The file utility performs a series of tests on each file supplied by file and, optionally, on each file listed in ffile in an attempt to classify it. If the file is not a regular file, its file type is identified. The file types directory, FIFO, block special, and character special are identified as such. If the file is a regular file and the file is zero-length, it is identified as an empty file.
If file appears to be a text file, file examines the first 512 bytes and tries to determine its programming language. If file is an executable a.out, file prints the version stamp, provided it is greater than 0. If file is a symbolic link, by default the link is followed and file tests the file to which the symbolic link refers.
By default, file will try to use the localized magic file /usr/lib/locale/locale/LC_MESSAGES/magic, if it exists, to identify files that have a magic number. For example, in the Japanese locale, file will try to use /usr/lib/locale/ja/LC_MESSAGES/magic. If a localized magic file does not exist, file will utilize /etc/magic. A magic number is a numeric or string constant that indicates the file type. See magic(4) for an explanation of the format of /etc/magic.
If file does not exist, cannot be read, or its file status could not be determined, it is not considered an error that affects the exit status. The output will indicate that the file was processed, but that its type could not be determined.
The following options are supported:
Checks the magic file for format errors. For reasons of efficiency, this validation is normally not carried out.
Does not follow symbolic links.
ffile contains a list of the files to be examined.
Uses mfile as an alternate magic file, instead of /etc/magic.
See largefile(5) for the description of the behavior of file when encountering files greater than or equal to 2 Gbyte ( 231 bytes).
Determine if an argument is a binary executable file:
file "$1" | grep -Fq executable && printf "%s is executable.\n" "$1"
See environ(5) for descriptions of the following environment variables that affect the execution of file: LANG, LC_ALL, LC_CTYPE, LC_MESSAGES, and NLSPATH.
See attributes(5) for descriptions of the following attributes:
|ATTRIBUTE TYPE||ATTRIBUTE VALUE|
If the -h option is specified and file is a symbolic link, file prints the error message:
symbolic link to file