xxd

# Common flags:
# -p  ;; prints hex only (instead of the complex output)
# -ps ;; it is the same thing as -p
# -r  ;; reverse mode, converts hex to ascii
# -e  ;; print in little-endian order, by default with -p it just prints byte as they are (big-endian)
# TODO: Research on endianness used by hexdump, xxd, od, objdump
# objdump interprets by default as little-endian 
# The traditional BSD hexdump utility uses the platform's endianness, so the
# output you see means your machine is little-endian.

# To get hexdump of a file
xxd <filename>

# To get hexdump of a file (only hex bytes)
xxd -p <filename>

# To convert file/string to hex:
echo -n '42 is the solution' | xxd -p
# output: 34322069732074686520736f6c7574696f6e0a

# To convert file/string to hex (5 bytes per line)
echo -n '42 is the solution' | xxd -p -c 5

# To convert hex to bin/string:
echo -n '34322069732074686520736f6c7574696f6e0a' | xxd -p -r
# output: 42 is the solution

# To convert string to hex without newlines
echo -n "Ciao" | xxd -p | tr -d '\n'

# Get the hexdump of the first 120 bytes of a file 12 bytes per line
xxd -l 120 -c 12 <filename>

# Get the hexdump of the first 120 bytes after the address 0x30 of a file
xxd -l 120 -s 0x30 <filename>

# Get from byte 0x36 to 0x13 + 13 bytes showing only 13 bytes (chirurgic mode ;))
xxd -s 0x36 -l 13 -c 13

# Create a file with plenty of null bytes except the 200000 byte which will be 0x41
echo "200000: 41" | xxd -r > <newfile>

# Copy input_file to output_file and prepend 100 bytes of value 0x00.
xxd input_file | xxd -r -s 100 > output_file

# Separate with a space each 4 byte (also with 1 is useful)
xxd -g 4 <file>


# xxd as hex editor
## Patch a byte in the file <filetomodify>
## in this way we change the 0x00000036 byte with 0x35 in the file xxd.1
echo "0000036: 35" | xxd -r - <filetomodify>

## Check the modification
xxd -s 0x36 -l 13 -c 13 <filetomodify>