Write a program in c

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,/et c)

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) == fprint f (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 dont have to provide the functionality of launch ing 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 find ing the current process ID

getppid () for find ing 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:

- p should allow the user to select an user - defined prompt. Otherwise, the default should be

my 257 sh> .

Shell functions to be implemented separately : exit, pid, ppid, cd.

For implementing exit from the shell, use the raise() signal handler.

cd prints the cu rrent working directory; whereas cd will change the current working directory.

All other shell commands will need a child process using fork() and then call ing execvp().

No input will be greater than 50 characters.

Only the interactive system need s to be implemented (batch system is not needed)

Background process execution (using &) is NOT required .

Each time a child process is created, e valuate its exit status and print it out.

^C should not take us out of the shell; use a signal handler .