If you’ve ever had a file without an extension lying around your filesystem or a file that says it’s a jpeg but refuses to show itself in a graphics application, the file command is for you. The command ships with just about every *nix under the sun (including MacOS/OSX).

Basic usage

$ file <the_file>

For example, on my M1 Macbook Pro (MacOS 12.x) running file on /bin/bash produces:

/bin/bash: Mach-O universal binary with 2 architectures: [x86_64:Mach-O 64-bit executable x86_64
- Mach-O 64-bit executable x86_64] [arm64e:Mach-O 64-bit executable arm64e
- Mach-O 64-bit executable arm64e]
/bin/bash (for architecture x86_64):	Mach-O 64-bit executable x86_64
/bin/bash (for architecture arm64e):	Mach-O 64-bit executable arm64e

While on my x86_64 Linux machine the same command produces:

/bin/bash: ELF 64-bit LSB pie executable, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, BuildID[sha1]=9483da49f2e17070c1df9a75d509e09211e96769, for GNU/Linux 4.4.0, not stripped

Some useful switches include:

  • -z <filename> - determine the file types of files inside a zipped archive
  • -I <filename> - determine the mime type of said file

Go forth and conquer!