tests each argument in an attempt to classify it. There are three sets of tests, performed in this order: filesystem tests, magic 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 adding local definitions to /etc/magic, make sure to preserve these keywords Users 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'
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 In sys/stat.h .
The magic 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 In elf.h , In a.out.h and possibly In 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 a `magic' 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 /etc/magic and the the compiled magic file /usr/share/misc/magic.mgc or the files in the directory /usr/share/misc/magic if the compiled file does not exist. In addition, if $HOME/.magic.mgc or $HOME/.magic exists, it will be used in preference to the system magic files.
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, 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 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. In 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 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.
$ 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
/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, 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.
Altered by Reuben Thomas (rrt@sc3d.org), 2007 to 2008, to improve MIME 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.
The list of contributors to the `magic' directory (magic files) is too long to include here. You know who you are; thank you. Many contributors are listed in the source files.
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.
There must be a better way to automate the construction of the Magic file from all the glop in Magdir. What is it?
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.
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. `Fromas 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 manual page, and particularly this section, is too long.
This Debian version adds a number of new magic entries. It can be obtained from every site carrying a Debian distribution (that is ftp.debian.org and mirrors).