file - determine file type
file [ -bciknsvzL ] [ -f namefile ] [ -m magicfiles ] file ...
file -C [ -m magicfile ]
This manual page documents version 3.37-3.1 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 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 adding local definitions to /etc/magic, 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 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 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 particular 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. 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 /etc/magic and 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-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''.
Do not prepend filenames to output lines.
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.
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.
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).
Don't stop at the first match, keep going.
-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.
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.
Print the version of the program and exit.
Try to look inside compressed files.
This option causes symlinks to be followed, as the like-named option in ls(1). (on systems that support symbolic links).
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 filesystem 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.
Print a help message and exit.
Print version information and exit.
Default compiled list of magic numbers
Default list of magic numbers
Default list of magic numbers, used to output mime types when the -i option is specified.
Local additions to magic wisdom.