bash-pure, 120 bytes

Result

AC

Code [DL]

a=()
while read -n 1 c;do a[$c]=$((a[c]+1));done
for((i=0;i<10;i++));do for((j=0;j<${a[i]};j++));do echo -n $i;done;done

stdin

9511394619472572745908531919207444548330099690442668264226652971014204699624536332061383703972499126

stdout

0000000001111111112222222222222333333333344444444444444555555566666666666677777778888999999999999999

stderr

/bin/bash: warning: setlocale: LC_ALL: cannot change locale (en_US.UTF-8)

strace