--- /dev/null
+.TH FILE 1 "Public Domain"
+.SH NAME
+.I file
+\- determine file type
+.SH SYNOPSIS
+.B file
+[
+.B -c
+]
+[
+.B -f
+file ]
+[
+.B -m
+magicfile ]
+file ...
+.SH DESCRIPTION
+.I File
+performs a series of tests on 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 program prints the
+.I first
+type that matches the file, and goes on to the next file argument if any.
+.PP
+The type printed will usually contain one of the words
+.B text
+(the file contains only ASCII characters and is
+moderately safe to read on an ASCII terminal),
+.B executable
+(the file contains the result of compiling a program
+in a form understandable to some \s-1UNIX\s0 kernel or another),
+or
+.B 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
+.I /etc/magic
+or the program itself,
+.B "preserve these keywords" .
+People depend on knowing that all the files in a directory
+that are readable contain the word ``text''.
+Don't do as one computer vendor did \- change ``shell commands text''
+to ``shell script''.
+.PP
+The filesystem tests are based on examining the return from a
+.I 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 and symbolic links on 4.2BSD, named pipes (fifos) on System V)
+are intuited if they are defined in
+the appropriate system header file
+.I sys/stat.h .
+.PP
+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)
+.I a.out
+file, whose format is defined in
+.I a.out.h
+and possibly
+.I 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 \s-1UNIX\s0 operating system
+that the file is a binary executable, and which of several types thereof.
+The concept of `magic number' has been applied by extention to data files.
+Any file with some invariant identifier at a fixed
+offset into the file can usually be described in this way.
+The information in these files is read from the magic file
+.I /etc/magic .
+.PP
+If an argument appears to be
+.SM ASCII ,
+.I file
+attempts to guess its language.
+The language tests look for particular strings that can appear
+anywhere in the first few blocks of the file.
+For example, the keyword
+.I dimension
+indicates that the file is most likely a \s-1FORTRAN\s0 program,
+just as the keyword
+.I struct
+indicates a C program.
+These tests are less reliable than the previous
+two groups, so they are performed last.
+The keywords used in the language tests are stored in the
+source file
+.I names.h .
+The routine that handles the language tests also tests for some miscellany
+(such as
+.I tar
+archives) and determines whether an unknown file should be
+labelled as `ascii text' or `data'.
+.PP
+Use
+.B -m
+.I file
+to specify an alternate file of magic numbers.
+The option
+.B -c
+causes a checking printout of the parsed form of the magic file.
+This is usually used in conjunction with
+.B -m
+to debug a new magic file.
+.PP
+The
+.B -f
+.I file
+option specifies that the names of the files to be examined
+is to come from
+.I file
+rather than (or in addition to) the argument list.
+.SH FILES
+.I /etc/magic
+\- default list of magic numbers
+.SH SEE ALSO
+.IR File (5)
+\- description of magic file format.
+.IR Strings (1), " od" (1)
+\- tools for examining non-textfiles.
+.SH STANDARDS CONFORMANCE
+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.
+.SH HISTORY
+There has been a
+.I file
+command in every UNIX since at least Research Version 6
+(man page dated January, 1975).
+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 significantly
+more flexible.
+.PP
+This program, based on the System V version,
+was written by Ian Darwin without looking at anybody else's source code
+(I looked at one later, and was glad I hadn't!).
+.SH NOTICE
+This program is copyright \(co 1986, Ian Darwin.
+.B "Unmodified copies
+of the source may be freely copied for any purpose
+provided this notice is maintained.
+.B "Redistribution of modified source copies is prohibited;
+redistribute the original source with your changes as diff listings.
+Send the author your changes; if I like 'em I'll incorporate them.
+.PP
+Author's addresses:
+Postal: Box 603, Station F, Toronto, CANADA M4Y 2L8;
+uUCp: {utzoo|ihnp4}!darwin!ian;
+InterNet: ian@sq.com.
+.PP
+A few files (such as
+.I is_tar.c ,
+.I strtok.c ,
+and
+.I strtol.c )
+are not covered by the above restrictions.
+.SH BUGS
+You can't use `-' (or a null argument list) to determine the file type
+of the standard input, since the program insists on doing a
+.I stat (2)
+call to glean some information about the file.
+.PP
+.I File
+uses several algorithms that favor speed over accuracy,
+thus it is often misled about the contents of ASCII files.
+.PP
+The support for ASCII files (primarily for programming languages)
+is simplistic, inefficient and requires recompilation to update.
+.PP
+The magic file should be compiled into a binary
+(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.
+But then there would have to be yet another magic number for the
+.I magic.out
+file.
projects / file / commitdiff