The notes to write Linux shell

1. Convert hex byte to decimal format
a=0x30
b=`printf %i $a`
echo $b

2. Convert ASCII code byte to the char.
a=0x61
b=`printf %i $a | awk '{printf("%c\n", $1)}'`
echo $b

3. Convert the char to ASCII code byte
a="abcde"
a_len=`expr length "$a"`

for (( i=1; i<=$a_len; i=i+1  ))
do
                index=$i
                each_char=`expr substr $a $i 1`
                each_char_ascii=`printf %s $each_char | od -A n -t x1 | tr -d ' '`
                echo "$each_char_ascii "


done

4.