Printing decimal to ascii character, my command does not output as intended

Question

I wanted to output a string of all the ascii characters with the following command

for i in `seq 32 127`; do printf "%c" $i; done

The output of the above command is:

33333334444444444555555555566666666667777777777..............

It's the first (from the left) digit of each number.

Looking through this site I came across the answer to my problem How to print all printable ASCII chars in CLI?, however I still don't understand why my original snippet does not output the ascii characters as intended.

Answer 1

You can't directly print the ascii codes by using the printf "%c" $i like in C.

You have to first convert the decimal value of i into its octal value and then you have to print it using using printf and putting \ in front of their respective octal values.

To print A, you have to convert the decimal 65 into octal, i.e. 101, and then you have to print that octal value as:

printf "\101\n"

This will print A.

So you have to modify it to :

for i in `seq 32 127`; do printf \\$(printf "%o" $i);done;

But by using awk you can directly print like in C language

awk 'BEGIN{for(i=32;i<=127;i++)printf "%c",i}';echo

Answer 2

%c Interprets the associated argument as char: only the first character of a given argument is printed

You seem to already have a way to print them, but here is one variant.

for i in `seq 32 127`; do printf "\x$(printf "%x" $i) $i"; done

Answer 3

You need printf, but only once; you can replace one use of printf by the simpler and more efficient echo plus Bash escape sequences:

With hexagesimals:

for i in `seq 32 127`; do
  echo -ne \\x$(printf %02x $i)
done

With octals:

for i in `seq 32 127`; do
  echo -ne \\0$(printf %03o $i)
done

Answer 4

If, like me, you prefer keeping the printf format string as constant as possible, change:

for i in `seq 32 127`; do printf "\\$(printf %o "$i")"; done
for i in `seq 32 127`; do printf "\x$(printf %x "$i")"; done  # not POSIX

to

for i in `seq 32 127`; do printf %b "\0$(printf %o "$i")"; done
for i in `seq 32 127`; do printf %b "\x$(printf %x "$i")"; done  # not POSIX

(Incidentally, the POSIX standard for printf doesn't include \x among the escape sequences allowed in the format string.)

I prefer keeping the printf format string as constant as possible because of separation of concerns. The format string's concern is formatting; the arguments are for data that varies. Also a problem could occur if an expansion in the format string includes things that could be interpreted as format specifiers, as discussed here in the context of C programming. Also discussed here: ShellCheck linting rule SC2059 Don't use variables in the printf format string.