Week 2 The Crunchy Shell to the Soft and Chewy Kernel…

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Week 2The Crunchy Shell

to the Soft and Chewy Kernel…

Sarah Diesburg8/3/2010COP4610 / 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?

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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

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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

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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

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How /proc?

Looking to our good friend bash…

$> cat /proc/cpuinfo

(Hint – your viewproc command

should do something similar…)

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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()

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Shell Basics (Project 1)

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Inside main()

Continuous loop Parse user input Make something happen

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Inside main()

while(1)

{

}

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Inside main()

while(1)

{

*/ Get user input */

}

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Inside main()

while(1)

{

*/ Get user input */

*/ Exit? */

}

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Inside main()

while(1)

{

*/ Get user input */

*/ Exit? */

*/ Do something with input */

}

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Inside main()

while(1)

{

*/ Get user input */

*/ Exit? */

*/ Do something with input */

*/ Reset the shell */

}

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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

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Standard I/O Stream File 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

sarah@trogdor:~$ echo $PATH/usr/local/bin:/usr/bin:/bin:/usr/games

May be set by you or other shell scripts (like .bashrc)

sarah@trogdor:~$ export TEST=hellosarah@trogdor:~$ echo $TESThello

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Environmental Variables

char *getenv(const char *name);

Returns value of an environmental variable Returns NULL if not found

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Environmental Variables

Important examples $PATH $USER $PWD

(Hint: may want to use these in building the shell prompt)

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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

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Parsing Example

ls -l a

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Too much whitespace!

Parsing Example

ls -l a

ls –la

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Parse out the whitespace

Parsing Example

ls -l a

ls –la

/bin/ls -la

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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

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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!

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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!

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Fork

Child pid==0 Parent pid==something else

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#include <stdio.h>#include <unistd.h>#include <sys/types.h>

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!

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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

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Execv()

char *command = “/bin/ls”;

char *argv[] =

{“/bin/ls”, “-l”, NULL};

execv(command,argv);

Execv() replaces running image of child with a new process!

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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!

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Zombie?

Process that shows up with a “Z” status or the word <defunct>

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()

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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

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waitpid()

Comment waitpid() line to see a defunct process for 10 seconds through ‘ps’

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#include <stdio.h>#include <unistd.h>#include <sys/types.h>#include <stdlib.h>

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

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Next Recitation

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

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Any Questions?

Time for some of my demos?