1 Week 2 The Crunchy Shell to the Soft and Chewy Kernel… Sarah Diesburg 8/3/2010 COP4610 / CGS5765

Why is the Shell Important? Shells provide us with a way to interact with the core system  Executes programs on our behalf  Shows us our stuff  No OS should be without one! Can think of a shell as “built around” a component So what are some examples of shells? 2

How do we Crack the Shell? In other words, how will our shell interact with the soft and chewy kernel?  /proc file system  System calls 3

What is /proc? Virtual file system created to pass information via files to and from the kernel  Inside /proc directory Often used in kernel debugging or when a developer does not want to create a new system call 4

Why /proc? You will be reading and displaying a bunch of goodies from the kernel through the /proc interface – Date – Up Time – Idle Time – CPU Info – Memory Info – Kernel Version – Terminal Process Details 5

How /proc? Looking to our good friend bash… $> cat /proc/cpuinfo (Hint – your viewproc command should do something similar…) 6

System Calls What are they again? The traditional way to ask the OS to do something on the user’s behalf Some important ones  Fork()  Execv() 7

Shell Basics (Project 1) 8

Inside main() Continuous loop  Parse user input  Make something happen 9

Inside main() while(1) { } 10

Inside main() while(1) { */ Get user input */ } 11

Inside main() while(1) { */ Get user input */ */ Exit? */ } 12

Inside main() while(1) { */ Get user input */ */ Exit? */ */ Do something with input */ } 13

Inside main() while(1) { */ Get user input */ */ Exit? */ */ Do something with input */ */ Reset the shell */ } 14

I/O Streams Examples  scanf() reads from standard input  fscanf() reads from any I/O stream  printf() prints to standard output  fprintf() prints to any I/O stream 15 Standard I/O StreamFile descriptor Standard input (stdin)0 Standard output (stdout)1 Standard error (stderr)2

Environmental Variables Gives programs specific information about your environemnt, such as your execution paths echo $PATH /usr/local/bin:/usr/bin:/bin:/usr/games May be set by you or other shell scripts (like.bashrc) export TEST=hello echo $TEST hello 16

Environmental Variables char *getenv(const char *name); Returns value of an environmental variable Returns NULL if not found 17

Environmental Variables Important examples  $PATH  $USER  $PWD (Hint: may want to use these in building the shell prompt) 18

Command Line Parsing Standard input (stdin) is the source of input data for command line programs Parsing can be done in multiple stages  Strip the whitespace  Interpret the command  Resolve the pathname  Variable expansion  I/O redirection  Final execution 19

Parsing Example ls -l a 20 Too much whitespace!

Parsing Example ls -l a ls –la 21 Parse out the whitespace

Parsing Example ls -l a ls –la /bin/ls -la 22 Resolve the pathname

Resolving Pathnames? You may not just pass ‘ls’ to the execute command  What is ‘ls’? You must search all of the users paths stored in the $PATH environmental variable 23

Finding full pathname for ‘ls’ $PATH=/usr/local/bin:/usr/bin:/bin Does /usr/local/bin/ls exist?  No Does /usr/bin/ls exist?  No Does /bin/ls exist?  Yes! 24

Processes Our shell process must continually run  …but we need to execute other stuff on the user’s behalf How can we create “children” processes to do our work? Fork! 25

Fork Child pid==0 Parent pid==something else 26 #include int main() { pid_t pid; if ((pid = fork()) == 0) { printf(“I am a child with pid %d\n”, pid); } else { printf(“I am the parent with pid %d\n”, pid); } return 0; }

Exec Once we have created a child process, we need the child to execute a command for us Exec has many forms  Execl  Execlp  Execle  Execv  Execvp Must use execv() for project 1! 27

Execv() Takes two arguments  Absolute pathname  Array of string arguments, ending with NULL What is an absolute pathname?  Full execution path starting from root “/”  /bin/ls 28

Execv() char *command = “/bin/ls”; char *argv[] = {“/bin/ls”, “-l”, NULL}; execv(command,argv); Execv() replaces running image of child with a new process! 29

Wait up? How does our parent process know to wait until the child is done?  waitpid() Performing a wait allows the system to release the resources associated with the child  If child is not waited on, it will become a zombie! 30

Zombie? Process that shows up with a “Z” status or the word Child process has terminated, but parent has not waited on it Child process stays allocated on the system until  Wait() or waitpid() is called by the parent  The parent exits, and init “adopts” the zombie processes and performs a wait() 31

waitpid() int waitpid(pid_t pid, int *status, int options); pid – type of children to wait on  For this project, pid==0 to mean wait for any child process created by our parent *status – returns the status of the child process options – return if additional things have happened to the child 32

waitpid() Comment waitpid() line to see a defunct process for 10 seconds through ‘ps’ 33 #include int main() { pid_t pid; if ((pid = fork()) == 0) { printf(“I am a child with pid %d\n”, pid); } else { printf(“I am the parent\n”); waitpid(-1, status, 0); sleep(10); } return 0; }

In Summary Pieces necessary for some of project 1  Part 1 – Command line parsing  Part 2 – Environmental variables and expansion  Part 3 – Command execution  Part 7 – Built-ins including /proc Hint: chdir() may be of some use  Part 8 – The prompt via parsing environmental variables and updating $PWD 34

Next Recitation Part 4 – Input/output redirection Part 5 – Pipes Part 6 – Background processing 35

Any Questions? Time for some of my demos?