sh.c

// Shell.

#include "types.h"
#include "user.h"
#include "fcntl.h"
#include "param.h"

// Parsed command representation
#define EXEC  1
#define REDIR 2
#define PIPE  3
#define LIST  4
#define BACK  5

#define MAXARGS 10

struct cmd {
  int type;
};

struct execcmd {
  int type;
  char *argv[MAXARGS];
  char *eargv[MAXARGS];
};

struct redircmd {
  int type;
  struct cmd *cmd;
  char *file;
  char *efile;
  int mode;
  int fd;
};

struct pipecmd {
  int type;
  struct cmd *left;
  struct cmd *right;
};

struct listcmd {
  int type;
  struct cmd *left;
  struct cmd *right;
};

struct backcmd {
  int type;
  struct cmd *cmd;
};

int fork1(void);  // Fork but panics on failure.
void panic(char*);
struct cmd *parsecmd(char*);

// Execute cmd. Never returns.
#if __GNUC__ >= 12
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Winfinite-recursion"
#endif
void
runcmd(struct cmd *cmd)
{
  int p[2];
  struct backcmd *bcmd;
  struct execcmd *ecmd;
  struct listcmd *lcmd;
  struct pipecmd *pcmd;
  struct redircmd *rcmd;

  if(cmd == 0)
    exit();

  switch(cmd->type){
  default:
    panic("runcmd");

  case EXEC:
    ecmd = (struct execcmd*)cmd;
    if(ecmd->argv[0] == 0)
      exit();

    execvpe(ecmd->argv[0], ecmd->argv, environ);
    printf(2, "exec %s failed\n", ecmd->argv[0]);
    break;

  case REDIR:
    rcmd = (struct redircmd*)cmd;
    close(rcmd->fd);
    if(open(rcmd->file, rcmd->mode) < 0){
      printf(2, "open %s failed\n", rcmd->file);
      exit();
    }
    runcmd(rcmd->cmd);
    break;

  case LIST:
    lcmd = (struct listcmd*)cmd;
    if(fork1() == 0)
      runcmd(lcmd->left);
    wait();
    runcmd(lcmd->right);
    break;

  case PIPE:
    pcmd = (struct pipecmd*)cmd;
    if(pipe(p) < 0)
      panic("pipe");
    if(fork1() == 0){
      close(1);
      dup(p[1]);
      close(p[0]);
      close(p[1]);
      runcmd(pcmd->left);
    }
    if(fork1() == 0){
      close(0);
      dup(p[0]);
      close(p[0]);
      close(p[1]);
      runcmd(pcmd->right);
    }
    close(p[0]);
    close(p[1]);
    wait();
    wait();
    break;

  case BACK:
    bcmd = (struct backcmd*)cmd;
    if(fork1() == 0)
      runcmd(bcmd->cmd);
    break;
  }
  exit();
}
#if __GNUC__ >= 12
#pragma GCC diagnostic pop
#endif

int
getcmd(char *buf, int nbuf)
{
  printf(2, "$ ");
  memset(buf, 0, nbuf);
  gets(buf, nbuf);
  if(buf[0] == 0) // EOF
    return -1;
  return 0;
}

int
is_validc(char c)
{
  return (c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z');
}

char *
substitute(char *arg)
{
  int arg_len = 0;
  while (arg[arg_len] != '\0') {
    arg_len++;
  }

  char *result = malloc(ENVVAL_MAX + 1);
  if (result == 0) {
    return 0;
  }

  int result_index = 0;
  int i = 0;

  while (i < arg_len) {
    if (arg[i] == '$') {
      i++;
      char env_var_name[ENVKEY_MAX + 1];
      int env_var_name_index = 0;

      while (is_validc(arg[i]) && env_var_name_index < 31) {
        env_var_name[env_var_name_index++] = arg[i++];
      }
      env_var_name[env_var_name_index] = '\0';
      char *env_var_value = getenv(env_var_name);

      if (env_var_value != 0) {
        int env_var_value_len = 0;
        while (env_var_value[env_var_value_len] != '\0') {
          result[result_index++] = env_var_value[env_var_value_len++];
        }
      }
    } else {
      result[result_index++] = arg[i++];
    }
  }
  result[result_index] = '\0';
  return result;
}

int
main(void)
{
  static char buf[100];
  int fd;

  // Ensure that three file descriptors are open.
  while((fd = open("console", O_RDWR)) >= 0){
    if(fd >= 3){
      close(fd);
      break;
    }
  }

  // Read and run input commands.
  while(getcmd(buf, sizeof(buf)) >= 0){
    if(strncmp(buf, "cd ", 3) == 0) {
      // Chdir must be called by the parent, not the child.
      buf[strlen(buf)-1] = 0;  // chop \n
      if(chdir(buf+3) < 0)
        printf(2, "cannot cd %s\n", buf+3);
      continue;
    }

