Read/write stdin/out for bash interpreter linux, fork - execl - c

Question

I've been trying to write a program that will send and receive commands to a bash shell (/bin/sh). Like a wrapper program around a bash shell. So, I could write to stdin "cd ~/Desktop", then write again "ls" and I will receive a listing of the files on the desktop. I can't get it working though. On the second write command in this code, it will echo back whatever I wrote to stdin. I've also tried using popen() but that only provides output, not allowing me to write to stdin. Could someone please help solve this problem? Thanks
void main()
{
// Create a pipe and fork
//
int fd[2];
int p = pipe(fd);
pid_t pid = fork();
if (pid > 0)
{
// Read from the pipe and output the result
//
//close(fd[1]);
char buf[1024] = { 0 };
read(fd[0], buf, sizeof(buf));
printf("1 - %s\n", buf);
write (fd[1], "ifconfig", strlen ("ifconfig") );
// problem is here, read is returning echo'd bytes from write()
read(fd[0], buf, sizeof(buf));
printf("2 - %s\n", buf);
// Wait for child to terminate
int status;
wait(&status);
}
else if (pid == 0)
{
// Redirect stdout and stderr to the pipe and execute the shell
// command
//
dup2(fd[0], STDIN_FILENO);
dup2(fd[1], STDOUT_FILENO);
dup2(fd[1], STDERR_FILENO);
//close(fd[0]);
execl("/bin/sh", "exec sh", "-c", "ls", (char*) NULL );
}
}
EDIT - Updated code per 1st answer, now there is no output from the 2nd read() call
void main()
{
// Create a pipe and fork
//
int fd[2];
int ChildToParent[2], ParentToChild[2];
pipe (ParentToChild);
pipe (ChildToParent);
pid_t pid = fork();
if (pid > 0)
{
// In parent process
// Read the output of the child from child_to_parent[0]
// We don't need child_to_parent[1] so close it
close(ChildToParent[1]);
// Write output to the child using parent_to_child[1]
// We don't need parent_to_child[0] so close it
close(ParentToChild[0]);
// Read from and write to the child process...
char buf[1024] = { 0 };
read(ChildToParent[0], buf, sizeof(buf));
printf("1 - %s\n", buf);
write(ParentToChild[1], "whoami", strlen ("whoami") );
memset (buf, 0, 1024);
// this call to read returns nothing
read(ChildToParent[0], buf, sizeof(buf));
printf("2 - %s\n", buf);
}
else if (pid == 0)
{
// Redirect stdout and stderr to the pipe and execute the shell
// command
//
// child_to_parent[1] is were we write output, it's the
// new standard output, child_to_parent[0] can be closed
dup2 (ChildToParent[1], STDOUT_FILENO);
close(ChildToParent[0]);
// parent_to_child[0] is where we read input from, it's the
// new standard input, parent_to_child[1] can be closed
dup2 (ParentToChild[0], STDIN_FILENO);
close(ParentToChild[1]);
//close(fd[0]);
execl("/bin/sh", "exec sh", "-c", "ls", (char*) NULL );
}
}

Answer 1

Remember that pipes are a one-way communication stream. You can't use it for two-way communication between two processes. For that you need two pipes, one in each direction.
Perhaps something like this simple example:
// Pipe for the child process to write to the parent process
int child_to_parent[2];
// Pipe for the parent process to write to the child process
int parent_to_child[2];
// Create the TWO pipes
pipe(child_to_parent);
pipe(parent_to_child);
pid_t pid = fork();
if (pid > 0)
{
// In parent process
// Read the output of the child from child_to_parent[0]
// We don't need child_to_parent[1] so close it
close(child_to_parent[1]);
// Write output to the child using parent_to_child[1]
// We don't need parent_to_child[0] so close it
close(parent_to_child[0]);
// Read from and write to the child process...
}
else if (pid == 0)
{
// In child process
// child_to_parent[1] is were we write output, it's the
// new standard output, child_to_parent[0] can be closed
dup2(child_to_parent[1], STDOUT_FILENO);
close(child_to_parent[0]);
// parent_to_child[0] is where we read input from, it's the
// new standard input, parent_to_child[1] can be closed
dup2(parent_to_child[0], STDIN_FILENO);
close(parent_to_child[1]);
// Do whatever the child is supposed to do
}