CMSC257-Assignment 4: Sample Unix Shell Solved

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> ”.

o   Shell functions to be implemented separately: exit, pid, ppid, cd.  o For implementing “exit” from the shell, use the raise() signal handler.

o   “cd”  prints the current working directory; whereas “cd <path>” will change the current working directory.

o   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.