Issues with file redirection when building a simple shell

Question

I'm trying to implement file redirection in my simple shell program. The issue is that when I enter a prompt when I run my shell program to test it (e.g ./test1 > test2 ) instead of executing the command and going onto the next prompt, it sort of lulls and keeps taking in input to redirect to my test2 file. Can't figure out what to do.

for(int i = 1; i < arg_no; i++){
        if (strcmp(arg[i], ">") == 0){
                fd = open(arg[i+1], O_WRONLY | O_CREAT,  S_IRUSR|S_IWUSR|S_IRGRP);
                if (fd < 0) {
                    perror("open");
                    exit(1);
                }
                close(STDOUT_FILENO);
                if (dup(fd) < 0) {
                    perror("dup");
                    exit(1);
                }
                close(fd);
                arg[i] = NULL;
               break;
        }
        else if(strcmp(arg[i], ">&") == 0){
                fd = open(arg[i+1], O_WRONLY | O_CREAT, 0644);
                dup2(fd, STDOUT_FILENO);
                dup2(fd, STDERR_FILENO);
                close(fd);
                arg[i] = NULL;
        }
        else if(strcmp(arg[i], ">>") == 0){
                fd = open(arg[i+1], O_WRONLY | O_CREAT | O_APPEND, 0644);
                dup2(fd, STDOUT_FILENO);
                close(fd);
                arg[i] = NULL;
        }
        else if(strcmp(arg[i] , ">>&") == 0){
                fd = open(arg[i+1], O_WRONLY | O_CREAT | O_APPEND, 0644);
                dup2(fd, STDOUT_FILENO);
                dup2(fd, STDERR_FILENO);
                close(fd);
                arg[i] = NULL;
        }
        else if(strcmp(arg[i], "<") == 0){
                fd = open(arg[i+1], O_RDONLY);
                dup2(fd, STDIN_FILENO);
                close(fd);
                arg[i] = NULL;
        }
    }

My printf statements after dup(fd) don't have any effect, so I'm assuming the program isn't able to close(fd) which is why this is occurring. I'm just not sure why this is or how to force it to close.

Nevermind, my redirection was supposed to take place in my child process, I had it in my parent process. — iknn, Apr 08 '23 at 23:40
You should probably delete the question. It was a bit unclear. — pmacfarlane, Apr 08 '23 at 23:41
@iknn if you have solved the problem, you can add an answer on your own. If you think the question is answered elsewhere, you can close it as a duplicate. — Ajay Brahmakshatriya, Apr 09 '23 at 00:52
Voting to close as "nonreproduceable/typo" since it was essentially in part of the code that is not present here. — Antti Haapala -- Слава Україні, Apr 09 '23 at 06:25

pmacfarlane · Answer 1 · 2023-04-08T23:23:18.873

1

If you type this into your Linux shell:

./test1 > test2

Then test1 gets executed with argc == 1 and no additional arguments. In particular, the > test2 is not passed to your program. The actual shell you are typing at removes these and does the file redirection for you.

If your program needs to interpret strings containing < and > you'll need to enclose them in parentheses, e.g.

./test1 "> test2"

You'll have to deal with the fact that this parameter is a single string, rather than a series of argv[] pointers.

edited Apr 08 '23 at 23:23

answered Apr 08 '23 at 23:11

pmacfarlane

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I don't think OP is dealing with command line arguments. `arg` seems to be a sequence of strings they read from the std input – Ajay Brahmakshatriya Apr 09 '23 at 00:50
@AjayBrahmakshatriya In hindsight, I think you are right. I think the question should probably be closed or answered by OP, but I'll leave my answer there until that happens. – pmacfarlane Apr 09 '23 at 07:34

Issues with file redirection when building a simple shell

1 Answers1