In C programming:

I'm working on a shell and I have a function to parse and a function to expand variables. I need help with the execute_command function near the bottom of the main. Thank you!

Here's what I have so far:

#include

#include

#include

#include

#include

#include

#define MAX_ARGS 512

#define MAX_INPUT 2048

// Function to expand environment variables in a given string

void expand_vars(char* input) {

char* var_start;

char* var_end;

char* var_value;

char var_name[MAX_INPUT];

char output[MAX_INPUT];

char* out_pos = output;

while ((var_start = strchr(input, '$')) != NULL) {

if (*(var_start + 1) == '$') {

sprintf(out_pos, "%d", getpid());

out_pos += strlen(out_pos);

input = var_start + 2;

continue;

}

if (*(var_start + 1) == '?') {

sprintf(out_pos, "%d", WEXITSTATUS(EXIT_SUCCESS));

out_pos += strlen(out_pos);

input = var_start + 2;

continue;

}

if (*(var_start + 1) == '!') {

sprintf(out_pos, "%d", getpid());

out_pos += strlen(out_pos);

input = var_start + 2;

continue;

}

var_end = strpbrk(var_start + 1, " \t \"\'$\\");

if (var_end == NULL) {

var_end = var_start + strlen(var_start);

}

strncpy(var_name, var_start + 1, var_end - var_start - 1);

var_name[var_end - var_start - 1] = '\0';

var_value = getenv(var_name);

if (var_value == NULL) {

var_value = "";

}

*out_pos++ = '"';

strcpy(out_pos, var_value);

out_pos += strlen(var_value);

*out_pos++ = '"';

input = var_end;

}

strcpy(out_pos, input);

strcpy(input, output);

}

// Function to parse a command line into tokens

int parse_command(char* input, char** args, char** input_file, char** output_file, int* background) {

int i = 0;

char* token;

char* saveptr;

// Check for input redirection

if ((token = strstr(input, "<")) != NULL && (token == input || *(token-1) == ' ' || *(token-1) == '\t')) {

*token = '\0';

token++;

*input_file = strtok_r(token, " \t ", &saveptr);

expand_vars(*input_file);

}

// Check for output redirection

if ((token = strstr(input, ">")) != NULL && (token == input || *(token-1) == ' ' || *(token-1) == '\t')) {

*token = '\0';

token++;

*output_file = strtok_r(token, " \t ", &saveptr);

expand_vars(*output_file);

}

// Check for background execution

if ((token = strstr(input, "&")) != NULL && (token == input + strlen(input) - 1)) {

*background = 1;

*token = '\0';

}

// Parse the command and its arguments

while ((token = strtok_r(input, " \t ", &saveptr)) != NULL) {

input = NULL;

args[i] = (char*)malloc(strlen(token) + 1);

strcpy(args[i], token);

// Expand any environment variables in each argument

expand_vars(args[i]);

i++;

// Execute the command

execvp(args[0], args);

perror("execvp");

exit(EXIT_FAILURE);

} else {

// Parent process

// Wait for the child to complete if not running in the background

if (!background) {

waitpid(pid, &status, 0);

if (WIFEXITED(status)) {

exit_code = WEXITSTATUS(status);

}

}

}

return exit_code;

}

int main(int argc, char** argv) {

char* args[MAX_ARGS];

char input[MAX_INPUT];

char* input_file;

char* output_file;

int background;

int exit_code = 0;

while (1) {

input_file = NULL;

output_file = NULL;

background = 0;

printf("shell> ");

fflush(stdout);

// Read the command line from input

if (fgets(input, MAX_INPUT, stdin) == NULL) {

break;

}

// Parse the command line into tokens

if (parse_command(input, args, &input_file, &output_file, &background) < 0) {

continue;

}

// THIS FUNCTION

exit_code = execute_command(args, input_file, output_file, background);

// Free memory allocated for args

for (int i = 0; args[i] != NULL; i++) {

free(args[i]);

}

}

return exit_code;

}