Description

 

 

What is a shell?

• Command line interpreter – You type “ls /etc”
  • The shell invokes the first parameter as a command, with the remainder as the parameters – eg: exec(ls,”/etc”)
• Built-in commands
  • Most commands are separate executable programs
• ls, rm, mv, make, gcc
  • Some commands are interpreted by the shell • cd, exit, pid, ppid.

 

Interactive vs Batch

• Interactive
  • User types commands in, hits return to invoke them
• Batch
  • shell reads from an input file
• What is the difference?
  • where the commands come from
• You need to implement the Interactive shell model.

 

Input/Output

• C has 3 standard files prepared for you
  • stdin = input
  • stdout = output
  • stderr = error output
• printf(“foo”) == fprintf(stdout,”foo”)
• scanf(“%s”,str) == fscanf(stdin,”%s”, str)
• fprintf(stderr,”Panic!”) prints an error message separately

 

Process Control

• Your shell should execute the next command line after the previous one terminates – you must wait for any programs that you launch to finish
• You don’t have to provide the functionality of launching multiple simultaneous commands with “;” separating them

 

Hints

• A shell is a loop
  • read input
  • execute program
  • wait program
  • repeat

 

• Useful routines
  • fgets() for string input
  • strtok() for parsing
  • exit() for exiting the shell
  • getpid() for finding the current process ID
  • getppid() for finding the parent process ID
  • getcwd() for getting the current working directory
  • getenv()/setenv()
  • chdir() for changing directories

 

• Executing commands
  • fork() creates a new process
  • execvp() runs a new program and does path processing
  • wait(), waitpid() waits for a child process to terminate

 

Requirements:

• <executable> -p <prompt> should allow the user to select an user-defined prompt. Otherwise, the default should be “my257sh> ”.
  • Shell functions to be implemented separately: exit, pid, ppid, cd. o For implementing “exit” from the shell, use the raise() signal handler.
  • “cd” prints the current working directory; whereas “cd <path>” will change the current working directory.
  • All other shell commands will need a child process using fork() and then calling execvp().
• No input will be greater than 50 characters.
• Only the interactive system needs to be implemented (batch system is not needed)
• Background process execution (using &) is NOT required.
• Each time a child process is created, evaluate its exit status and print it out.
• ^C should not take us out of the shell; use a signal handler. Hint: you can use the same signal handler code from the slides.