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?
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