NAME

file − Determines file type

SYNOPSIS

file [-c] [-f file_list] [-m magic_file] file ... 

The file command reads input files and performs a series of tests on each one. It then attempts to classify them by type and writes the file types to standard output. 

DESCRIPTION

The file command uses the /etc/magic file to identify files that have some sort of a magic number (that is, any file containing a numeric or string constant that indicates its type). 

If a file appears to be plain text, file examines the first 512 bytes and tries to determine what kind of text it is.  If the first 512 bytes only contain ASCII characters, file returns either ASCII text or English text.  If the file contains other characters (that is, European or Asian extended characters), file returns data. 

If a file does not appear to be plain text, file attempts to distinguish a binary data file from a text file that contains extended characters.  If the file is an a.out file and the version number is greater than zero, file displays the version stamp. 

For character special files, part of the identification is information about the devices the system shows as active. In particular, file returns device-specific information such as controller type and unit, device type and unit, and status (offline, write locked, density, errors).  The general categories currently implemented are disk, tape, and terminal devices. The supported terminal devices include Local Area Terminals (LAT) but not Local Area Network (LAN) pseudo-terminals. 

The following example shows how the file command identifies a device. Note that the output is shown on two lines due to space considerations, but appears on one line on a DEC OSF/1 system. 

 % file /dev/rrz0c

 /dev/rrz0c: character special (8/2) SCSI #0
 RZ24 disk #0 (SCSI ID #0)

On DEC OSF/1 systems, the file command recognizes OSF core files.  For example:

  % file core

  core:   core dump, generated from ’mwm’

The amount and type of information the file command returns can depend on the permissions of the file being queried. For example, most special device files have permissions that allow access only by root and non-root users cannot open them. The file command has to open the device and only root has the proper permissions.  Thus, if the file command is issued by a non-root user, it can report only information it can determine without gaining access to the device. 

The file command also uses internal tables to decode certain types of files. The following example shows the keywords the file command uses to locate troff, C code, and assembler code. 

char  ∗troff[] = {    /∗ new troff intermediate lang ∗/
      "x","T","res","init","font","202","V0","p1",0};
char  ∗c[] = {
      "int","char","float","double","struct","extern",0};
char  ∗as[] = {
      "globl","byte","align","text","data","comm",0};

FLAGS

−cChecks the magic file (/etc/magic by default) for format errors.  This validation is not normally done.  File typing is not done under this flag. 

−f file_list
Reads file_list for a list of files to examine. 

−m magic_file
Specifies magic_file as the magic file (/etc/magic by default). 

RESTRICTIONS

The file command often does a poor job of distinguishing C programs, shell scripts, English text, and ASCII text. In addition, it does not recognize certain programming languages, including Modula, Pascal, and Lisp. 

EXAMPLES

     1.To display the type of information a file contains, enter:

file myfile

This displays the file type of myfile (directory, data, ASCII text, C program source, archive, and so on). 

     2.To display the type of each file named in a list of filenames, enter:

file -f filenames

This displays the type of each file with a name that appears in filenames.  Each filename must appear alone on a line. 

To create filenames, enter:

ls > filenames

Then edit filenames as desired. 

FILES

/etc/magicFile type database

RELATED INFORMATION

Programmer’s Guide