The IEEE 1003.1 (POSIX.1) standard contains the following normative text in the extended description for printf():
The argument operands shall be treated as strings if the corresponding conversion specifier is b , c , or s , and shall be evaluated as if by the strtod() function if the corresponding conversion specifier is a , A , e , E , f , F , g , or G . Otherwise, they shall be evaluated as unsuffixed C integer constants, as described by the ISO C standard, with the following extensions:
* 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.
….
The embolding is mine.
I never took much notice of this sentence until recently when I happened to come across this extension being used to print out the numeric value of a character in the underlying codeset.
A simple example of what this means might clarify things for you:
$ printf "%X\n" "A" 0 $ printf "%X\n" A 0 $ printf "%X\n" \"A 41 $ printf "%X\n" \'A 41 $
Note that both the Korn shell and the Bash shell also support trailing single and double quotes.
$ printf "%X\n" \"A\" 41 $ printf "%X\n" \'A\' 41
The Korn shell also supports the following format
$ printf "%X\n" L\'A 41
One use for this functionality is to print out the ASCII codeset values in hexadecimal format for the characters in a string as shown in the following example:
#!/bin/ksh93
str="Hi there Vi Castillo!"
for (( i=0; i < ${#str}; i++ ))
do
c=${str:$i:1}
if [[ $c == ' ' ]]
then
printf "[%s] 0x%X\n" " " \'\ \'
else
printf "[%s] 0x%X\n" "$c" \'$c\'
fi
done
Note that spaces must be escaped with a slash otherwise ksh93 emits a warning about an invalid string constant. Bash is silent but outputs two zeros.
Here is the output from this example:
[H] 0x48 [i] 0x69 [ ] 0x20 [t] 0x74 [h] 0x68 [e] 0x65 [r] 0x72 [e] 0x65 [ ] 0x20 [V] 0x56 [i] 0x69 [ ] 0x20 [C] 0x43 [a] 0x61 [s] 0x73 [t] 0x74 [i] 0x69 [l] 0x6C [l] 0x6C [o] 0x6F [!] 0x21
This example also works in the Bash shell.
Please let me know if you find any other interesting applications of this functionality.
