Implementing a command interpreter in C

Question

I must write a command interpreter in C. It must:

handle command options and parameters
support commands without options and parameters
allow redirecting one command to another (for example: ls -a | wc) with a max of 3 redirects

We can also assume a fixed maximum of arguments (say MAXARG).

At this moment I have written this code, but I do not know why some commands do not work (e.g. cd). Please help me.

#include <stdio.h>
#include <string.h>
#include <unistd.h>

void  parse(char *line, char **argv)
{
 while (*line != '\0') {       
      while (*line == ' ' || *line == '\t' || *line == '\n' || *line=='|')
           *line++ = '\0';     
      *argv++ = line;         
      while (*line != '\0' && *line != ' ' && 
             *line != '\t' && *line != '\n') 
           line++;            
 }
 *argv = '\0';               
 }


 int main(int argc,char *argv[])
  {
  int child_pid;
  char * arg[10];       
   int child_status;
  char str[10];
  do
  {
  printf(">> ");
  gets(str);
  parse(str,arg);
  if(!strcmp(arg[0],"end"))
  {
    break;
   }
  child_pid = fork();
  if(child_pid == 0)
  {
    execvp(*arg,arg);
    printf("Unknown command\n");


   }
   else 
  {
    wait(&child_status);
    printf("koniec\n");
  }
 }
  while(1==1);
    return 0;
 }

I must write Command Interpreter in C I thought Pinochet was dead — Ed Heal
– Ed Heal, Commented Dec 14, 2013 at 20:40

user2485710 · Accepted Answer · 2013-12-14 20:42:34Z

2

you need a parser/lexer to inspect your input, such tools are already available, a few names: YACC, Bison, Flex .

Regarding your <command> | <command> situation that is a pipe and is a functionality provided by the linux kernel space and if you want that you need to code using the linux kernel api for the pipes.

answered Dec 14, 2013 at 20:42

user2485710

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2 Comments

Damian Over a year ago

I am absolutly green in Linux and C. Can you give any example?

user2485710 Over a year ago

@user2900036 you just have to search, those are among the most popular topics when programming under linux/unix, you will find anything with any search engine, even on SO stackoverflow.com/questions/5823838/… !

unxnut · Accepted Answer · 2013-12-14 20:44:01Z

2

For cd, you should not do a fork/exec because then, you will only change the working directory of the child which will not be the same as changing the working directory of the parent. For a command like cd, you must use chdir(2) after parsing it without doing a fork.

answered Dec 14, 2013 at 20:44

unxnut

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2 Comments

Damian Over a year ago

Where the program will know when to use and when not to fork execvp

unxnut Over a year ago

That is for you to decide when you analyze the problem. It may help to study how the current shells are implemented. The shells have a lot of builtin commands and as the programmer, you will have to decide which ones to do without going through fork/exec. For example, the arithmetic functions, control functions (if/then/for/while/case) are not to be implemented using the fork/exec mechanism. Any user command for which you have binary executable will typically be implemented with fork/exec.

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