Editing File (command) (section)

{{Short description|Shell command for reporting file type information}}
{{Redirects here|File (software)|the file management program bundled with [[iOS 11]]|Files (software)}}
{{lowercase title}}
{{Infobox software
| name                   = file
| logo                   = <!-- [[File: ]] -->
| logo caption           = 
| screenshot             = File-example-command-gimp.gif
| caption                = Example usage of {{code|file}}
| collapsible            = 
| author                 = 
| developer              = [[AT&T Bell Laboratories]]
| released               = {{Start date|1973|df=yes}} as part of [[Unix Research Version 4]]; {{Start date|1986|df=yes}} open-source reimplementation
| discontinued           = 
| latest release version = {{wikidata|property|preferred|references|edit|P348|P548=Q2804309}}                                             | latest release date    = {{Start date and age|{{wikidata|qualifier|preferred|single|P348|P548=Q2804309|P577}}|df=yes}}
| latest preview version = 
| latest preview date    = <!-- {{Start date and age|YYYY|MM|DD|df=yes/no}} -->
| repo                   = {{URL|https://github.com/file/file}}
| programming language   = [[C (programming language)|C]]
| operating system       = [[Unix]], [[Unix-like]], [[Plan 9 from Bell Labs|Plan 9]], [[IBM i]]
| platform               = [[Cross-platform]]
| size                   = 
| language               = 
| genre                  = [[File type]] detector
| license                = [[BSD license]], [[Common Development and Distribution License|CDDL]]<br />Plan 9: [[MIT License]]
| alexa                  = 
| website                = {{URL|https://darwinsys.com/file/}}
}}
'''<code>file</code>''' is a [[Shell (computing)|shell]] [[command (computing)|command]] for reporting the type of data contained in a [[computer file|file]]. It is commonly supported in [[Unix]] and [[Unix-like]] [[operating systems]].

As the command uses relatively quick-running [[heuristic]]s to determine [[file type]], 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 [[Single UNIX Specification]] (SUS) requires the command to exhibit the following behavior with respect to the file specified via the [[command-line interface|command-line]]:
# If the file cannot be read, or its [[Unix file type]] is undetermined, the command will report that the file was processed but its type was undetermined
# The command must be able to determine the types [[folder (computing)|directory]], [[named pipe|FIFO]], [[Unix domain socket|socket]], block [[device file|special file]], and character special file
# A zero-length file is reported as such
# An initial part of file is considered and the command is to use position-sensitive tests
# The entire file is considered and the command is to use context-sensitive tests
# Otherwise, the file is reported as a data file

Position-sensitive tests are normally implemented by matching various locations within the file against a textual database of [[Magic number (programming)|magic number]]s (see the Usage section). This differs from other simpler methods such as [[file extension]]s and schemes like [[MIME]].

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 <code>/etc/magic</code>, <code>/usr/share/file/magic</code> or similar.