Diff for "BashFAQ/071"

Differences between revisions 1 and 16 (spanning 15 versions)

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)
   }
  
   #Another version doing the octal conversion with arithmetic 
   # faster as it avoids a subshell
  chr () {
    printf \\$(($1/64*100+$1%64/8*10+$1%8))
   } 

   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"
   }

   # examples:

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

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

-  ⇤ ← Revision 1 as of 2007-05-03 00:06:43 → 
  Size: 1714
  Editor: redondos
  Comment:
+   ← Revision 16 as of 2009-10-17 07:22:24 → ⇥
  Size: 1903
  Editor: ppp089210051033
  Comment: remove all the blah, add the faster version with maths in the text
-Deletions are marked like this.
+Additions are marked like this.
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-[[Anchor(faq71)]]
+<<Anchor(faq71)>>
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-This task is quite easy while using the {{{printf}}} builtin. You can either write two simple functions as shown below or use the plain {{{printf}}} constructions alone.
+If you have a known octal or hexadecimal value (at script-writing time), you can just use `printf`:
 Line 7:
+   # 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
-Line 9:
+Line 18:
-Line 13:
+Line 22:
+      #Another version doing the octal conversion with arithmetic 
   # faster as it avoids a subshell
  chr () {
    printf \\$(($1/64*100+$1%64/8*10+$1%8))
   }
-Line 18:
+Line 33:
-   hex() {
+   # hex() - converts ASCII character to a hexadecimal value
   # unhex() - converts a hexadecimal value to an ASCII character

   hex() {
-Line 22:
+Line 40:
+   unhex() {
      printf \\x"$1"
   }
-Line 23:
+Line 45:
-Line 27:
+Line 49:
+The {{{ord}}} function above is quite tricky.
-Line 28:
+Line 51:
-The {{{ord}}} function above is quite tricky. It can be re-written in several other ways (use that one that will best suite your coding style or your actual needs).

 ''Q:  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''

 ''A:  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 [http://www.opengroup.org/onlinepubs/009695399/utilities/printf.html printf()] to know more) -- mjf''

{{{
   ord() {
     printf '%d' \"$1\"
   }
}}}

Or:

{{{
   ord() {
     printf '%d' \'$1\'
   }
}}}

Or, rather:

{{{
   ord() {
     printf '%d' "'$1'"
   }
}}}

Etc. All of the above {{{ord}}} functions should work properly. Which one you choose highly depends on particular situation.
+ . ''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 [[http://www.opengroup.org/onlinepubs/009695399/utilities/printf.html|printf()]] to know more) -- mjf''