Diff for "BashFAQ/071"

Differences between revisions 2 and 10 (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)
   }
 
   # another faster version 
   # the octal conversion is done using maths, potentially avoiding a subshell and a fork
   chr() { 
     printf "\0$(( $1 % 8 + 10 * ( $1 / 8 ) + 20 ))"; 
   }

   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 2 as of 2007-09-15 18:50:34 → 
  Size: 1946
  Editor: irc2samus
  Comment:
+   ← Revision 10 as of 2009-10-16 07:47:19 → ⇥
  Size: 1931
  Editor: pgas
  Comment: move the faster solution with the other examples, using printf for more portability
-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:
+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 14:
+Line 21:
+   # another faster version 
   # the octal conversion is done using maths, potentially avoiding a subshell and a fork
   chr() { 
     printf "\0$(( $1 % 8 + 10 * ( $1 / 8 ) + 20 ))"; 
   }
-Line 18:
+Line 31:
+   # hex() - converts ASCII character to a hexadecimal value
   # unhex() - converts a hexadecimal value to an ASCII character
-Line 20:
+Line 36:
+   }

   unhex() {
      printf \\x"$1"
-Line 28:
+Line 48:
-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 50:
- ''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 52:
- ''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.



There is also an alternative when printing characters by their ascii value that is using escape sequences like:
{{{
   echo $'\x27'
}}}
which prints a literal ' (there 27 is the hexadecimal ascii value of the character).
+  ''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''