What is this shell doing? - c

Question

I was presented with this code as an example of how to make a 'mini shell' but I have no idea what it is doing-- I understand most things in the main function, but the functions above it initializing I'm really quite lost on.
Any basics on how a shell works and some of the lower level functions in C would be much appreciated.
log_t Log;
void printLog(log_t *l){
log_entry_t *tmp = l->tail;
while(tmp != NULL){
printf("%s\n", tmp->value);
tmp = tmp->prev;
}
}
int countArg(char *line){
unsigned int i, count = 0;
for(i = 0; i < strlen(line); i++){
if(line[i] == ' ')
count++;
}
return count + 1;
}
char **makeargv(char *line){
char *buff;
char **retv;
unsigned int i, count, arg_num = 0;
buff = malloc((strlen(line) + 1) * sizeof(char));
strcpy(buff, line);
arg_num = countArg(buff);
retv = malloc((arg_num + 1) * sizeof(char*));
retv[arg_num] = NULL;
retv[0] = buff;
for(i = 0, count = 1; buff[i]!='\0'; i++){
if(buff[i] == ' '){
buff[i]='\0';
retv[count] = buff + i + 1;
count++;
}
}
return retv;
}
int main()
{
log_init(&Log);
char * line;
while(1){
size_t size;
char * cwd = NULL;
char * query = NULL;
char * match = NULL;
int f_exit = 0,
f_nbi=0,
f_match=0,
f_sys = 0,
f_path=0;
// print out the requested prompt
cwd = getcwd(NULL,0);
printf("$(pid=%d)%s$ ", getpid(), cwd );
free(cwd);
line = NULL;
fflush(stdout);
getline(&line, &size, stdin);
line[strlen(line)-1] = '\0';
while(1){
if(strcmp(line, "exit") == 0){
if(f_match) printf("%s matches %s\n", query, line);
f_exit = 1;
printf("Command executed by pid=%d\n",getpid());
log_destroy(&Log);
break;
}
else if(strncmp(line, "cd ", 3) == 0){
if(f_match) printf("%s matches %s\n", query, line);
printf("Command executed by pid=%d\n",getpid());
log_push(&Log, line);
if(chdir(line + 3) != 0){
printf("%s: No such file or directory\n",(line + 3));
}
break;
}
else if(strcmp(line, "!#") == 0){
if(f_match) printf("%s matches %s\n", query, line);
printf("Command executed by pid=%d\n",getpid());
printLog(&Log);
break;
}
else if(strstr(line,"!")==line){
if(f_match) printf("%s matches %s\n", query, line);
query = line+1;
match = log_search(&Log, query);
if(match==NULL){
printf("Command executed by pid=%d\n",getpid());
printf("No Match\n");
break;
}
else{
f_match = 1;
printf("Command executed by pid=%d\n",getpid());
line = malloc((strlen(match)+1)*sizeof(char));
strcpy(line, match);
continue;
}
}
else{
f_nbi = 1;
log_push(&Log,line);
if(strstr(line,"/"))
f_path = 1;
break;
}
}
if(f_exit)
break;
if(f_nbi){
pid_t pid = fork();
if(pid == 0){ //child
if(f_match){
printf("%s matches %s\n", query, line);
free(query-1);
}
printf("Command executed by pid=%d\n",getpid());
char **argv = makeargv(line);
f_sys = 0;
if(f_path)
f_sys = execv(argv[0],&argv[0]);
else
f_sys = execvp(argv[0],&argv[0]);
if(f_sys == -1)
printf("%s: not found\n",line);
free(argv[0]);
free(argv);
log_destroy(&Log);
exit(0);
}
else { //parent
waitpid(pid, NULL, WUNTRACED);
if(f_match)
free(query-1);
}
}
free(line);
line = NULL;
}
if(line != NULL){
free(line);
line = NULL;
}
return 0;
}

Answer 1

The printLog() function is simply printing all the data from a linked list of records in a structure type given the name log_t by a typedef. It starts at the tail of the list and works backwards. It is not clear whether the list is single-linked or double-linked, but the prev usually only appears in a double-linked list.
The countArgs() function is a crude way of determining how many arguments are present on a command line. It is crude because it does not take into account quotes or multiple adjacent spaces. However, those merely mean it overestimates the number of arguments, which is not serious. It doesn't recognize tabs as separators, which is unusual for a shell.
The makeargv() function splits a command line into a series of space separated words and returns the list of words to the calling function. It effectively parses the command line, making sure there's a null pointer at the end of the command line. Again, it is simplistic, but sufficient. It is also inconsistently named compared with the other two functions.