LinuxGuide.it > Linux Man Page: "file"
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Namefile - determine file type
Synopsisfile [-bchikLnNprsvz] [-f namefile] [-F separator] [-m magicfiles] file file -C [-m magicfile]
DescriptionThis manual page documents version 4.21 of the file 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, make sure to 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
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 #include
<a.out.h> 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, nonISO 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 #include <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’’.
Options
-b, --brief
apptype
ascii
compress
elf
fortran
soft
tar
token
troff
Files
EnvironmentThe environment variable MAGIC can be used to set the default magic number file name. If that variable is set, then file will not attempt to open $HOME/.magic. file adds “.mime” and/or “.mgc” to the value of this variable as appropriate. The environment variable POSIXLY_CORRECT controls (on systems that support symbolic links), if file will attempt to follow symlinks or not. If set, then file follows symlink, otherwise it does not. This is also controlled by the -L and -h options.
See Alsomagic(5) , strings(1) , od(1) , hexdump(1)
Standards ConformanceThis program is believed to exceed the System V Interface Definition of FILE(CMD), as near as one can determine from the vague language contained therein. Its behavior is mostly compatible with the System V program of the same name. This version knows more magic, however, so it will produce different (albeit more accurate) output in many cases. 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,
in an existing magic file would have to be changed to
In addition, in this version, if a pattern string contains a backslash, it must be escaped. For example
in an existing magic file would have to be changed to
SunOS releases 3.2 and later from Sun Microsystems include a file 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
Magic DirectoryThe magic file entries have been collected from various sources, mainly USENET, and contributed by various authors. Christos Zoulas (address below) will collect additional or corrected magic file entries. A consolidation of magic file entries will be distributed periodically. 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 ).
Examples
$ file file.c file /dev/{wd0a,hda}
$ file -s /dev/wd0{b,d} $ file -s /dev/hda{,1,2,3,4,5,6,7,8,9,10}
$ file -i file.c file /dev/{wd0a,hda}
HistoryThere has been a file command in every UNIX since at least Research Version 4 (man page dated November, 1973). The System V version introduced one significant major change: the external list of magic number types. This slowed the program down slightly but made it a lot more flexible. 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.
Legal NoticeCopyright (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. The files tar.h and is_tar.c were written by John Gilmore from his public-domain tar(1) program, and are not covered by the above license.
BugsThere must be a better way to automate the construction of the Magic file from all the glop in Magdir. What is it? 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. 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. 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.
AvailabilityYou can obtain the original author’s latest version by anonymous FTP on ftp.astron.com in the directory /pub/file/file-X.YZ.tar.gz
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