bash-pure, 94 bytes

Result

AC

Code [DL]

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

8374217079768265197411523063165145518160440154346506902290608848388611561605375447946123878650376858

stdout

0000000000111111111111122222233333333444444444445555555555556666666666666677777777788888888888899999

stderr

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

strace