Creating Shell in C - Piping and External Commands - c

Question

I am trying to create a small shell using C. At the moment I am trying to figure out piping and external commands. I got stuck in them both even after looking at various youtube videos.
I referred to MAN and even Advanced Linux Programming
What can I change to improve and make the implementation work?
This a part of the checking of commands, args := tokenisation by whitespace, commLHS := will be used to store args before | and commRHS will be used to store args after | and indexT refers to the number of arguments inputted
else if((check4pipe(args, commLHS, commRHS, indexT) != 0))
{
return runPipeComm(commLHS, commRHS);
//fprintf(stderr, "%s: command not found\n", args[0]);
}
This will execute External Commands
void externalCommands(char **args)
{
// fork-plus-exec pattern
// https://www.percona.com/community-blog/2021/01/04/fork-exec-wait-and-exit/
/*
First we Fork
Then we Exec
Then we Wait
Then we Exit
*/
int status;
pid_t pip = fork();
if (pip == -1)
{
perror("Error - fork()");
}
else if (pip == 0)
{
//If PID is the child process
//Launches the process.
if (execvp(args[0], args) < 0)
{
perror("Error - execvp()");
}
}
else
{ //If PID is the parent process.
//Waits for the child process and returns exit code if waitpid() is successful.
if(waitpid(pip, &status, WUNTRACED) == -1)
{
perror("Error occured during waitpi");
}
else
{
//set_exitcode(status); //Sets the exitcode environment variable.
}
}
}
This is to check for | in args inputted by user after tokenisation.
int check4pipe(char **args, char **pipeLHS, char **pipeRHS, int indexT)
{
bool foundPipe = false;
for(int i = 0; i < indexT; i++)
{
if(strcmp(args[i], "|") == 0)
{
foundPipe = true;
memcpy(pipeLHS, args, (i+1) * sizeof(char*));
pipeLHS[i] = NULL;
memcpy(pipeLHS, args+(i+1), ((indexT-i)+1) * sizeof(char*));
pipeRHS[i]= NULL;
}
else
{
continue;
}
}
if(foundPipe == true)
{
return 1;
}
else
{
return 0;
}
}
This will run the pipe commands
int runPipeComm(char **commLHS, char **commRHS)
{
int userPipe[2];
pid_t pip1; // Pipe ID 1
pid_t pip2; // Pipe ID 2
if(pipe(userPipe) < 0)
{
perror("Error Occurred while piping: ");
}
// Start Process
pip1 = fork();
if(pip1 == -1)
{
perror("Error Occurred while forking: ");
}
else if(pip1 == 0)
{
dup2(userPipe[1], STDOUT_FILENO);
close(userPipe[1]);
//run
exit(0);
}
else
{
pip2 = fork();
if(pip2 == -1)
{
perror("Error Occurred while forking: ");
}
else if(pip2 == 0)
{
dup2(userPipe[0], STDOUT_FILENO);
close(userPipe[1]);
//run
exit(0);
}
else
{
close(userPipe[0]);
close(userPipe[1]);
wait(NULL);
wait(NULL);
}
}
return 1;
}

Answer 1

You forgot to change some things when copying snippets within your program.
wrong:
memcpy(pipeLHS, args+(i+1), ((indexT-i)+1) * sizeof(char*));
pipeRHS[i]= NULL;
right:
memcpy(pipeRHS, args+(i+1), (indexT-(i+1)) * sizeof (char*));
pipeRHS[indexT-(i+1)] = NULL;
wrong:
dup2(userPipe[0], STDOUT_FILENO);
close(userPipe[1]);
right:
dup2(userPipe[0], STDIN_FILENO);
close(userPipe[0]);
After the first //run line, add the missing
execvp(*commLHS, commLHS);
perror("Error - execvp()");
After the second //run line, add the missing
execvp(*commRHS, commRHS);
perror("Error - execvp()");
Finally, the write end of the pipe must be closed in the parent process, so move the close() there:
close(userPipe[1]);
pip2 = fork();