Ignoring spaces in a string unless it's in quotes - c

Question

char *args[32];
char **next = args;
char *temp = NULL;
char *quotes = NULL;
temp = strtok(line, " \n&");
while (temp != NULL) {
if (strncmp(temp, "\"", 1) == 0) {
//int i = strlen(temp);
printf("first if");
quotes = strtok(temp, "\"");
} else if (strncmp(temp, "\"", 1) != 0) {
*next++ = temp;
temp = strtok(NULL, " \n&");
}
}
I'm having trouble with trying to understand with how to still keep spaces if a part of the string is surrounded with quotes. For example, if I want execvp() to execute this: diff "space name.txt" sample.txt, it should save diff at args[0], space name.txt at args[1] and sample.txt at args[2].
I'm not really sure on how to implement this, I've tried a few different ways of logic with if statements, but I'm not quite there. At the moment I am trying to do something simple like: ls "folder", however, it gets stuck in the while loop of printing out my printf() statement.
I know this isn't worded as a question - it's more explaining what I'm trying to achieve and where I'm up to so far, but I'm having trouble and would really appreciate some hints of how the logic should be.

Answer 1

Instead of using strtok process the string char by char. If you see a ", set a flag. If flag is already set - unset it instead. If you see a space - check the flag and either switch to next arg, or add space to current. Any other char - add to current. Zero byte - done processing.
With some extra effort you'll be able to handle even stuff like diff "file \"one\"" file\ two (you should get diff, file "one" and file two as results)

Answer 2

I'm confused even to understand what you try to do. Are you trying to tokenize the input string into space separated tokens?
Just separate the input string on spaces and when you encounter a double quote char you need a second inner loop which handles quoted strings.
There is more to quoted strings than to search for the closing quote. You need to handle backslashes, for example backslashed escaped quotes and also backslash escaped backslashes.
Just consider the following:
diff "space name \" with quotes.txt\\" foo
Which refers to a (trashy) filename space name " with quotes.txt\. Use this as a test case, then you know when you are done with the basics. Note that shell command line splitting is a lot more crazy than that.

Answer 3

Here is my idea:
Make two pointers A and B, initially pointing at first char of the string.
Iterate through the string with pointer A, copying every char into an array as long as it's not a space.
Once you have reached a ", take the pointer B starting from the position A+1 and go forward until you reach the next ", copying everything including space.
Now repeat from number 2, starting from the char B+1.
Repeat as long as you haven't reached \0.
Note: You'll have to consider what to do if there are nested quotes though.
You can also use a flag (int 1 || 0) and a pointer to denote if you're in a quote or not, following 2 separate rules based on the flag.

Answer 4

Write three functions. All of these should return the number of bytes they process. Firstly, the one that handles quoted arguments.
size_t handle_quoted_argument(char *str, char **destination) {
assert(*str == '\"');
/* discard the opening quote */
*destination = str + 1;
/* find the closing quote (or a '\0' indicating the end of the string) */
size_t length = strcspn(str + 1, "\"") + 1;
assert(str[length] == '\"'); /* NOTE: You really should handle mismatching quotes properly, here */
/* discard the closing quote */
str[length] = '\0';
return length + 1;
}
... then a function to handle the unquoted arguments:
size_t handle_unquoted_argument(char *str, char **destination) {
size_t length = strcspn(str, " \n");
char c = str[length];
*destination = str;
str[length] = '\0';
return c == ' ' ? length + 1 : length;
}
... then a function to handle (possibly repetitive) whitespace:
size_t handle_whitespace(char *str) {
int whitespace_count;
/* This will count consecutive whitespace characters, eg. tabs, newlines, spaces... */
assert(sscanf(str, " %n", &whitespace_count) == 0);
return whitespace_count;
}
Combining these three should be simple:
size_t n = 0, argv = 0;
while (line[n] != '\0') {
n += handle_whitespace(line + n);
n += line[n] == '\"' ? handle_quoted_argument(line + n, args + argv++)
: handle_unquoted_argument(line + n, args + argv++);
}
By breaking this up into four separate algorithms, can you see how much simpler this task becomes?

Answer 5

So here is where I read in the line:
while((qtemp = fgets(line, size, stdin)) != NULL ) {
if (strcmp(line, "exit\n") == 0) {
exit(EXIT_SUCCESS);
}
spaceorquotes(qtemp);
}
Then I go to this: (I haven't added my initializers, you get the idea though)
length = strlen(qtemp);
for(i = 0; i < length; i++) {
position = strcspn(qtemp, " \"\n");
while (strncmp(qtemp, " ", 1) == 0) {
memmove(qtemp, qtemp+1, length-1);
position = strcspn(qtemp, " \"\n");
} /*this while loop is for handling multiple spaces*/
if (strncmp(qtemp, "\"", 1) == 0) { /*this is for handling quotes */
memmove(qtemp, qtemp+1, length-1);
position = strcspn(qtemp, "\"");
stemp = malloc(position*sizeof(char));
strncat(stemp, qtemp, position);
args[i] = stemp;
} else { /*otherwise handle it as a (single) space*/
stemp = malloc(position*sizeof(char));
strncat(stemp, qtemp, position);
args[i] = stemp;
}
//printf("args: %s\n", args[i]);
length = strlen(qtemp);
memmove(qtemp, qtemp+position+1, length-position);
}
args[i-1] = NULL; /*the last position seemed to be a space, so I overwrote it with a null to terminate */
if (execvp(args[0], args) == -1) {
perror("execvp");
exit(EXIT_FAILURE);
}
I found that using strcspn helped, as modifiable lvalue suggested.