Project 2A - The Astro Home Page
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1. Store the C Strings in an array of character pointers declared like this char argv 100 Set the first location in the array after your C Strings to NULL You are going to pass the argv array to the execvp system call This system call does NOT take argc as an argument but the call still needs to detect the end of the argv array To detect the end of the argv array execvp looks for the first array entry that has the value of NULL 3 Inthe next debug version use argv 0 to find the executable file In this version simply print the filename e Construct a simple version that can find only command files that are in the current directory e Enhance your program so that it can find command files that are specified with an absolute pathname e Enable your program to search directories according to the string that is stored in the shell PATH environment variable You can access all of the environment variables by adding the following two lines before main in your program include lt unistd h gt extern char environ The last item in the array is a NULL C String 4 Create a child process to execute the command include lt iostream gt using namespace std introduces namespace std int main void CIS 3207 Project 2 Page 7 string theLine int argc char argv 100 Initialization stuff while true cout lt lt MyShell gt getline cin theLine cout lt lt Command was lt lt theLine lt lt2. By default a pipe employs asynchronous send and blocking receive operations Optionally the blocking receive operation may be changed to be a non blocking receive Pipes are FIFO first in first out buffers designed with an API that resembles as closely as possible a low level file I O interface A pipe may contain a system defined maximum number of bytes at any given time usually 4KB A process can send data by writing it into one end of the pipe and another can receive the data by reading the other end of the pipe Information Flow Through a Pipe A pipe is represented in the kernel by a file descriptor just like a file A process that wants to create a pipe calls the kernel with a call of the following form int thePipe 2 pipe thePipe A blocking receive means that the receive cannot complete until a new character is available to read An asynchronous send means that a write can take place at any time and will be accepted CIS 3207 Project 2 Page 5 The kernel creates the pipe as a kernel FIFO data structure with two file identifiers In this example code thePipe 0 is a file pointer an index into the process s open file table to the read end of the pipe and thePipe 1 is file pointer to the write end of the pipe For two or more processes to use pipes for IPC interprocess communication a common ancestor of the processes must create the pipe prior to creating the processes Because the fork command creates a child that con
3. and stderr These three file identifiers correspond to the C objects cin cout and cerr If the process reads from stdin using cin then the data that it receives will be directed from the keyboard to the stdin file descriptor Similarly data received from stdout using cout and stderr using cerr are mapped to the terminal display The user can redefine stdin or stdout whenever a command is entered If the user provides a filename argument to the command and precedes the filename with a less than character lt then the shell will substitute the designated file for stdin this is called redirecting the input from the designated file The user can redirect the output for the execution of a single command by preceding a filename with the right angular brace character gt character For example a command such as cis linux1 gt we lt main cpp gt program stats will create a child process to execute the we command Before it launches the command however it will redirect stdin so that it reads the input stream from the file main cpp and redirect stdout so that it writes the output stream to the file program stats The shell can redirect I O by manipulating the child process s file descriptors A newly created child process inherits the open file descriptors of its parent specifically the same keyboard for stdin and the terminal display for stdout and stderr This expands on why concurrent processes read and write the sam
4. be modified at any time by using the set command The PATH environment variable whose value can be viewed by typing echo PATH at the bash shell is an ordered list of absolute pathnames specifying where the shell should search for command files If the login file has a line such as 1 Bash User Manual http www gnu org software bash manual bashref html Bash Guide for Beginners http tldp org LDP Bash Beginners Guide html index html CIS 3207 Project 2 Page 2 set path bin usr bin for c shell for bash check out this link http www fnal gov docs UNIX unix_at_fermilab htmldoc rev1997 uatf 62 html then the shell will first look in the current directory since the first full pathname is then in bin and finally in usr bin If no file with the same name as the command can be found in any of the specified directories then the shell notifies the user that it is unable to find the command 5 Prepare the parameters The shell simply passes the parameters to the command as the argv array of pointers to strings on the top of stack 6 Execute the command The shell must execute the executable program in the specified file UNIX shells have always been designed to protect the original process from crashing when it executes a program That is since a command can be any executable file then the process that is executing the shell must protect itself in case the executable file contains a fatal error Somehow the shell wants to
