Diff for "BashFAQ/071"

Differences between revisions 1 and 9 (spanning 8 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)
   }
 
   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

This version of chr executes much faster than the printf version above (about 1/40 to less than 1/150 the time when run in a loop):

   chr() { echo -en "\0$(( $1 % 8 + 10 * ( $1 / 8 ) + 20 ))"; }

   for p in chr newchr; do time for i in {1..4000}; do $p 65 >/dev/null; done; done

   System1                     System2
   real    0m46.824s           real    1m33.814s
   user    0m4.624s            user    0m8.540s
   sys     0m33.290s           sys     1m23.978s

   real    0m1.340s            real    0m0.512s
   user    0m1.096s            user    0m0.389s
   sys     0m0.124s            sys     0m0.096s

-  ⇤ ← Revision 1 as of 2007-05-03 00:06:43 → 
  Size: 1714
  Editor: redondos
  Comment:
+   ← Revision 9 as of 2009-10-14 23:48:06 → ⇥
  Size: 2397
  Editor: DennisWilliamson
  Comment: Added a faster version of chr
-Deletions are marked like this.
+Additions are marked like this.
 Line 1:
-[[Anchor(faq71)]]
+<<Anchor(faq71)>>
 Line 3:
-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:
+Line 6:
+   # 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 18:
+Line 25:
+   # hex() - converts ASCII character to a hexadecimal value
   # unhex() - converts a hexadecimal value to an ASCII character
-Line 20:
+Line 30:
+   }

   unhex() {
      printf \\x"$1"
-Line 28:
+Line 42:
-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).
+The {{{ord}}} function above is quite tricky.
-Line 30:
+Line 44:
- ''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''
+ ''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''
-Line 32:
+Line 46:
- ''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''
+  ''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''

This version of {{{chr}}} executes much faster than the {{{printf}}} version above (about 1/40 to less than 1/150 the time when run in a loop):
-Line 35:
+Line 51:
-   ord() {
     printf '%d' \"$1\"
   }
+   chr() { echo -en "\0$(( $1 % 8 + 10 * ( $1 / 8 ) + 20 ))"; }
-Line 40:
+Line 54:
-Or:
+{{{
   for p in chr newchr; do time for i in {1..4000}; do $p 65 >/dev/null; done; done
-Line 42:
+Line 57:
-{{{
   ord() {
     printf '%d' \'$1\'
   }
+   System1                     System2
   real    0m46.824s           real    1m33.814s
   user    0m4.624s            user    0m8.540s
   sys     0m33.290s           sys     1m23.978s

   real    0m1.340s            real    0m0.512s
   user    0m1.096s            user    0m0.389s
   sys     0m0.124s            sys     0m0.096s
-Line 47:
+Line 66:
-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.