[SOLVED] Generate all permutations of up to `n` letters

Generate all permutations of up to `n` letters

I'd like to generate all permutations of letters up to length n

For example, for the argument 2 I'd like obtain a list like

a
aa
..
az
...
z
za
..
zz

I tried using a for loop to generate n increasingly larger brace expansions by repeating {a..z} ^1 and appending it to a variable. But this doesn't seem to work.

OUT=""

# loop from 1 to first argument
for ((i=1; i<=$1; i++))
do
    OUT+=$(echo $(printf '{a..z}%.0s' {1..$i}))
done

OUT=$(echo $OUT | sort)

echo $OUT

Solution

Chained brace expansions don't scale well. You're better off with a function like this:

# Usage: perms n [prefix]
perms() {
  if (($1 < 1)); then
    return
  fi

  for pfix in "$2"{a..z}; do
    printf '%s\n' "$pfix"
    perms $(($1 - 1)) "$pfix"
  done
}

$ perms 2
a
aa
ab
ac
...

But if you insist on it, this is how you should do it:

# Usage: perms n
perms() {
  printf -v brace_exp '{a..z}%*s' $(($1 - 1))
  brace_exp=${brace_exp// /'{,{a..z}}'}
  eval "printf '%s\n' $brace_exp"
}