How to write a simple shell program

Today I want to talk about how to write a simple shell program.

First of all you should know that the shell program is an interact program in most mainstream Linux/Unix system. It performs a sequence of read/evaluate steps. The read step reads a command line from the user. The evaluate step parses the command and runs the program on behalf of the user. And I will show you a simple implement of the shell program, which called shell_simple.

The code is as follows, Here we omit the error check to simplify the code:

//shell_simple.cvoid eval(const char *command);bool parseline(char *buf, char *argv[]);int main() {    char *command;    while(1) {        printf("> ");        fgets(command, MAXLINE, stdin))//never use gets        if(feof(stdin))            exit(0);        eval(command);    }    return 0;}void eval(const char *command) {    char buf[MAXLINE];    char *argv[MAXARGS];    strcpy(buf,command);    int bg; //if bg != 0, run in the background    bg = parseline(buf, argv);    if(strcmp(argv[0],"exit"))        exit(0);    pid_t pid;    if((pid = fork（）) == 0) {   // child process        if(0 > exectv(argv[0], argv, environ)) {            fprintf(stderr, "%s : Command not exist\n", argv[0]);            return;        }    }     if(!bg) {  // parent wait for abort of child         int status;        waitpid(pid, &status, 0);    } else {        printf("[%d] %s\n",pid, command);    }}bool parseline(char *buf, char *argv[]) {    char *space_pointer;    buf[strlen(buf) - 1] = ' '; // replace the tailing '\n' with space    while(*buf == ' ') // ignore leading spaces        ++ buf;    int argc = 0;    while(NULL != (space_pointer = strchr(buf, ' '))) { // find the pos of the first space        argv[argc++] = buf;        space_pointer = '\0';        buf = space_pointer + 1;        while(*buf == ' ')            ++ buf;    }    argv[argc] = NULL;    if（argc == 0)        return 1;    int bg;    if((bg = (argv[argc - 1] == '&')) != 0) { // should run in background        argv[--argc] = NULL;    }    return bg;}

This is the simple implement of shell program , it has some drawbacks e.g. it must use the full path to execute a command and it does not reap the child process. I will improve it in the future.

Welcome to communicate with me on this~