c-gcc, 109 bytes

Result

AC

Code [DL]

char c,s[10],i=100;main(){while(i--){read(0,&c,1);s[c-48]++;}for(i=0;i<10;i++){while(s[i]--)printf("%d",i);}}

stdin

1027056877460801681536487532922594057860813570825536862029897888503293366469898184197851330921728901

stdout

0000000000011111111122222222223333333334444455555555556666666666777777777888888888888888889999999999

stderr

/volume/CODE.c:1:20: warning: return type defaults to ‘int’ [-Wimplicit-int]
 char c,s[10],i=100;main(){while(i--){read(0,&c,1);s[c-48]++;}for(i=0;i<10;i++){while(s[i]--)printf("%d",i);}}
                    ^
/volume/CODE.c: In function ‘main’:
/volume/CODE.c:1:38: warning: implicit declaration of function ‘read’ [-Wimplicit-function-declaration]
 char c,s[10],i=100;main(){while(i--){read(0,&c,1);s[c-48]++;}for(i=0;i<10;i++){while(s[i]--)printf("%d",i);}}
                                      ^
/volume/CODE.c:1:87: warning: array subscript has type ‘char’ [-Wchar-subscripts]
 char c,s[10],i=100;main(){while(i--){read(0,&c,1);s[c-48]++;}for(i=0;i<10;i++){while(s[i]--)printf("%d",i);}}
                                                                                       ^
/volume/CODE.c:1:93: warning: implicit declaration of function ‘printf’ [-Wimplicit-function-declaration]
 char c,s[10],i=100;main(){while(i--){read(0,&c,1);s[c-48]++;}for(i=0;i<10;i++){while(s[i]--)printf("%d",i);}}
                                                                                             ^
/volume/CODE.c:1:93: warning: incompatible implicit declaration of built-in function ‘printf’
/volume/CODE.c:1:93: note: include ‘<stdio.h>’ or provide a declaration of ‘printf’

strace