Bash string handling (char at index and concatenation)

Question

I'm trying to learn bash string handling. How do I create a bash script which is equivalent to this Java code snippet?

String symbols = "abcdefg12345_";
for (char i : symbols.toCharArray()) {
    for (char j : symbols.toCharArray()) {
     System.out.println(new StringBuffer().append(i).append(j));
    }
}

The output of the above code snippet starts with:

aa
ab
ac
ad
ae
af

And ends with:

_g
_1
_2
_3
_4
_5
__

My goal is to have a list of allowed characters (not necessarily the ones above) and print out all the permutations of length 2. If it is possible I would like a solution which relies solely on bash and doesn't require anything else installed.

Edit: Just a little follow up question: Is there a way to do this with a string without spaces separating sub-strings? Like LIST="abcdef12345_"?

Answer 1

That is so simple, Bash does it in the input parser. No code required. Try:

echo {a,b,c,d,e,f,g,1,2,3,4,5,_}{a,b,c,d,e,f,g,1,2,3,4,5,_}

You might need a second pass to split it into lines, though.

Or, you could of course use a couple of nested loops like in your example:

LIST="a b c d e f 1 2 3 4 5 _" ; for a in $LIST ; do for b in $LIST ; do echo $a$b ; done ; done

Answer 2

for i in a b c d e f g 1 2 3 4 5 _; do
    for j in a b c d e f g 1 2 3 4 5 _; do
        echo $i$j
    done
done

man bash is your friend. It has large sections on its variable replacement and internal commands.

Answer 3

Fits one line nicely:

 for i in `echo {a,b,c,d,e,f,g,1,2,3,4,5,_}{a,b,c,d,e,f,g,1,2,3,4,5,_}`; do echo $i; done

Answer 4

Just some variation of how you can split the items generated. Personally, i like to use tr for this job. :

echo {a,b,c,d,e,f,g,1,2,3,4,5,_}{a,b,c,d,e,f,g,1,2,3,4,5,_} | tr " " "\n"