5. launch the executable so that even if the executable contains a fatal error which destroys the process executing it then the shell will remain unharmed by fork The Bourne shell uses multiple processes to accomplish this by using the UNIX style system calls fork execvp and wait fork The fork system call creates a new process that is a copy of the calling process except that it has its own copy of the memory its own process ID with the correct relationships to other processes and its own pointers to shared kernel entities such as file descriptors After fork has been called two processes will execute the next statement after the fork in their own address spaces they are the parent and the child If the call succeeds then in the parent process fork returns the process ID of the newly created child process and in the child process fork returns a zero value execvp The execvp system call changes the program that a process is currently executing It has the form execvp char path char argv The path argument is the pathname of a file that contains the new program to be executed The argv array is a list of parameter strings When a process encounters the execvp system call the next instruction it executes will be the one at the entry point of the new executable file Thus the kernel performs a considerable amount of work in this system call It must e find the new executable file e load the file i
6. out txt rather than sending it to the screen which is stdout 2 Also allow your user to use the pipe operator I to execute two processes concurrently with stdout from the first process being redirected as the stdin for the second process For example MyShell gt cat out txt Hello World MyShell gt cat out txt we l MyShell gt The cat command sends the contents of the out txt file to stdout The wc I command counts the number of characters words and lines typed into stdin The two commands can be strung together using the pipe Operator so that the output of cat sent to stdout is connected to the input of wc received from stdin You can design your program so that you only have to handle redirection OR pipes in one command line but not both Attacking Problem C Don t worry about mixing the amp redirection operators lt and gt and pipe operator in the same command Concentrate on getting the redirection operators to work first The key to getting these to work is to understand the I O Redirection discussion in this document Watch out for file permission problems with the redirection operators needed to bone up on the man pages for file to get these flags correct The default permissions for the file open for write meant subsequent redirects to the same file didn t work of because permission problems int newstdin open inputFileName c_str O_RDONLY int n
7. Project Two You will learn how to write a UNIX shell program This will give you the opportunity to learn how child processes are created to perform large grained work and how the parent process can follow up on a child process s work INTRODUCTION A shell or command line interpreter program is a mechanism with which each interactive user can send commands to the OS and by which the OS can respond to the user Whenever a user has successfully logged in to the computer the OS causes the user process assigned to the login port to execute a specific shell The OS does not ordinarily have a built in window interface Instead it assumes a simple character oriented interface in which the user types a string of characters terminated by pressing the Enter or Return key and the OS responds by typing lines of characters back to the screen If the human computer interface is to be a graphical windows interface then the software that implements the window manager subsumes the shell tasks that are the focus of this exercise Thus the character oriented shell assumes a screen display with a fixed number of lines usually 25 and a fixed number of characters usually 80 per line Once the shell has initialized its data structures and is ready to start work it clears the 25 line display and prints a prompt in the first few character positions on the first line Linux systems are usually configured to include the machine name as part of the prompt For examp
8. ct given to previous students and at other Universities You learn nothing from using code developed by others We will not tolerate code or documentation that is not your own Some helpful books available in the Temple Library C Cookbook http libproxy temple edu 2249 059600298xX C in a Nutshell http libproxy temple edu 2249 059600298X Core C A Software Engineering Approach http libproxy temple edu 2249 0130857297 CIS 3207 Project 2 Page 10
9. e keyboard and display The shell can change the child s file descriptors so that it reads and writes to files rather than to the keyboard and display Each process has its own file descriptor table in the kernel When the process is created the first entry in this table by convention refers to the keyboard stdin and the second two refer to the terminal display Next the C runtime environment and the kernel manage stdin stdout and stderr so that cin stdin 0 Keyboard cout stdout 1 Terminal Display cerr stderr 2 Terminal Display CIS 3207 Project 2 Page 4 If you want to perform I O redirection you need to connect stdin to a file which can be read for input instead of the keyboard or stdout to a file which can accept output instead of the terminal display Let s consider stdin specifically The key to getting I O redirection to work for stdin is to replace the contents of the file descriptor entry for stdin currently the keyboard with the file descriptor entry for the file you want to use for input To accomplish this you need to access the file descriptors for stdin and the file you want to read from You know how to get the file descriptor for stdin that s just the number 0 But how do you get the file descriptor for the file you want to use for input Unfortunately you can t get the file descriptor directly from a C ifstream object Instead you have to issue the following Linux system call we are still using the exa
