Diff for "BashFAQ/071"

How do I convert an ASCII character to its decimal (or hexadecimal) value and back?

If you have a known octal or hexadecimal value (at script-writing time), you can just use printf:

   # POSIX
   printf '\x27\047\n'

This prints two literal ' characters (27 is the hexadecimal ASCII value of the character, and 47 is the octal value) and a newline.

If you need to convert characters (or numeric ASCII values) that are not known in advance (i.e., in variables), you can use something a little more complicated:

   # POSIX
   # chr() - converts decimal value to its ASCII character representation
   # ord() - converts ASCII character to its decimal value
 
   chr() {
     printf \\$(printf '%03o' $1)
   }
 
   ord() {
     printf '%d' "'$1"
   }

   # hex() - converts ASCII character to a hexadecimal value
   # unhex() - converts a hexadecimal value to an ASCII character

   hex() { 
      printf '%x' "'$1"
   }

   unhex() {
      printf \\x"$1"
   }

   todec() {
      echo $(( $1#$2 ))
   }

   # examples:
 
   chr $(ord A)    # -> A
   ord $(chr 65)   # -> 65

   todec 16 ffe    # -> 4094
   todec 2 100100  # -> 36

The ord function above is quite tricky.

• Tricky? Rather, it's using a feature that I can't find documented anywhere -- putting a single quote in front of an integer. Neat effect, but how on earth did you find out about it? Source diving? -- GreyCat

  • It validates The Single Unix Specification: "If the leading character is a single-quote or double-quote, the value shall be the numeric value in the underlying codeset of the character following the single-quote or double-quote." (see printf() to know more) -- mjf