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   file    ( 1 )

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Описание (Description)

This manual page documents version 5.40 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 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 magic files
     or the program itself, 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
     <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 <elf.h>, <a.out.h> and possibly <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 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 identifying these files is
     read from the compiled magic file /usr/local/share/misc/magic.mgc,
     or the files in the directory /usr/local/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, 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.
     <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, JSON
     files).

     Any file that cannot be identified as having been written in any of
     the character sets listed above is simply said to be 'data'.