FILE(1) BSD Reference Manual FILE(1)
file - determine file type
file [-bckLNnrsvz] [-F separator] [-f namefile] [-m magicfiles] file ... file [-m magicfiles] -C
The file utility 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 ASCII characters and is probably safe to read on an ASCII terminal), "executable" (the file contains the result of compiling a pro- gram 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 /etc/magic or the program itself, preserve these keywords. People depend on knowing that all the readable files in a directory have the word "text" printed. Don't do as Berkeley did; 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 <sys/stat.h>. The magic number tests are used to check for files with data in particu- lar fixed formats. The canonical example of this is a binary executable (compiled program) a.out file, whose format is defined in <a.out.h> and possibly <exec.h> in the standard include directory and is explained in a.out(5). These files have a "magic number" stored in a particular place near the beginning of the file that tells the UNIX operating system that the file is a binary executable, and which of several types thereof. The concept of magic number has been applied by extension to data files. Any file with some invariant identifier at a small fixed offset into the file can usually be described in this way. The information in these files is read from the magic file /etc/magic. If an argument appears to be an ASCII file, file attempts to guess its language. The language tests look for particular strings (cf names.h) that can appear anywhere in the first few blocks of a file. For example, the keyword .br indicates that the file is most likely a troff(1) input file, just as the keyword struct indicates a C program. These tests are less reliable than the previous two groups, so they are performed last. The language test routines also test for some miscellany (such as tar(1) archives) and determine whether an unknown file should be labelled as "ASCII text" or "data". The options are as follows: -b Do not prepend filenames to output lines (brief mode). -C For each magic number file, write a magic.mgc output file that contains a preparsed (compiled) version of it. -c Cause a checking printout of the parsed form of the magic file. This is usually used in conjunction with -m to debug a new magic file before installing it. -F separator Use the specified string as the separator between the filename and the file result returned. Defaults to ':'. -f namefile Read the names of the files to be examined from namefile (one per line) before the argument list. Either namefile or at least one filename argument must be present; to test the standard input, use '-' as a filename argument. -k Don't stop at the first match, keep going. -L Cause symlinks to be followed, as the like-named option in ls(1) (on systems that support symbolic links). -m magicfiles Specify an alternate list, magicfiles, of files containing magic numbers. This can be a single file or a colon-separated list of files. If a compiled magic file is found alongside, it will be used instead. -N Don't pad filenames so that they align in the output. -n Force stdout to be flushed after checking each file. This is only useful if checking a list of files. It is intended to be used by programs that want filetype output from a pipe. -r Don't translate unprintable characters to '\ooo'. Normally file translates unprintable characters to their octal representation (raw mode). -s Normally, file only attempts to read and determine the type of argument files which stat(2) reports are ordinary files. This prevents problems, because reading special files may have pecu- liar consequences. Specifying the -s option causes file to also read argument files which are block or character special files. This is useful for determining the filesystem types of the data in raw disk partitions, which are block special files. This op- tion also causes file to disregard the file size as reported by stat(2), since on some systems it reports a zero size for raw disk partitions. -v Print the version of the program and exit. -z Try to look inside files that have been run through compress(1).
MAGIC Default magic number files, separated by colon characters. file adds ".mgc" to the value of this variable as appropriate.
/etc/magic default list of magic numbers
compress(1), hexdump(1), ls(1), od(1), strings(1), a.out(5), magic(5)
This 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 behaviour 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, >10 string language impress (imPRESS data) in an existing magic file would have to be changed to >10 string language\ impress (imPRESS data) In addition, in this version, if a pattern string contains a backslash, it must be escaped. For example 0 string \begindata Andrew Toolkit document in an existing magic file would have to be changed to 0 string \\begindata Andrew Toolkit document SunOS releases 3.2 and later from Sun Microsystems include a file 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 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 mag- ic file entries. A consolidation of magic file entries will be distribut- ed periodically. The order of entries in the magic file is significant. Depending on what system you are using, the order that they are put to- gether may be incorrect. If your old file command uses a magic file, keep the old magic file around for comparison purposes (rename it to /etc/magic.orig).
There has been a file command in every UNIX since at least Research Ver- sion 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 F. Darwin 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 en- tries. Contributions to the '&' operator by Rob McMahon, 1989. Guy Harris made many changes from 1993 to the present. Primary development and maintenance from 1990 to the present by Christos Zoulas <email@example.com>. Altered by Chris Lowth, 2000, to optionally report MIME types. This required an al- ternative magic file, and is not available in OpenBSD. Altered by Eric Fischer, 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 /etc/magic file) is too long to include here. You know who you are; thank you.
Copyright (c) Ian F. Darwin, Toronto, Canada, 1986-1999. Covered by the standard Berkeley Software Distribution copyright; see the file LEGAL.NOTICE in the distribution. The files tar.h and is_tar.c were written by John Gilmore from his public-domain tar program, and are not covered by the above license.
There must be a better way to automate the construction of the Magic file from all the glop in Magdir. What is it? Better yet, the magic file should be compiled into binary (say, ndbm(3) or, better yet, fixed-length ASCII strings for use in heterogenous network environments) for faster startup. Then the program would run as fast as the Version 7 program of the same name, with the flexibility of the System V version. file uses several algorithms that favor speed over accuracy; thus it can be misled about the contents of ASCII files. The support for ASCII files (primarily for programming languages) is simplistic, inefficient and requires recompilation to update. There should be an "else" clause to follow a series of continuation lines. The magic file and keywords should have regular expression support. Their use of ASCII TAB as a field delimiter is ugly and makes it hard to edit the files, but is entrenched. It might be advisable to allow upper-case letters in keywords for e.g., troff(1) commands vs man page macros. Regular expression support would make this easy. The program doesn't grok FORTRAN. It should be able to figure FORTRAN by seeing some keywords which appear indented at the start of line. Regular expression support would make this easy. The list of keywords in ascmagic probably belongs in the Magic file. This could be done by using some keyword like '*' for the offset value. Another optimization would be to sort the magic file so that we can just run down all the tests for the first byte, first word, first long, etc, once we have fetched it. Complain about conflicts in the magic file en- tries. Make a rule that the magic entries sort based on file offset rath- er 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. This manual page, and particularly this section, is too long.
You can obtain the original author's latest version by anonymous FTP on ftp.astron.com in the directory /pub/file/file-X.YY.tar.gz. MirOS BSD #10-current July 10, 2007 3
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