building a shell - IO trouble_问答_开发者_运维开发者技术经验分享

开发者 https://www.devze.com 2023-04-10 13:14 出处：网络

I am working on a shell for a systems programming class. I have been having some trouble with the file redirection. I just got redirecting the output to work, e.x. \"ls > a\" however when I type a com

相关专题：file-io shell

I am working on a shell for a systems programming class. I have been having some trouble with the file redirection. I just got redirecting the output to work, e.x. "ls > a" however when I type a command like "cat < a" into my shell it deletes everything in the file. I feel like the problem stems from the second if statement- "fdin = open(_inputFile,777)"

If that is the case a link to a recommended tutorial / other examples would be much appreciated.

On a side note, I included the entire function, however at the point which it creates the pipe, I have not tested anything there yet. I don't believe it works properly either though, but that may be from a mistake in another file.

void Command:: execute(){



    if(_numberOfSimpleCommands == 0){

    prompt();
        return;
    }

    //save input/output
    int defaultin = dup(0);
    int defaultout = dup(1);

    //initial input
    int fdin;
    if(_inputFile){
        fdin = open(_inputFile,0777);
    }else{
        //use default input
        fdin = dup(defaultin);
    }

    //execution
    int pid;
    int fdout;
    for(int i = 0; i < _numberOfSimpleCommands; i++){
        dup2(fdin,0);
        close(fdin);

        //setoutput
        if(i == _numberOfSimpleCommands -1){
            if(_outFile){
                fdout = creat(_outFile,0666);
            }else{
                fdout = dup(defaultout);
            }
        }else{
            int fdpipe[2];
            pipe(fdpipe);
            fdout = fdpipe[0];
            fdin = fdpipe[1];
        }
        dup2(fdout,1);
        close(fdout);

 开发者_StackOverflow社区       //create child
        pid = fork();
        if(pid == 0){
            execvp(_simpleCommands[0]->_arguments[0],_simpleCommands[0]->_arguments);
            perror("-myshell");
            _exit(1);
        }
    }
    //restore IO defaults
    dup2(defaultin,0);
    dup2(defaultout,1);
    close(defaultin);
    close(defaultout);

    if(!_background){
        waitpid(pid,0,0);
    }
}

Your call open(_inputFile, 0777) is incorrect. The second argument to open is supposed to contain a bitwise or'd combination of values that specify access mode and file creation flags, among other things (O_RDONLY, O_WRONLY, etc). Since you're passing 0777, that probably ends up containing both O_CREAT and O_TRUNC, which causes _inputFile to be erased. You probably want open(_inputFile, O_RDONLY).