10. end should close the end so that end of file EOF conditions can be detected A named pipe can be used to allow unrelated processes to communicate with each other Typically in pipes the children inherit the pipe ends as open file descriptors In named pipes a process obtains a pipe end by using a string that is analogous to a filename but that is associated with a pipe This allows any set of processes to exchange information by using a public pipe whose end names are filenames When a process uses a named pipe the pipe is a system wide resource potentially accessible by any process Just as files must be managed so that they are not inadvertently shared among many processes at one time named pipes must be managed by using low level file system commands Problem A Write a C C program that will act as a shell command line interpreter for the Linux kernel Your shell program should use the same style as the bash shell for running programs In particular when the user types a line such as command parameter1 parameterN CIS 3207 Project 2 Page 6 your shell should parse the command line to build argv It should search the directory system in the order specified by the PATH environment variable for a file with the same name as the first identifier which may be a relative filename or a full pathname If the file is found then it should be executed with the optional parameter list as is done with bash If the file is not found
11. endl if theLine exit amp amp cin eof exit 0 Determine the command name and construct the parameter list argv 0 command name argv 1 first parameter argv 2 second parameter I isi argv N Nth parameter argv N 1 NULL to indicate end of parameter list Find the full path name for the file Launch the executable file with the specified parameters using the execvp command and the argv array Determining the Command Name and the Parameter List A program might be run from your shell with the command MyShell gt main foo 100 Before calling execvp you need to set up the argv array so that argv 0 will point to the C String main argv 1 will point to foo argv 2 will point to the string 100 and argv 3 will be set to NULL to indicate the end of elements in the argv array Your program then can interpret the first parameter argv 1 as say a filename and the second parameter argv 2 as say an integer record count The shell would simply treat the first word on the command line as a filename and the remaining words as strings Finding the Full Pathhame The user might have provided a full pathname as the command name word or only a relative pathname that is to be bound according to the value of the PATH environment variable A name that begins with a or is an absolute pathname that can be used to launch the execution Otherwise
12. ewstdout open outputFileName c_str O_WRONLY O_CREAT S_IRWXU S_IRWXG S_IRWXO To get command line pipes to work re read the Shell Pipes section carefully Since a pipe is just like a file there s no reason why you couldn t use I O Redirection and pipes together 1 Have the parent create a pipe Spawn a child Have the parent redirect stdout to the write portion of the pipe Have the parent execute the first command with excecvp Have the child redirect stdin to the read portion of the pipe SE iS CIS 3207 Project 2 Page 9 6 Have the child execute the second command with execvp 7 The output from the parent command should flow to the child command via the pipe and i o redirection you performed on the pipe Watch out for EOF problems with pipes couldn t get the child process to terminate on an eof condition because the parent didn t close the read portion of the pipe Make sure that for parent and child that you call the system call close for any portion of a pipe you aren t using General Hints You should learn to use man pages to get info about important system calls Sometimes the man page wanted didn t come up when typed man lt keyword gt For example man wait gave me lots of information about the bash shell not the system call wait But could find out about wait by typing man 2 wait figured this out by looking at the documentation If you see something like See also wait 2 It means use
13. le a Linux machine is named cis linux1 cis temple edu so the shell prints as its prompt string cis linux1 gt or bash gt depending on which shell am using The shell then waits for the user to type a command line in response to the prompt The command line could be a string such as cis linux1 gt Is al terminated with an lt ENTER gt or return character in Linux this character is represented internally by the NEWLINE character n When the user enters a command line the shell s job is to cause the OS to execute the command embedded in the command line Every shell has its own language syntax and semantics In the standard Linux shell bash a command line has the form command arg1 arg2 argN in which the first word is the command to be executed and the remaining words are arguments expected by that command The number of arguments depends on which command is being executed For example the directory listing command may have no arguments simply by the user s typing Is or it may have arguments prefaced by the negative character as in Is al where a and I are arguments The command determines the syntax for the arguments such as which of the arguments may be grouped as for the a and I in the Is command which arguments must be preceded by a character and whether the position of the argument is important y Other commands use a different argument pa
14. mple command line for the we command int newstdin open main cpp O_RDONLY The second argument to the open call is an open this file for reading only flag Now newstdin has a file descriptor for the file main cpp Next you have to replace the contents of the stdin file descriptor with the main cpp file descriptor Use the following sequence of Linux system calls close 0 dup newstdin close newstdin The close 0 call wipes out the contents of file descriptor table entry 0 which is the table entry for stdin The dup newstdin call copies the contents of the newstdin file descriptor table entry to the first empty table entry in the file descriptor table In this case that will be entry 0 Now stdin will use the file main cpp instead of the keyboard for input There are now two file descriptors which are linked to main cpp one from the open and one from the dup The final close newstdin cleans things up so that only the stdin file descriptor is linked to main cpp A similar set of calls is used if you want to redirect stdout to an output file int newstdout open program stats O_WRONLY O_CREAT S_IRWXU S_IRWXG S_IRWXO close 1 dup newstdout close newstdout Make sure that when you use this open call you include ALL of the flags I ve listed here Otherwise the redirection won t work Shell Pipes The pipe is a common IPC mechanism in Linux and other versions of UNIX
