ปฏ บ ต การ #3. A Simple Shell Interpreter

Transcription

1 ปฏ ต การ #3 A Simple Shell Interpreter Write a shell (read section and 1.4 in text book) that is similar to Figure 1-10 but contains enough code that it actually works so you can test it. You might also add some features such as redirection of input and output, pipes (read section in text book), and background jobs. Minimum requirement: at least one internal command, any external commands, and run foreground. while (1) /* repeat forever */ read_command(command, parameters); /* read input from terminal */ if (fork()!= 0) /* fork off child process */ /* Parent code. */ waitpid(-1, &status, 0); /* wait for child to exit */ else /* Child code. */ execve(command, parameters, 0); /* execute command */ Figure 1-10.A stripped-down shell. (Hint: For internal command, insert Program2 below in your command interpreter. Pseudo code of simple shell interpreter may be look like this: read_command(command,parameters,foregound ) fscanf(stdin, %s, command_and_options); if (eof(stdin)) strcpy(command, logout ); else command =? parameters =? foreground = yes or no; /* if there is & then foreground = no */

2 main() /* a simple shell intepreter */ char *prompt= %> ; while (1) /* repeat forever */ fprintf(stderr, %s,prompt); read_command(command, parameters,foreground); switch (command) case /* logout */ : case /* exit */ : exit(0); case /* copy */ : /* insert internal command copy (Program 2) here */ break; default : /* external command */ if (fork()!= 0) /* fork off child process */ /* Parent code. */ if (foreground) waitpid(-1, &status, 0); /* use old version wait(&status) if you have problem */ else /* Child code. */ execve(command, parameters, 0); /* execute command, environment = null */ fprintf(stderr, command not found\n ); exit(0); /* end switch */ /* end while */ OR this: (Reference :Maurice J. Bach, The Design of the Unix Operating System /* read command line until end of file */ while (read(stdin,buffer,numchars)) /* parse command line */ if (/* command line contains & */)

3 amper = 1; else amper = 0; /* for commands not part of the shell command language */ if (fork() == 0) /* redirection of IO? */ if (/* redirect output */ fd = creat(newfile,fmask); close(stdout); dup(fd); close(fd); /* stdout is now redirected */ if (/* piping */ pipe(fildes); if (fork() == 0) /* first component of command line */ close(stdout); dup(fildes[1]); close(fildes[1]); close(fildes[0]); /* stdout now goes to pipe */ /* child process does command */ execlp(command1, command1,0); /* 2 nd command component of command line */ close(stdin); dup(fildes[0]); close(fildes[0]); close(fildes[1]); /* standard input now comes from pipe */

4 execve(command2, command2,0); /* parent continue over here * waits for child to exit if required */ if (amper == 0) retid = wait(&status); Here are the example programs for system call fork() and execl(). You should use manual online (man) to see system call that you want such as man execve. Program 1: create process /* filename : myfork.c */ #include <sys/types.h> #include <unistd.h> main() pid_t child_pid; child_pid = fork(); if (child_pid == (pid_t)-1) printf("can not create child process\n"); if (child_pid!= 0) /* parent process */ printf("parent: child pid = %d, parent pid = %d\n", child_pid, getpid()); else /* child process */ printf("child: child pid = %d, parent pid = %d\n, getpid(), getppid()); Program 2 : copy file /* file name: copy.c */

5 #include <fcntl.h> char buffer[2048]; int version = 1; main(argc,argv) int argc; char *argv[]; int fdold,fdnew; if (argc!= 3) printf("need 2 arguments for copy program\n"); exit(1); fdold = open(argv[1],o_rdonly); if (fdold == -1) printf("cannot open file %s\n",argv[1]); exit(1); fdnew = creat(argv[2],0666); if (fdnew == -1) printf("cannot create file %s\n",argv[2]); exit(1); copy(fdold,fdnew); exit(0); copy(old,new) int old,new; int count;

6 while ((count = read(old, buffer, sizeof(buffer))) > 0) write(new, buffer, count); Program 3 : copy file using execl system call /* file name: mycopy.c */ main(argc, argv) int argc; char *argv[]; /* assume 2 args: source file and target file */ if (fork() == 0) execl("copy","copy",argv[1],argv[2],0); wait((int *) 0); printf("copy done\n"); How to search file from environment variable and open? Copy from BOB Ray Tracing (Note! It can handle both file and file). /* ษออออออออออออออออออออออออออออออออออออออออออออออออออออออออออออออออออออออออ อป Bob Ray Tracer File.C = functions to handle opening of files taking into consideration environment variables. Copyright 1988,1992 Christopher D. Watkins and Stephen B. Coy ALL RIGHTS RESERVED. This software is published, but is NOT Public Domain and remains the propery of ALGORITHM, Inc., Christopher D. Watkins and Stephen B. Coy. This software may not be reproduced or integrated into other packages without the prior written consent of Christopher D. Watkins and Stephen B. Coy.

7 ศอออออออออออออออออออออออออออออออออออออออออออออออออออออออออออออออออออออออออผ */ #include <stdio.h> #include <stdlib.h> #include <string.h> #define BOB_ENV "BOB" /* change to PATH here */ #define DELIMS ":" #define MAX_PATHS (32) static int static char num_paths; *paths[max_paths]; char *strdup(const char *string) char *tmp; /* */ if (string==null) return NULL; if (tmp=(char *)malloc(strlen(string)+1)) strcpy(tmp,string); return tmp; fprintf(stderr,"memory overflow\n"); exit(1); qstrcat() -- just like strcat() except ignores quotes qstrcat(char *d, char *s) /* find end of dest */

8 while(*d) d++; /* tack on source */ while(*s) if(*s!= '"') *d = *s; d++; s++; *d = *s; /* copy final NULL */ /* end of qstrcat() */ /* */ init_env() -- reads the environment and sets up the paths[] array. void init_env() char *path, *sub; num_paths = 1; paths[0] = ""; /* set first path to nothing */ sub = getenv(bob_env); path = strdup(sub); /* make a local copy to work with */ sub = strtok(path, DELIMS); while(sub && num_paths < MAX_PATHS) paths[num_paths] = strdup(sub); num_paths++; sub = strtok(null, DELIMS);

9 /* end of init_env() */ /* */ env_fopen() -- acts just like fopen except uses the paths taken from the envoronment variable BOB. Also handles filenames wrapped in quotes. FILE *env_fopen(char *name, char *mode) FILE *fp; int i; static char full_path[1024]; for(i=0; i<num_paths; i++) strcpy(full_path, paths[i]); if(i) strcat(full_path, "/"); qstrcat(full_path, name); fp = fopen(full_path, mode); if(fp) return fp; /* nothing worked so return NULL */ return NULL; /* end of env_fopen() */

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