bash-pure, 93 bytes

Result

AC

Code [DL]

while read -n1 c;do((a[c]++));done
for i in {0..9};do for((;a[i]--;));do echo -n $i;done;done

stdin

6263379678009080077660834380589855835767840637481663483071296009957743634401028006486270366717025104

stdout

0000000000000000001111122222333333333334444444445555556666666666666667777777777777888888888888999999

stderr

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

strace