15. nto the address space currently being used by the calling process overwriting and discarding the previous program e set the argv array and environment variables for the new program execution and start the process executing at the new program s entry point Various versions of execvp are available at the system call interface differing in the way that parameters are specified for example some use a full pathname for the executable file and others do not wait The wait system call is used by a process to block itself until the kernel signals the process to execute again for example because one of its child processes has terminated When the wait call returns as a result of a child process s terminating the status of the terminated child is returned as a parameter to the calling process Here is the code skeleton that a shell might use to execute a command when these three system calls are used Child if fork 0 execvp fullpathname argv Parent else CIS 3207 Project 2 Page 3 int status 0 wait amp status cout lt lt Child exited with status of lt lt status lt lt endl Putting a Process in the Background In the normal paradigm for executing a command the parent process creates a child process starts it executing the command and then waits until the child process terminates If the and amp operator is used to terminate the command line then the shell is expected to crea
16. s available sometimes hardcoded into the shell for example the single character string or gt When the shell is started it can look up the name of the machine on which it is running and prepend this string to the standard prompt character for example a prompt string such as cis linux1 gt The shell also can be designed to print the current directory as part of the prompt meaning that each time that the user types cd to change to a different directory the prompt string is redefined Once the prompt string is determined the shell prints it to stdout whenever it is ready to accept a command line 2 Get the command line To get a command line the shell performs a blocking keyboard input operation so that the process that executes the shell will be asleep until the user types a command line in response to the prompt Once the user types the command line and terminates it with a NEWLINE n character the command line string is returned to the shell 3 Parse the command The syntax for the command line is trivial The parser begins at the left side of the command line and scans until it sees a whitespace character such as space tab or NEWLINE The first word is the command name and subsequent words are the parameters 4 Find the file The shell provides a set of environment variables for each user These variables are first defined in the user s login file for the bash shell this is home lt username gt bashrc but they can
17. s original UNIX paper Ritchie and Thompson 1974 As described in the previous subsection the shell should accept a command line from the user parse the command line and then invoke the OS to run the specified command with the specified arguments This command line is a request to execute the command program i e the specified file that contains a program including programs that the user wrote Thus a programmer can write an ordinary C program compile it and have the shell execute it just like it was a UNIX command For example suppose that you write a C program in a file named main cpp and then compile and execute it with shell commands such as cis linux1 gt g c l o main o main cpp cis linux1 gt g o main o main cis linux1 gt main For the first command line the shell will find the g command the C compiler in the bin directory and then when the g command is executed pass it the string main cpp The C compiler will translate the C program that is stored in main cpp and write the resulting object file named main o in the current directory The next command links the object file into an executable The third command is simply the name of the file to be executed main without any parameters The shell finds the main file in the current directory specified by and then executes it Consider the following steps that a shell must take to accomplish its job 1 Print a prompt A default prompt string i
18. ssing syntax For example a g compiler command might look like cis linux1 gt g g o deviation S main cpp inout cpp Imath in which the arguments g o deviation S main cpp inout cpp and Imath are all passed to the C compiler g The shell relies on an important convention to accomplish its task The command for the command line is usually the name of a file that contains an executable program for example Is and g files stored in bin on most UNIX style machines In a few cases the command is not a filename but rather a command that is implemented within the shell For example cd change directory is usually implemented within the shell itself rather than in a file in bin Because the vast majority of the commands are implemented in files you can think of the command as actually being a filename in some directory on the machine This means that the shell s job is to 1 find the file CIS 3207 Project 2 Page 1 2 prepare the list of parameters for the command 3 cause the command to be executed using the parameters Many shell programs are used with UNIX variants including the original Bourne shell sh the C shell csh with its additional features over sh the Korn shell and the standard Linux shell bash All have followed a similar set of rules for command line syntax though each has a superset of features Basic UNIX Style Shell Operation The Bourne shell is described in Ritchie and Thompson
