The command cc x.c && a.out

cc x . c ; a . out - means execute the command cc x . c and then a . out. If x . c fails to compile successfully, then if there is any executable file a . out, it will he executed. So, execution of a . out, has nothing to do with the outcome of cc x . c. In the case of cc x.c && a . out , a . out will be executed only if x . c compiles successfully (i.e. re-turns 0 as the exit status).