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xxd

This tool is the most commonly available I have found for this type of task (available by default on both latest Ubuntu and macOS). You can remove the ascii readable part on the right if needed using -p and you can revert (change ascii input to binary data) using the -r function. Here are some simple example uses:

Converting to hex with ascii view:

echo example | xxd

Converting to a hexdump (no ascii view on the right):

echo example | xxd -p

Converting from a hexdump back to binary data:

echo 746573740a | xxd -p -r

You can get much more complex with this in shell scripts. I have actually used this and "dd" to scan for specific sequences and modify them in a predefined fashion all from a shell script using nothing but bash, dd, and xxd. You actually don't need dd for this either as you can "seek" to a specific location and write to that location the byte sequence you need. The biggest advantage to this approach is its easily scriptable.

There is also DHEX

apt-cache show dhex

ncurses based hex editor with diff mode

This is more than just another hex editor: It includes a diff mode,
which can be used to easily and conveniently compare two binary files.
Since it is based on ncurses and is themeable, it can run on any
number of systems and scenarios. With its utilization of search logs,
it is possible to track changes in different iterations of files
easily.

If you are not familiar with vim or emacs, this one doesn't seem to have much of a learning curve.

You might be able to use vi/vim as a hex editor too (it can call xxd).

Enter hex mode:

:%!xxd

Exit hex mode:

:%!xxd -r

Source: Using vi as a hex editor

I know this is an old question, but I was dissatisfied with all of the answers here. I was looking for a hex editor that allowed for me to create my own binary files (aka insert mode) and could handle very large files.

I came across tweak, which fulfills both of these requirements, as well as the OPs.

• 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. Tweak only has to take
  significant time when you genuinely need it to read the entire file.
  The only two operations with this property are searching, and saving
  the modified version of the file to disk. Everything else is instant.

Bless Hex Editor is a is a binary (hex) editor and currently provides the following features:

• Efficient editing of large data files and block devices.


• Multilevel undo - redo operations.


• Customizable data views.


• Fast data rendering on screen.


• Multiple tabs.


• Fast find and replace operations.


• A data conversion table.


• Advanced copy/paste capabilities.


• Highlighting of selection pattern matches in the file.


• Plugin based architecture.


• Export of data to text and html (others with plugins).


• Bitwise operations on data.


• A comprehensive user manual.

You can dounload it from here: http://packages.debian.org/sid/all/bless/download.

To install it, see How do I install a .deb file via the command line?

Need more?

• https://stackoverflow.com/questions/5498197/need-a-good-hex-editor-for-linux

Try hexed, it's made for use in scripts and make files.

This one is dead simple to use:

sudo apt-get install hexcurse

There is also ht. Install it as

sudo apt-get install ht

and then run it by typing hte.

I haven't tried it with really large files/partitions, though.