FILE(1) BSD General Commands Manual FILE(1)
file — determine file type
file [-bcdEhiklLNnprsSvzZ0] [–apple] [–exclude-quiet] [–extension] [–mime-encoding]
[–mime-type] [-e testname] [-F separator] [-f namefile] [-m magicfiles] [-P name=value]
file -C [-m magicfiles]
This manual page documents version 5.39 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 termi‐
nal), 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
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” 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 in‐
formation identifying these files is read from the compiled magic file /usr/share/misc/magic.mgc,
or the files in the directory /usr/share/misc/magic if the compiled file does not exist. In ad‐
dition, if $HOME/.magic.mgc or $HOME/.magic exists, it will be used in preference to the system
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 consti‐
tute printable text in each set. If a file passes any of these tests, its character set is re‐
ported. 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” be‐
cause, 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 in‐
dicates 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) ar‐
chives, 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”.
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