    // export
    if(strncmp(buf, "export", 6) == 0) {
      char* arg = buf + 6;
      while (*arg == ' ')
        arg++;
      arg[strlen(arg) - 1] = '\0'; // chop \n

      if (*arg == '\0') {
        for (int i = 0; environ[i] != 0; i++) {
          printf(1, "%s\n", environ[i]);
        }
        continue;
      }

      char* equal_sign = strchr(arg, '=');
      if (equal_sign == 0) {
        printf(2, "Invalid argument format for export: %s\n", arg);
        continue;
      }

      *equal_sign = '\0';
      char* name = arg;
      for (int i = 0; name[i] != '\0'; i++) {
        if (!is_validc(name[i])) {
          printf(2, "Invalid environment variable name: %s\n", name);
          continue;
        }
      }

      char* value = equal_sign + 1;
      if (*name == '\0') {
        printf(1, "Invalid environment variable name");
        continue;
      }

      char* evaluated_value = substitute(value);
      if (setenv(name, evaluated_value, 1) != 0) {
        printf(2, "Failed to set environment variable: %s\n", name);
      }
      continue;
    }

    // unset
    if(strncmp(buf, "unset", 5) == 0) {
      char* arg = buf + 5;
      while (*arg == ' ')
        arg++;
      arg[strlen(arg) - 1] = '\0'; // chop \n

      if (*arg == '\0') {
        printf(2, "Invalid argument format for unset: %s\n", arg);
        continue;
      }

      char* name = arg;
      for (int i = 0; name[i] != '\0'; i++) {
        if (!is_validc(name[i])) {
          printf(2, "Invalid environment variable name: %s\n", name);
          goto next;
        }
      }

      if (unsetenv(name) != 0) {
        printf(2, "Failed to unset environment variable: %s\n", name);
      }
      continue;
    }
    if(fork1() == 0)
      runcmd(parsecmd(buf));
    wait();
    next: ;
  }
  exit();
}

void
panic(char *s)
{
  printf(2, "%s\n", s);
  exit();
}

int
fork1(void)
{
  int pid;

  pid = fork();
  if(pid == -1)
    panic("fork");
  return pid;
}

//PAGEBREAK!
// Constructors

struct cmd*
execcmd(void)
{
  struct execcmd *cmd;

  cmd = malloc(sizeof(*cmd));
  memset(cmd, 0, sizeof(*cmd));
  cmd->type = EXEC;
  return (struct cmd*)cmd;
}

struct cmd*
redircmd(struct cmd *subcmd, char *file, char *efile, int mode, int fd)
{
  struct redircmd *cmd;

  cmd = malloc(sizeof(*cmd));
  memset(cmd, 0, sizeof(*cmd));
  cmd->type = REDIR;
  cmd->cmd = subcmd;
  cmd->file = file;
  cmd->efile = efile;
  cmd->mode = mode;
  cmd->fd = fd;
  return (struct cmd*)cmd;
}

struct cmd*
pipecmd(struct cmd *left, struct cmd *right)
{
  struct pipecmd *cmd;

  cmd = malloc(sizeof(*cmd));
  memset(cmd, 0, sizeof(*cmd));
  cmd->type = PIPE;
  cmd->left = left;
  cmd->right = right;
  return (struct cmd*)cmd;
}

struct cmd*
listcmd(struct cmd *left, struct cmd *right)
{
  struct listcmd *cmd;

  cmd = malloc(sizeof(*cmd));
  memset(cmd, 0, sizeof(*cmd));
  cmd->type = LIST;
  cmd->left = left;
  cmd->right = right;
  return (struct cmd*)cmd;
}

struct cmd*
backcmd(struct cmd *subcmd)
{
  struct backcmd *cmd;

  cmd = malloc(sizeof(*cmd));
  memset(cmd, 0, sizeof(*cmd));
  cmd->type = BACK;
  cmd->cmd = subcmd;
  return (struct cmd*)cmd;
}
//PAGEBREAK!
// Parsing

char whitespace[] = " \t\r\n\v";
char symbols[] = "<|>&;()";

int
gettoken(char **ps, char *es, char **q, char **eq)
{
  char *s;
  int ret;

  s = *ps;
  while(s < es && strchr(whitespace, *s))
    s++;
  if(q)
    *q = s;
  ret = *s;
  switch(*s){
  case 0:
    break;
  case '|':
  case '(':
  case ')':
  case ';':
  case '&':
  case '<':
    s++;
    break;
  case '>':
    s++;
    if(*s == '>'){
      ret = '+';
      s++;
    }
    break;
  default:
    ret = 'a';
    while(s < es && !strchr(whitespace, *s) && !strchr(symbols, *s))
      s++;
    break;
  }
  if(eq)
    *eq = s;

  while(s < es && strchr(whitespace, *s))
    s++;
  *ps = s;
  return ret;
}

int
peek(char **ps, char *es, char *toks)
{
  char *s;

  s = *ps;
  while(s < es && strchr(whitespace, *s))
    s++;
  *ps = s;
  return *s && strchr(toks, *s);
}

