c-gcc, 100 bytes

Result

AC

Code [DL]

char c,s[1],i=100;main(){while(i--){read(0,&c,1);s[c]++;}while(58-i++){while(s[i]--)write(1,&i,1);}}

stdin

8867664674962976670333157606173754542906155951668354632520949512338435161183927326312836653564869115

stdout

0000111111111112222222233333333333333444444445555555555555666666666666666666677777777888888899999999

stderr

/volume/CODE.c:1:19: warning: return type defaults to ‘int’ [-Wimplicit-int]
 char c,s[1],i=100;main(){while(i--){read(0,&c,1);s[c]++;}while(58-i++){while(s[i]--)write(1,&i,1);}}
                   ^
/volume/CODE.c: In function ‘main’:
/volume/CODE.c:1:37: warning: implicit declaration of function ‘read’ [-Wimplicit-function-declaration]
 char c,s[1],i=100;main(){while(i--){read(0,&c,1);s[c]++;}while(58-i++){while(s[i]--)write(1,&i,1);}}
                                     ^
/volume/CODE.c:1:51: warning: array subscript has type ‘char’ [-Wchar-subscripts]
 char c,s[1],i=100;main(){while(i--){read(0,&c,1);s[c]++;}while(58-i++){while(s[i]--)write(1,&i,1);}}
                                                   ^
/volume/CODE.c:1:79: warning: array subscript has type ‘char’ [-Wchar-subscripts]
 char c,s[1],i=100;main(){while(i--){read(0,&c,1);s[c]++;}while(58-i++){while(s[i]--)write(1,&i,1);}}
                                                                               ^
/volume/CODE.c:1:85: warning: implicit declaration of function ‘write’ [-Wimplicit-function-declaration]
 char c,s[1],i=100;main(){while(i--){read(0,&c,1);s[c]++;}while(58-i++){while(s[i]--)write(1,&i,1);}}
                                                                                     ^

strace