bash-pure, 99 bytes

Result

AC

Code [DL]

a=()
while read -n 1 c;do((a[c]++));done
for i in {0..9};do for((;a[i]--;));do echo -n $i;done;done

stdin

5197132445861483132846212875868642183482897576394490447660976964082908049925094507115778295983566437

stdout

0000000111111112222222223333333444444444444445555555556666666666677777777778888888888888999999999999

stderr

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

strace