Multi Piping bash-style in C - c

Question

I know there are many threads that talk about this problem but I don't really understand the way it can be done.
I'm trying to make a shell that can execute a linux command sucha as ps | grep | less
I've donne the parsing by puting every command and its args in a simply linked list.
Here's my implementation that doesn't work. Hope that's clear enough.
if ((son = fork()) < 0)
return printerr_sys("Unable to fork", 0);
if (son == 0)
{
if (first > 1 && data->format[first - 1] &&
is_directing_elt(data->format[first - 1]) == DIRECT_TPIPE)
dup2(tube_p[0], STDIN_FILENO);
first = make_argv(data, first, &argv);
if (next)
{
dup2(tube_v[1], STDOUT_FILENO);
close(tube_v[0]);
}
if (execvp(argv[0], argv) < 0)
return printerr_cmd(argv[0], 1);
}
else
{
if (next)
{
close(tube_v[1]);
cmdline_executer(data, next, tube_v);
}
waitpid(son, &(data->lastcmd), WUNTRACED);
data->lastcmd = WEXITSTATUS(data->lastcmd);
}
return TRUE;
My questions are:
What would be the correct implementation?
Is it possible to do it with recursion?
Do I need to fork from right to left or left to right (logically it give the same result)?

Answer 1

Here's a part of a UNIX Shell I had to implement in C for Operating System subject in my Computer Science career.
/* Executes the command 'buffer' assuming that doesn't contain redirections */
void execute_only_pipes(char* buffer)
{
char *temp = NULL, *pipeCommands[MAX_PIPES], *cmdArgs[MAX_ARGUMENTS];
int newPipe[2], oldPipe[2], pipesCount, aCount, i, status;
pid_t pid;
pipesCount = -1; /* This variable will contain how many pipes the command contains */
/* Counting the number of pipes and splitting them into pipeCommands */
do
{
temp = strsep(&buffer, "|");
if(temp != NULL)
{
if(strlen(temp) > 0)
{
pipeCommands[++pipesCount] = temp;
}
}
} while(temp);
cmdArgs[++pipesCount] = NULL;
for(i = 0; i < pipesCount; i++) /* For each command */
{
aCount = -1;
/* Parsing command & arguments */
do
{
temp = strsep(&pipeCommands[i], " ");
if(temp != NULL)
{
if(strlen(temp) > 0)
{
/* If a parameter is ~, then replace it by /home/user */
if (!strcmp(temp, "~"))
strcpy(temp, home);
cmdArgs[++aCount] = temp;
}
}
} while(temp);
cmdArgs[++aCount] = NULL;
/* If there still are commands to be executed */
if(i < pipesCount-1)
{
pipe(newPipe); /* just create a pipe */
}
pid = fork();
if(pid == 0) /* Child */
{
/* If there is a previous command */
if(i > 0)
{
close(oldPipe[1]);
dup2(oldPipe[0], 0);
close(oldPipe[0]);
}
/* If there still are commands to be executed */
if(i < pipesCount-1)
{
close(newPipe[0]);
dup2(newPipe[1], 1);
close(newPipe[1]);
}
/* Execute it */
int res = execvp(cmdArgs[0], cmdArgs);
if (res == -1)
{
printf("Error. Command not found: %s\n", cmdArgs[0]);
}
exit(1);
}
else /* Father */
{
/* If there is a previous command */
if(i > 0)
{
close(oldPipe[0]);
close(oldPipe[1]);
}
/* do we have a next command? */
if(i < pipesCount-1)
{
oldPipe[0] = newPipe[0];
oldPipe[1] = newPipe[1];
}
/* wait for last command process? */
if(i == pipesCount-1)
{
waitpid(pid, &status, 0);
}
}
}
}
It might be a little buggy (I'm not checking if fork() < 0, etc) but the main idea is correct.
> Is it possible to do it with recursion?
Most of the time I try to avoid recursion, if I can write a similar understandable code without using it.

Answer 2

Processes run independently, so you need to set up the pipe for at least the first pair of commands before you fork, but you're doing that in the child (son == 0). You could code a recursive solution that, as long as there are at least two commands left, creates a pipe, then forks, then runs the first command.