Question

I am working on a custom shell for a systems programming class. We were instructed to implement the built-in setenv() and unsetenv() commands with a hint of check man pages for putenv().

My issue is that setenv(char*, char*, int) and putenv(char*) do not seem to be working at all. My code for executing a command entered is as follows:

//... skipping past stuff for IO redirection
pid = fork();
if(pid == 0){
    //child
    if(!strcmp(_simpleCommands[0]->_arguments[0],"printenv")){
        //check if command is "printenv"
        extern char **environ;
        int i;
        for(i = 0; environ[i] != NULL; i++){
            printf("%s\n",environ[i]);
        }
        exit(0);
    }
    if(!strcmp(_simpleCommands[0]->_arguments[0],"setenv")){
        //if command is "setenv" get parameters char* A, char* B
        char * p = _simpleCommands[0]->_arguments[1];
        char * s = _simpleCommands[0]->_arguments[2];

        //putenv(char* s) needs to be formatted A=B; A is variable B is value
        char param[strlen(p) + strlen(s) + 1];
        strcat(param,p);
        strcat(param,"=");
        strcat(param,s);
        putenv(param);
        //setenv(p,s,1);
        exit(0);
    }
    if(!strcmp(_simpleCommands[0]->_arguments[0],"unsetenv")){
        //remove environment variable
        unsetenv(_simpleCommands[0]->_arguments[0]);
        exit(0);
    }
    //execute command
    execvp(_simpleCommands[0]->_arguments[0],_simpleCommands->_arguments);
    perror("-myshell");
    _exit(1);
}

//omitting restore IO defaults...

If I run printenv it works properly, but if I try to set a new variable using either putenv() or setenv() my printenv() command returns the exact same thing, so it does not appear to be working.

As a side note, the problem may not be with the functions or how I called them, because my shell is executing the commands as though it had to format a wildcard (* or ?) which I am not sure should happen.

Answer 1

You appear to be calling fork unconditionally before examining the command line. But some shell built-in commands need to run in the parent process, so that their effect persists. All the built-ins that manipulate the environment fall in this category.

As an aside, I wouldn’t try to use the C library’s environment manipulation functions if I were writing a shell. I’d use three-argument main, copy envp into a data structure under my full control, and then feed that back into execve. This is partially because I’m a control freak, and partially because it’s nigh-impossible to do anything complicated with setenv and/or putenv and not have a memory leak. See this older SO question for gory details.