Hex editor
From Sega Retro
A hex editor (or binary file editor or byte editor) is a type of computer program that allows a user to manipulate binary (normally non-plain text) computer files. Hex editors that were designed to edit sector data from floppy or hard disks were sometimes called sector editors.
Using a hex editor a user can see or edit the raw and exact contents of a file as opposed to the interpretation of the same content that other, higher level application software may associate with the file format. For example, this could be raw image data, in contrast to the way image editing software would interpret the same file.
In most hex editor applications the data of the computer file is represented as hexadecimal values grouped in two groups of 8 bytes and one group of 16 ASCII characters, nonprintable characters normally are represented by a dot "." in the ASCII part.
The standard Unix shell command used to display (though not edit) a file in hexadecimal and octal is od.