CMSC 257 Assignment 5 Sample Unix Shell solved

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 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.


• 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


• 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


• -p should allow the user to select an user-defined prompt. Otherwise, the default should be “257sh> ”. 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 ” 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.


you can use the same signal handler code from the slides.