In this post, I will explain how I recreated a shell, called crash. This shell incorporates all
commands in a normal UNIX system as well as additional commands which will be described later. To
start though, we first have to create the string parsing system.
Parsing and Tokenization
when parsing, we eliminate whitespace and focus on combining characters into tokens that can be interpereted by the kernel.
c
void parse_and_eval(char *s) {
assert(s);
const char *toks[MAXLINE+1];
while (*s != '\0') {
bool end = false;
bool bg = false;
int t = 0;
while (*s != '\0' && !end) {
while (*s == '\n' || *s == '\t' || *s == ' ') ++s;
if (*s != ';' && *s != '&' && *s != '\0') toks[t++] = s;
while (strchr("&;\n\t ", *s) == NULL) ++s;
switch (*s) {
case '&':
bg = true;
end = true;
break;
case ';':
end = true;
break;
}
if (*s) *s++ = '\0';
}
toks[t] = NULL;
eval(toks, bg);
}
}