Implementing Pipes in a C shell (Unix)

You should be able to implement pipes and output redirection with your shell, but there are a few things I noticed:

• Your code for reading input, parsing, and output are mixed together, you may want to separate this functionality.
• strtok won't work very well as a parser for shell commands. It will work for very simple commands, but you may want to look into creating or finding a better parser. A command like echo "hello world" will be problematic with your current parsing method.
• You may want to create a simple structure for holding your parsed commands.

Here is some pseudocode to get you started:

#define MAX_LINE 10000#define MAX_COMMANDS 100#define MAX_ARGS 100// Struct to contain parsed inputstruct command{    // Change these with IO redirection    FILE *input; // Should default to STDIN    FILE *output; // Should default to STDOUT    int num_commands;    int num_args[MAX_COMMANDS]; // Number of args for each command    char* command_list[MAX_COMMANDS]; // Contains the programs to be run    char* args_list[MAX_COMMANDS][MAX_ARGS]; // The args for each command    boolean background_task;    boolean append;}int main(){    char input[MAX_LINE];    while (1)    {        struct command cmd;        print_prompt();        read_input(input);        parse_input(input, &cmd);        execute(&cmd);    }}

Good luck with this project!

Pipes and redirections are different, actually. To implement a redirection (such as >>) you have to use dup2 indeed. First, open the desired file with appropriate flags (for >> they'll be O_WRONLY|O_CREAT|O_APPEND). Second, using dup2, make stdout (file descriptor 1) a copy of this newly opened fd. Finally, close newly opened fd.

To create a pipe, you'll need a pipe syscall. Read its manpage, it contains example code. Then you'll also need dup2 to make file descriptors returned by pipe be stdin for one process and stdout for another, respectively.