More Exec

• int execvp(const char *file, char *const argv[]);
  int execvpe(const char *file, char *const argv[], char *const envp[]);
      

  • Looks at the environment variable PATH
  • If this is not available a very restricted path is used (/usr/bin:/bin), this does not include . as he says in the book.
  • . in the search path is dangers, why?
  • He cautions not to use the exec*p* function in anywhere programs that are set-user-ID or set-group-ID
• He provides a set of examples for using various forms of exec.
• Scripts
  • This is built into the exec command.
  • The following must be true
    • The file must have execute permission.
    • The file must start with #!/path/to/interpreter [opt-args]
  • This eventually will execute /path/to/interpreter [opt-args] script args
    • So opt-args are arguments to the interpreter, not the script.
  • If you use the exec*p* function, and the interpreter is missing, it will try to execute the script with /bin/sh
• The int system(const char *command); command.
  • Forks,
  • Executes the command
    • execl("/bin/sh", "sh", "-c", command, (char *) NULL);
  • waits
  • Returns
    • 0 if no shell is available.
    • 127 if the shell fails to execute
    • Termination code of the last command executed.
  • You don't have to do the fork, exec, wait.
  • This probably does more fork/exec pairs than you want.
  • This should not be used for suid or sgid programs.
• He provides a sample implementation for system.