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.Bd -literal -offset indent
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0 string \ebegindata Andrew Toolkit document
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.Ed
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.Pp
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in an existing magic file would have to be changed to
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.Bd -literal -offset indent
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0 string \e\ebegindata Andrew Toolkit document
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.Ed
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.Pp
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SunOS releases 3.2 and later from Sun Microsystems include a
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.Nm
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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
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.Sq &
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operator, used as,
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for example,
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.Bd -literal -offset indent
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>16 long&0x7fffffff >0 not stripped
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.Ed
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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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.Nm
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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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.Pa __MAGIC__.orig ).
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.Sh EXAMPLES
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.Bd -literal -offset indent
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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
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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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.Ed
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.Sh HISTORY
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There has been a
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.Nm
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command in every
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.Dv UNIX 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 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 <ian@darwinsys.com>
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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 `&' 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
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.Fl i
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option to output mime type strings, using an alternative
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magic file and internal logic.
|
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.Pp
|
|
Altered by Eric Fischer (enf@pobox.com), July, 2000,
|
|
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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Altered by Reuben Thomas (rrt@sc3d.org), 2007 to 2008, to improve MIME
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support and merge MIME and non-MIME magic, support directories as well
|
|
as files of magic, apply many bug fixes and improve the build system.
|
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.Pp
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|
The list of contributors to the
|
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.Dq magic
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directory (magic files)
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is too long to include here.
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You know who you are; thank you.
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Many contributors are listed in the source files.
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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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.Dv tar.h
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and
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.Dv is_tar.c
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|
were written by John Gilmore from his public-domain
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.Xr tar 1
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program, and are not covered by the above license.
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.Sh BUGS
|
|
.Pp
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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.
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What is it?
|
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.Pp
|
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.Nm
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|
uses several algorithms that favor speed over accuracy,
|
|
thus it can be misled about the contents of
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text
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files.
|
|
.Pp
|
|
The support for text files (primarily for programming languages)
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|
is simplistic, inefficient and requires recompilation to update.
|
|
.Pp
|
|
The list of keywords in
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.Dv ascmagic
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probably belongs in the Magic file.
|
|
This could be done by using some keyword like
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.Sq *
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for the offset value.
|
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.Pp
|
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Complain about conflicts in the magic file entries.
|
|
Make a rule that the magic entries sort based on file offset rather
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|
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 manual page, and particularly this section, is too long.
|
|
.Sh RETURN CODE
|
|
.Nm
|
|
returns 0 on success, and non-zero on error.
|
|
.Sh AVAILABILITY
|
|
You can obtain the original author's latest version by anonymous FTP
|
|
on
|
|
.Dv ftp.astron.com
|
|
in the directory
|
|
.Dv /pub/file/file-X.YZ.tar.gz
|