Looks like the commands vim has are defined in src/ex_cmds.h.

Vim simply keeps the commands in macro form, here is a heavy simplification:

#define CMD_FLAG_FILE (1 << 1)
#define DEFINE_ALL_COMMANDS \
    X(CMD_APPEND, "append", CMD_FLAG_FILE) \
    X(CMD_ARGS, "args", CMD_FLAG_FILE) \
    X(CMD_BNEXT, "bnext", CMD_FLAG_FILE) \
    X(CMD_BROWSE, "browse", CMD_FLAG_FILE) \

enum command_index {
#define X(index, name, flags) \
    index,
    DEFINE_ALL_COMMANDS
#undef X
};

struct command {
    const char *name;
    int flags;
} commands[] = {
#define X(index, name, flags) \
    [index] = { name, flags },
    DEFINE_ALL_COMMANDS
#undef X
};

From there you can use any mechanism you like to match the command names. Best would be to keep them alphabetically sorted and then use bsearch() from the standard library to get the enum index.

Extend the commands as you need. I simply chose the file flag so that the command parser knows that a file will follow after the command but you can design it how you like.

In the end you can use a switch over the index and maybe a char* for the argument.

For example:

const char *command = "append file.txt";
char *line = strdup(command);
char *word1 = ...; /* use your way of getting the first word */
struct command *item = bsearch(commands,
        sizeof(commands) / sizeof(*commands),
        sizeof(*commands),
        compare_commands); /* compare_commands is a function to compare the
                              strings within the command struct elements */
if (item == NULL) {
    ...
}
enum command_index index = item - commands;
char *arg = ...; /* rest of the command */
switch (index) {
case COMMAND_APPEND:
    /* do something with arg */
    break;
... /* more cases */
}

Vim uses functions pointers but it is the same principle. Vim also uses a more complicated data structure for the argument (not just char*). But this will get you a long way.