/*
  Nick Urbanik <nicku@vtc.edu.hk>
  (very) simple shell.
*/

#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <libgen.h>
#include <stdlib.h>
#include <ctype.h>
#include <sys/wait.h>

void print_prompt( const char *prog )
{
        printf( "%s-> ", prog );
}

/*
  Read the command into a buffer line[].
  Then set an array of pointers, one to the start of each word.
  A word starts at the first non-whitespace character after
  either the beginning of the line, or whitespace.
  Terminate the array of pointers to arguements with a NULL
  pointer, as required for execv*().

  We also null-terminate each argument, as required.
  Handle specially the case where the command is exit.
  Exit if get end of file.
 */

#define MAX_ARGS 64
#define MAX_LINE_LEN 1024
char *args[ MAX_ARGS + 1 ];

void built_in_commands( void )
{
        if ( strcmp( args[ 0 ], "exit" ) == 0 ) exit( 0 );
}

void read_command( void )
{
        static char line[ MAX_LINE_LEN ];
        int i = 0, j = 0, inspace = 1;
        if ( fgets( line, MAX_LINE_LEN, stdin ) == NULL ) exit( 1 );
        while ( line[ i ] != '\0' && j < MAX_ARGS ) {
                if ( isspace( line[ i ] ) ) {
                        if ( ! inspace ) line[ i ] = '\0';
                        ++i;
                        inspace = 1;
                        continue;
                }
                if ( inspace ) {
                        args[ j++ ] = &line[ i ];
                        inspace = 0;
                }
                else ++i;
        }
        args[ j ] = NULL;
        built_in_commands();
}

int main( int argc, char **argv )
{
        char *prog_name = basename( *argv );
        print_prompt( prog_name );
        read_command();
        for ( ;; ) {
                int pid = fork();
                if ( pid < 0 ) {
                        perror( prog_name );
                        exit( 2 );
                }
                if ( pid == 0 ) {
                        int rv = execvp( args[ 0 ], args );
                        if ( rv == -1 ) perror( prog_name );
                        exit( 1 ); 
                }
                wait( NULL );
                print_prompt( prog_name );
                read_command();
        }
}