Bash: Split string into character array

I have a string in a Bash shell script that I want to split into an array of characters, not based on a delimiter but just one character per array index. How can I do this? Ideally it would not use any external programs. Let me rephrase that. My goal is portability, so things like sed that are likely to be on any POSIX compatible system are fine.

Try

echo "abcdefg" | fold -w1

Edit: Added a more elegant solution suggested in comments.

echo "abcdefg" | grep -o .

You can access each letter individually already without an array conversion:

$ foo="bar"
$ echo ${foo:0:1}
b
$ echo ${foo:1:1}
a
$ echo ${foo:2:1}
r

If that's not enough, you could use something like this:

$ bar=($(echo $foo|sed  's/\(.\)/\1 /g'))
$ echo ${bar[1]}
a

If you can't even use sed or something like that, you can use the first technique above combined with a while loop using the original string's length (${#foo}) to build the array.

Warning: the code below does not work if the string contains whitespace. I think Vaughn Cato's answer has a better chance at surviving with special chars.

thing=($(i=0; while [ $i -lt ${#foo} ] ; do echo ${foo:$i:1} ; i=$((i+1)) ; done))

If your string is stored in variable x, this produces an array y with the individual characters:

i=0
while [ $i -lt ${#x} ]; do y[$i]=${x:$i:1};  i=$((i+1));done

As an alternative to iterating over 0 .. ${#string}-1 with a for/while loop, there are two other ways I can think of to do this with only bash: using =~ and using printf. (There's a third possibility using eval and a {..} sequence expression, but this lacks clarity.)

With the correct environment and NLS enabled in bash these will work with non-ASCII as hoped, removing potential sources of failure with older system tools such as sed, if that's a concern. These will work from bash-3.0 (released 2005).

Using =~ and regular expressions, converting a string to an array in a single expression:

string="wonkabars"
[[ "$string" =~ ${string//?/(.)} ]]       # splits into array
printf "%s\n" "${BASH_REMATCH[@]:1}"      # loop free: reuse fmtstr
declare -a arr=( "${BASH_REMATCH[@]:1}" ) # copy array for later

The way this works is to perform an expansion of string which substitutes each single character for (.), then match this generated regular expression with grouping to capture each individual character into BASH_REMATCH[]. Index 0 is set to the entire string, since that special array is read-only you cannot remove it, note the :1 when the array is expanded to skip over index 0, if needed. Some quick testing for non-trivial strings (>64 chars) shows this method is substantially faster than one using bash string and array operations.

The above will work with strings containing newlines, =~ supports POSIX ERE where . matches anything except NUL by default, i.e. the regex is compiled without REG_NEWLINE. (The behaviour of POSIX text processing utilities is allowed to be different by default in this respect, and usually is.)

Second option, using printf:

string="wonkabars"
ii=0
while printf "%s%n" "${string:ii++:1}" xx; do 
  ((xx)) && printf "\n" || break
done 

This loop increments index ii to print one character at a time, and breaks out when there are no characters left. This would be even simpler if the bash printf returned the number of character printed (as in C) rather than an error status, instead the number of characters printed is captured in xx using %n. (This works at least back as far as bash-2.05b.)

With bash-3.1 and printf -v var you have slightly more flexibility, and can avoid falling off the end of the string should you be doing something other than printing the characters, e.g. to create an array:

declare -a arr
ii=0
while printf -v cc "%s%n" "${string:(ii++):1}" xx; do 
    ((xx)) && arr+=("$cc") || break
done

The most simple, complete and elegant solution:

$ read -a ARRAY <<< $(echo "abcdefg" | sed 's/./& /g')  

and test

$ echo ${ARRAY[0]}
  a

$ echo ${ARRAY[1]}
  b

Explanation: read -a reads the stdin as an array and assigns it to the variable ARRAY treating spaces as delimiter for each array item.

The evaluation of echoing the string to sed just add needed spaces between each character.

We are using Here String (<<<) to feed the stdin of the read command.