File command on Linux (check file type)

The file command in Linux is used to determine the type of file entered as a parameter based on the Magic Number (first two bytes).

Unlike Windows, file extensions mean nothing in Linux. The file command performs three types of checks to determine the file type:

File system testing;
Magic Number testing;
Language testing

The first file system test is done to determine whether the file is an ordinary file, a directory, a link, a device, a connection socket, etc. In Linux, absolutely everything is a file. The type of file in the file system determines whether it is a regular file or another special type.

The second Magic Number test checks the first two bytes of the file to determine its type. There is a convention in computing that states that the first two bytes of the file must contain a code that indicates its type. For example, scripts start with the code “#!” , followed by the full path of the interpreter who will interpret and execute the script.

The third test, once it has been determined that the file is a program, script, or source code, indicates the program’s language.

Examples:

$ file test1.c test1.c: C source, UTF-8 Unicode text

$ file test1 test1: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked (uses shared libs), for GNU/Linux 2.6.32, buildID [sha1] =f63857e5ce1a259a5228b1ea15ced24867583793, not stripped

$ file backup.sh backup.sh: Bourne-Again shell script, ASCII text executable

$ file supermarket.xz supermarket.xz: XZ compressed data

$ file MAPA_MENTAL_LPI.pdf MAPA_MENTAL_LPI.pdf: PDF document, version 1.6

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File command on Linux (check file type)

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