tweak (ncurses-based hex editor)

Tweak is a hex editor. It allows you to edit a file at very low level,
letting you see the full and exact binary contents of the file. It can be
useful for modifying binary files such as executables, editing disk or CD
images, debugging programs that generate binary file formats incorrectly,
and many other things.

Tweak uses a complex data structure based on B-trees, designed to make
almost all editing operations extremely fast, even when they are working
on huge amounts of data.

Tweak supports insert mode (not particularly useful if you're editing
an executable file or a filesystem image, but can be extremely handy in
other file formats such as PNG).

Cutting, copying and pasting within the file you are editing is extremely
efficient. No matter how big the chunk of data you are moving around -
even if it's a 200Mb section of a CD image - Tweak will always perform
the operation effectively instantly.

Tweak supports lazy loading of the input file: rather than sucking it
all into memory straight away, it simply remembers which parts of the
editing buffer are copies of which parts of the input file and refers to
the file on disk when it needs to.