[linux] How to create a hex dump of file containing only the hex characters without spaces in bash?

How do I create an unmodified hex dump of a binary file in Linux using bash? The od and hexdump commands both insert spaces in the dump and this is not ideal.

Is there a way to simply write a long string with all the hex characters, minus spaces or newlines in the output?

I think this is the most widely supported version (requiring only POSIX defined tr and od behavior):

cat "$file" | od -v -t x1 -A n | tr -d ' \n'

This uses od to print each byte as hex without address without skipping repeated bytes and tr to delete all spaces and linefeeds in the output. Note that not even the trailing linefeed is emitted here. (The cat is intentional to allow multicore processing where cat can wait for filesystem while od is still processing previously read part. Single core users may want replace that with < "$file" od ... to save starting one additional process.)

It seems to depend on the details of the version of od. On OSX, use this:

od -t x1 -An file |tr -d '\n '

(That's print as type hex bytes, with no address. And whitespace deleted afterwards, of course.)

Format strings can make hexdump behave exactly as you want it to (no whitespace at all, byte by byte):

hexdump -ve '1/1 "%.2x"'

1/1 means "each format is applied once and takes one byte", and "%.2x" is the actual format string, like in printf. In this case: 2-character hexadecimal number, leading zeros if shorter.

tldr;

$ od -t x1 -A n -v <empty.zip | tr -dc '[:xdigit:]' && echo 
504b0506000000000000000000000000000000000000
$

Explanation:

Use the od tool to print single hexadecimal bytes (-t x1) --- without address offsets (-A n) and without eliding repeated "groups" (-v) --- from empty.zip, which has been redirected to standard input. Pipe that to tr which deletes (-d) the complement (-c) of the hexadecimal character set ('[:xdigit:]'). You can optionally print a trailing newline (echo) as I've done here to separate the output from the next shell prompt.

References:

• https://pubs.opengroup.org/onlinepubs/9699919799/utilities/od.html
• https://pubs.opengroup.org/onlinepubs/9699919799/utilities/tr.html

The other answers are preferable, but for a pure Bash solution, I've modified the script in my answer here to be able to output a continuous stream of hex characters representing the contents of a file. (Its normal mode is to emulate hexdump -C.)

xxd -p file

Or if you want it all on a single line:

xxd -p file | tr -d '\n'

Perl one-liner:

perl -e 'local $/; print unpack "H*", <>' file