struct cmd *parseline(char**, char*);
struct cmd *parsepipe(char**, char*);
struct cmd *parseexec(char**, char*);
struct cmd *nulterminate(struct cmd*);

struct cmd*
parsecmd(char *s)
{
  char *es;
  struct cmd *cmd;

  es = s + strlen(s);
  cmd = parseline(&s, es);
  peek(&s, es, "");
  if(s != es){
    printf(2, "leftovers: %s\n", s);
    panic("syntax");
  }
  nulterminate(cmd);
  return cmd;
}

struct cmd*
parseline(char **ps, char *es)
{
  struct cmd *cmd;

  cmd = parsepipe(ps, es);
  while(peek(ps, es, "&")){
    gettoken(ps, es, 0, 0);
    cmd = backcmd(cmd);
  }
  if(peek(ps, es, ";")){
    gettoken(ps, es, 0, 0);
    cmd = listcmd(cmd, parseline(ps, es));
  }
  return cmd;
}

struct cmd*
parsepipe(char **ps, char *es)
{
  struct cmd *cmd;

  cmd = parseexec(ps, es);
  if(peek(ps, es, "|")){
    gettoken(ps, es, 0, 0);
    cmd = pipecmd(cmd, parsepipe(ps, es));
  }
  return cmd;
}

struct cmd*
parseredirs(struct cmd *cmd, char **ps, char *es)
{
  int tok;
  char *q, *eq;

  while(peek(ps, es, "<>")){
    tok = gettoken(ps, es, 0, 0);
    if(gettoken(ps, es, &q, &eq) != 'a')
      panic("missing file for redirection");
    switch(tok){
    case '<':
      cmd = redircmd(cmd, q, eq, O_RDONLY, 0);
      break;
    case '>':
      cmd = redircmd(cmd, q, eq, O_WRONLY|O_CREATE, 1);
      break;
    case '+':  // >>
      cmd = redircmd(cmd, q, eq, O_WRONLY|O_CREATE, 1);
      break;
    }
  }
  return cmd;
}

struct cmd*
parseblock(char **ps, char *es)
{
  struct cmd *cmd;

  if(!peek(ps, es, "("))
    panic("parseblock");
  gettoken(ps, es, 0, 0);
  cmd = parseline(ps, es);
  if(!peek(ps, es, ")"))
    panic("syntax - missing )");
  gettoken(ps, es, 0, 0);
  cmd = parseredirs(cmd, ps, es);
  return cmd;
}

struct cmd*
parseexec(char **ps, char *es)
{
  char *q, *eq;
  int tok, argc;
  struct execcmd *cmd;
  struct cmd *ret;

  if(peek(ps, es, "("))
    return parseblock(ps, es);

  ret = execcmd();
  cmd = (struct execcmd*)ret;

  argc = 0;
  ret = parseredirs(ret, ps, es);
  while(!peek(ps, es, "|)&;")){
    if((tok=gettoken(ps, es, &q, &eq)) == 0)
      break;
    if(tok != 'a')
      panic("syntax");

    if (q[strlen(q) - 1] == '\n') {
      q[strlen(q) - 1] = '\0';
    }

    if (cmd->argv[argc] != 0) {
      free(cmd->argv[argc]);
    }
    cmd->argv[argc] = substitute(q);

    char *eargv = strchr(cmd->argv[argc], ' ');
    if(eargv == 0) {
      eargv = cmd->argv[argc] + strlen(cmd->argv[argc]);
    }
    cmd->eargv[argc] = eargv;

    argc++;
    if(argc >= MAXARGS)
      panic("too many args");
    ret = parseredirs(ret, ps, es);
  }
  cmd->argv[argc] = 0;
  cmd->eargv[argc] = 0;
  return ret;
}

// NUL-terminate all the counted strings.
struct cmd*
nulterminate(struct cmd *cmd)
{
  int i;
  struct backcmd *bcmd;
  struct execcmd *ecmd;
  struct listcmd *lcmd;
  struct pipecmd *pcmd;
  struct redircmd *rcmd;

  if(cmd == 0)
    return 0;

  switch(cmd->type){
  case EXEC:
    ecmd = (struct execcmd*)cmd;
    for(i=0; ecmd->argv[i]; i++)
      *ecmd->eargv[i] = 0;
    break;

  case REDIR:
    rcmd = (struct redircmd*)cmd;
    nulterminate(rcmd->cmd);
    *rcmd->efile = 0;
    break;

  case PIPE:
    pcmd = (struct pipecmd*)cmd;
    nulterminate(pcmd->left);
    nulterminate(pcmd->right);
    break;

  case LIST:
    lcmd = (struct listcmd*)cmd;
    nulterminate(lcmd->left);
    nulterminate(lcmd->right);
    break;

  case BACK:
    bcmd = (struct backcmd*)cmd;
    nulterminate(bcmd->cmd);
    break;
  }
  return cmd;
}