file (command)

file is <a href="/facts/Shell_(computing)/Sa67elS7">shell</a> <a href="/facts/Command_(computing)/zUrvQbFy">command</a> for reporting the type of data contained in a <a href="/facts/Computer_file/FjFH3Ufm">file</a>. It is commonly supported in <a href="/facts/Unix/0pNL2seI">Unix</a> and <a href="/facts/Unix-like/YAKrAxzd">Unix-like</a> <a href="/facts/Operating_systems/8XTuvMh2">operating systems</a>.
As the command uses relatively quick-running <a href="/facts/Heuristic/BcBEkv9c">heuristics</a> to determine <a href="/facts/File_type/qrXbwJaH">file type</a>, it can report misleading information. The command can be fooled, for example, by including a magic number in the content even if the rest of the content does not match what the magic number indicates. The command report cannot be taken as completely trustworthy.
The <a href="/facts/Single_UNIX_Specification/w1ba5bDt">Single UNIX Specification</a> (SUS) requires the command to exhibit the following behavior with respect to the file specified via the <a href="/facts/Command-line_interface/R9JpGmXQ">command-line</a>:

<ol><li>If the file cannot be read, or its <a href="/facts/Unix_file_type/CplllKdU">Unix file type</a> is undetermined, the command will report that the file was processed but its type was undetermined</li>
<li>The command must be able to determine the types <a href="/facts/Folder_(computing)/VLtLcEXY">directory</a>, <a href="/facts/Named_pipe/g78xY9lC">FIFO</a>, <a href="/facts/Unix_domain_socket/fHCylnmJ">socket</a>, block <a href="/facts/Device_file/t8ISnXft">special file</a>, and character special file</li>
<li>A zero-length file is reported as such</li>
<li>An initial part of file is considered and the command is to use position-sensitive tests</li>
<li>The entire file is considered and the command is to use context-sensitive tests</li>
<li>Otherwise, the file is reported as a data file</li></ol>
Position-sensitive tests are normally implemented by matching various locations within the file against a textual database of <a href="/facts/Magic_number_(programming)/Xg7mVI7V">magic numbers</a> (see the Usage section). This differs from other simpler methods such as <a href="/facts/File_extension/cJgfAym4">file extensions</a> and schemes like <a href="/facts/MIME/P8PDYuts">MIME</a>.
In the System V implementation, the Ian Darwin implementation, and the OpenBSD implementation, the command uses a database to drive the probing of the lead bytes. That database is stored as a file that is located in /etc/magic, /usr/share/file/magic or similar.

file (command) open-in-new

file (command)