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FILE (1) | General commands | Unix Manual Pages | :man

NAME

file - determine file type

CONTENTS

Synopsis
Description
Options
Files
Environment
See Also
Standards Conformance
Magic Directory
Examples
History
Legal Notice
Bugs
Availability

SYNOPSIS

file [-bcikLnNprsvz] [-f namefile] [-F separator] [-m magicfiles] file -C [-m magicfile]

DESCRIPTION

This manual page documents version 4.12 of the file utility which tests each argument in an attempt to classify it. There are three sets of tests, performed in this order: file system 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/misc/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. Do not do as Berkeley did and change ""shell commands text"" to ""shell script"". Note that the file /usr/share/misc/magic is built mechanically from a large number of small files in the subdirectory Magdir in the source distribution of this program.

The file system 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 is 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 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
.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 '"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/share/misc/magic.mgc, or /usr/share/misc/magic if the compile file does not exist.

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 Ns -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).

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
Do not prepend filenames to output lines (brief mode).
-c -, --checking-printout
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.
-C -, --compile
Write a magic.mgc output file that contains a pre-parsed version of file.
-f -, --files-from 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.
-F -, --separator separator
Use the specified string as the separator between the filename and the file result returned. Defaults to ‘:’.
-i -, --mime
Causes the file command to output mime type strings rather than the more traditional human readable ones. Thus it may say ""text/plain; charset=us-ascii"" rather than ""ASCII text"". In order for this option to work, file changes the way it handles files recognised by the command itself (such as many of the text file types, directories etc), and makes use of an alternative magic file. (See FILES section, below).
-k -, --keep-going
Do not stop at the first match, keep going.
-L -, --dereference
option causes symlinks to be followed, as the like-named option in ls(1). (on systems that support symbolic links).
-m -, --magic-file list
Specify an alternate list 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. With the -i or --mime option, the program adds .mime to each file name.
-n -, --no-buffer
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.
-N -, --no-pad
Do not pad filenames so that they align in the output.
-p -, --preserve-date
On systems that support utime(3) or utimes(2), attempt to preserve the access time of files analyzed, to pretend that file never read them.
-r -, --raw
Do not translate unprintable characters to \ooo. Normally file translates unprintable characters to their octal representation.
-s -, --special-files
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 peculiar 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 file system types of the data in raw disk partitions, which are block special files. This option 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 -, --version
Print the version of the program and exit.
-z -, --uncompress
Try to look inside compressed files.
--help
Print a help message and exit.

FILES

/usr/share/misc/magic.mgc Default compiled list of magic numbers
/usr/share/misc/magic Default list of magic numbers
/usr/share/misc/magic.mime.mgc Default compiled list of magic numbers, used to output mime types when the -i option is specified.
/usr/share/misc/magic.mime Default list of magic numbers, used to output mime types when the -i option is specified.
/etc/magic Local additions to magic wisdom.

ENVIRONMENT

The environment variable MAGIC can be used to set the default magic number file name. file adds .mime and/or .mgc to the value of this variable as appropriate.

SEE ALSO

hexdump(1), od(1), strings(1), magic(5)

STANDARDS CONFORMANCE

file(1)

MAGIC DIRECTORY

EXAMPLES

HISTORY

LEGAL NOTICE

BUGS

ndbm(3) troff(1)

AVAILABILITY


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