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".