file Command

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Purpose

Determines the file type.

Syntax

To Classify the File Type

file [ -m MagicFile] [ -d ] [ -h ] [ -i ] [ -M MagicFile ] [ -f FileList] [File...]

To Check the Magic File for Format Errors

file -c [ -m MagicFile]

Description

The file command reads the files specified by the File parameter or the FileList variable, performs a series of tests on each file, and attempts to classify them by type. The command then writes the file types to standard output. The file can be regular file, directory, FIFO(named pipe), block special, character special, symbolic link or sockets type.

  • If it is a regular file and of zero length, it is identified as an empty file.
  • If the file is a symbolic link, by default, the link is followed by file the symbolic link refers to.

If a file appears to be in ASCII format, the file command examines the first 1024 bytes and determines the file type. If a file does not appear to be in ASCII format, the file command further attempts to distinguish a binary data file from a text file that contains extended characters.

If the File parameter specifies an executable or object module file and the version number is greater than 0, the file command displays the version stamp. The ld command explains the use of a.out files.

If the language environment is the C programming language, 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 type.

However, if the language environment is some language other than the C programming language, the file command uses the /usr/lib/nls/msg/<language_env.>/magic.cat file to identify files with a magic number.

If the file does not exist, cannot be read or its file status could not be determined then, it is not considered as an error that affects the exit status. The output indicates that the file was processed but the type could not be determined.

When the -i flag is used, the following format shall be used to identify each operand, file specified: 
"%s: %s\n", file, type
The values for type are unspecified except that in the POSIX locale, if file is identified as one of the types listed in the following table, type shall contain (but is not limited to) the corresponding string. Each space shown in the strings shall be exactly one space.
Table 1. File Utility Output Strings
If file is a: type shall contain the string:
Directoy directory
FIFO fifo
Socket socket
Block special block special
Character special character special
Executable binary executable
Empty regular file empty
Symbolic link symbolic link to
ar archive library archive
Extended cpio format cpio archive
Extended tar format tar archive
Shell script commands text
C-language source c program text
FORTRAN source fortran program text
If file is identified as a symbolic link, the following alternative output format shall be used: 
"%s: %s %s\n", file, type, contents of link"
If the file named by the file operand does not exist or cannot be read, the string cannot open shall be included as part of the type field, but this shall not be considered an error that affects the exit status. If the type of the file named by the file operand cannot be determined, the string data shall be included as part of the type field, but this shall not be considered an error that affects the exit status.

Flags

Item Description
-c Checks the specified magic file (the /etc/magic file, by default) for format errors. This validation is not normally done. File typing is not done under this flag.
-d Applies any default system tests to the file.
-f FileList Reads the specified file list. The file must list one file per line and must not contain leading or trailing spaces.
-h When a symbolic link is encountered, identifies the file as a symbolic link. If the -h flag is not specified and file is a symbolic link that refers to a nonexistent file, file shall identify the file as a symbolic link, as if the -h flag had been specified.
-i If a file is a regular file, does not attempt to classify the type of the file further, but identifies the file as specified in Description.
-m MagicFile Specifies the file name of the magic file (the /etc/magic file, by default).
-M MagicFile Specifies the name of a file containing tests that shall be applied to a file in order to classify it. No default system tests shall be applied.

Exit Status

This command returns the following exit values:

Item Description
0 Successful completion.
>0 An error occurred.

Examples

  1. To display the type of information a file contains, enter: 
    file myfile
    This displays the file type of myfile (such as directory, data, ASCII text, C-program source, and archive).
  2. To display the type of each file named in a list of file names, enter:
    file -f filenames
    This displays the type of each file named in the filenames list. Each file name must appear alone on a line.
Note: To get customized messages from the file command, use a separate magic file with the -m option. It is not advisable to edit the read-only /etc/magic file.

Files

Item Description
/usr/bin/file Contains the file command.
/etc/magic Contains the file type database.