Convert ascii code to hexadecimal in UNIX shell script

There's a printf tool that simulates the C function; normally it's at /usr/bin/printf, but a lot of shells implement built-ins for it as well. You can use %02x to get the hex representation of a character, but you need to make sure you pass a string that includes the character in single-quotes (Edit: It turns out just a single-quote at the beginning is sufficient):

printf "%%%02x\n" "'-"   # Outputs %2d

You can make a shell function for convenience:

function hex() {
    printf "%%%02x\n" "'$1"
}

hex -   # Outputs %2d
hex _   # Outputs %5f
hex .   # Outputs %2e

Try od:

$ echo -n "-_." | od -A n -t x1

2d 5f 2e

-A n means do not print offsets and -t x1 means that the type of the input is hexadecimal integers of 1 byte.

xxd is a hex-dump utility shipped as part of the vim-common package
It has a handy -p option which outputs a continuous 'plain/postscript' hexdump style.. This can eaisly be reversed via -r... -u will output upper-case Hex-Digits.

$ echo -n "M" |xxd -p    # 1 ASCII char= 1 UTF-8 byte; Unicode Codepoint: U+0041
4d

$ echo -n "〶" |xxd -p -u # 1 CJK char= 3 UTF-8 bytes; Unicode Codepoint: U+3036  
E380B6

$ echo -n "Dump a string" |xxd -p -u  
44756D70206120737472696E67

$ echo -n "Dump and Revert" |xxd -p |xxd -r  
Dump and Revert