Diff for "BashFAQ/071"

Differences between revisions 14 and 16 (spanning 2 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 14 as of 2009-10-16 10:13:35 → 
  Size: 3544
  Editor: pgas
  Comment: yet another chr
+   ← 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.
 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 47:
+Line 53:
-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
}}}
  
This version is faster as it executes without a subshell, it seems to only work strictly on ascii chars <127 while the printf version is happy with chars up to 255 and also for only a subset of ascii ie >64 decimal.
Some versions avoiding a subshell:
{{{
oldchr () {  printf \\$(printf '%03o' $1) ;}

#posix
chr () {
    set -- $(($1 / 64)) $(($1 % 64))
    set -- $1  $(($2 / 8)) $(($2 % 8))
    printf \\"${1}${2}${3}"
}

#bash only
chr_bash () {
    local temp
    printf -v temp  '%03o' $1
    printf \\$temp
}

#test
for i in {1..255} ;do [[ "$(oldchr $i)" = "$(chr $i)" ]] || echo $i;done
for i in {1..255} ;do [[ "$(oldchr $i)" = "$(chr_bash $i)" ]] || echo $i;done
for p in oldchr chr chr_bash; do echo $p:;time for i in {1..4000}; do $p 65 >/dev/null; done; done

}}}
the timings:
{{{
$ bash  chr
oldchr:

real    0m14.350s
user    0m5.004s
sys     0m9.248s
chr:

real    0m0.422s
user    0m0.059s
sys     0m0.216s
chr_bash:

real    0m0.400s
user    0m0.042s
sys     0m0.189s

}}}

Yet another version probably faster:
{{{
chr () {
    printf \\$(($1/64*100+$1%64/8*10+$1%8))
}


}}}