Pipe implementation on C++ - c++

Question

I built a simple shell in order to execute pipe command like % ls | cat | number.When I execute normal pipe command, it works.But when I entering unknown command like% ls | las,I want to output error msg like Unknown command:[las],but the outputs of the program becomes weird.Here's my code about pipe:
void pipe_cmd(){
int index = 0;
char* cmds[command_num][100];
char* cmd[100];
while (index < command_num)
{
for(int i = 0; i < argc[index]; i++){
cmd[i] = const_cast<char*>(command[index][i].c_str());
cmds[index][i] = cmd[i];
}
cmds[index][argc[index]] = NULL;
index++;
}
int fd[2*(command_num - 1)];
for (int i = 0; i < command_num - 1; i++){
if(pipe(fd + 2 * i) == -1){
perror("Pipe failed");
exit(1);
}
}
pid_t pid;
for (int i = 0; i < command_num; i++)
{
pid = fork();
if(pid == 0){
if (i != 0){
dup2(fd[2 * (i - 1)], 0);
}
if(i != command_num - 1){
dup2(fd[2 * i + 1], 1);
}
for(int j = 0; j < 2*(command_num - 1);j++){
close(fd[j]);
}
if(execvp(cmds[i][0],cmds[i]) == -1){
cerr << "Unknown command: " << "[" << cmds[i][0] << "]." << endl;
}
} else{
if (i != 0){
close(fd[2 * (i - 1)]);
close(fd[2 * (i - 1) + 1]);
}
}
}
waitpid(pid, NULL, 0);
}
with my outputs look like:
% ls | las
Unknown command: [las].
% Unknown command: [README.md].
% Unknown command: [bin].
% % % Unknown command: [npshell.cpp].
% Unknown command: [test.html].

Answer 1

Before executing the command with execvp you have to check if all commands are at $PATH if it doesn't exist, which means the command entered by the user is not valid.
Here's an example for checking if a command exists or not, I have used vectors and string here as you are using c++.
#include <iostream>
#include <stdlib.h>
#include <string.h>
#include <sys/stat.h>
#include <vector>
std::vector<std::string> split_line(char *command) {
std::vector<std::string> commands;
char *token = strtok(command, " ");
while (token != NULL) {
commands.push_back(token);
token = strtok(NULL, " ");
}
return commands;
}
bool does_command_exists(std::vector<std::string> path, std::string command) {
char *c_command = const_cast<char *>(command.c_str());
std::vector<std::string> args = split_line(c_command);
// converting vector to char** (required for execvp)
char **argv = (char **)malloc(sizeof(char *) * args.size());
for (size_t i = 0; i < args.size(); i++) {
argv[i] = const_cast<char *>(args[i].c_str());
}
struct stat stats;
for (auto x : path) {
x = x.append("/");
x = x.append(argv[0]);
if (stat(x.c_str(), &stats) == 0) {
return true;
}
}
return false;
}
int main(void) {
std::vector<std::string> commands{"ls -l", "sort", "Something"};
char *env = getenv("PATH");
char *token = strtok(env, ":");
std::vector<std::string> path;
while (token != NULL) {
path.push_back(token);
token = strtok(NULL, ":");
}
for (auto command : commands) {
if (!does_command_exists(path, command)) {
std::cout << "Command: " << command << " does not exist "
<< "\n";
return -1;
}
}
// All commands are valid, now other stuff can be done
return 0;
}