19. tains a copy of the open file table that is the child has access to all of the files that the parent has already opened the child inherits the pipes that the parent created To use a pipe it needs only to read and write the proper file descriptors For example suppose that a parent creates a pipe It then can create a child and communicate with it by using a code fragment such as the following int pid int thePipe 2 pipe thePipe pid fork Parent if pid gt 0 char message Hello char messageLen 6 write thePipe 1 message messageLen cout lt lt Parent sent lt lt message lt lt endl Child else if pid 0 char message 6 int messageLen 6 read thePipe 0 message messageLen cout lt lt Child received lt lt message lt lt endl Pipes enable processes to copy information from one address space to another one process to another by using the UNIX file model The pipe read and write ends can be used in most system calls in the same way as a file descriptor for reading and writing a file Further the information written to and read from the pipe is a byte stream UNIX pipes do not explicitly support messages though two processes can establish their own protocol and data structures to provide structured messages Also library routines are available that can be used with a pipe to communicate via messages A process that does not intend to use a pipe
20. te the child process and start it executing on the designated command but not have the parent wait for the child to terminate That is the parent and the child will execute concurrently While the child executes the command the parent prints another prompt to stdout and waits for the user to enter another command line If the user starts several commands each terminated by an amp and each takes a relatively long time to execute then many processes can be running at the same time When a child process is created and starts executing as its own program both the child and the parent expect their stdin stream to come from the user via the keyboard and for their stdout stream to be written to the character terminal display Notice that if multiple child processes are running concurrently and all expect the keyboard to define their stdin stream then the user will not know which child process will receive data on its stdin if data is typed to the keyboard do they all receive the same data Similarly if any of the concurrent processes write characters to stdout those characters will be written to the terminal display wherever the cursor happens to be positioned The kernel makes no provision for giving each child process its own keyboard or terminal unlike a windows system which controls the multiplexing and demultiplexing through explicit user actions 1 0 Redirection A process when created has three default file identifiers stdin stdout
21. the syntax above You should figure out how to get the error from a system call If a system call fails you can find out WHY it failed in and English friendly way by calling strerror with the global variable errno Add this to your code include lt errno h gt include lt string h gt extern int errno make a system call that fails cout lt lt strerror errno lt lt endl DELIVERABLES You will submit your project to the Assignment Project 2 Shell in Blackboard You will include e All the files necessary to compile link and run your program solutions Organize them by the Problem The final program that you produce should allow us to test the solutions to A B and C e An ELECTRONIC document describing how to run the program you created Call this document README TXT All of these files should be placed in a directory called lt username gt project2 Use the tar command to place all the files in a single file called lt username gt project2 tar Attach the resulting tar file with your assignment submission e You are to create a document describing your problem solution and program implementation and program documentation This document should also include your descriptions of testing methods and results of applying the tests Submit this document along with your assignment submission e REMEMBER This is to be your work This is not a unique project that is in the general sense it is a proje
22. then an error should be printed Use the execvp system call to execute the file that contains the command You will also need to become familiar with the Linux fork and wait functions When the command has completed executing your shell should prompt the user for another command Here s an example of the shell running a simple hello world program followed by the Is command MyShell gt main Hello World MyShell gt Is main cpp main o main makefile MyShell gt Attacking Problem A The introduction to this exercise generally describes how a shell behaves It also implicitly provides a plan of attack summarized here This plan describes several debugging versions that you can use for Part A then apply this information to the other parts of the problem as required 1 Organize the shell to initialize variables and then to perform an endless loop until the shell detects an EOF condition When in the shell and you are reading from stdin an EOF condition can be CTRL D character typed Keyword exit typed Develop a very simple version that prints the prompt character and then waits for the user to type a command After it reads the command it should print the command to stdout 2 Refine your simple shell so that it parses the command typed by the user The parser should do the following Convert each distinct string in the command into a C String Store the number of strings in the command in the integer variable argc
23. your program must search each directory in the list specified by the PATH environment variable to find the relative pathname You can figure out if the file exists in a particular directory by trying to open the file If the open fails then the file does not exist Launching the Command The final step in executing the command is to fork a child process to execute the specified command and then to cause the child to execute that command The following code skeleton will accomplish this Child if fork 0 execvp lt fullpathnameofcommand gt argv Parent else CIS 3207 Project 2 Page 8 int status 0 wait amp status cout lt lt Child exited with status of lt lt status lt lt endl Problem B Add functionality to the shell from Problem A so that a user can use the amp operator as a command terminator A command terminated with amp should be executed concurrently with the shell rather than the shell s waiting for the child to terminate before it prompts the user for another command This can be called executing the launched program in the background Problem C 1 Modify your shell program so that the user can redirect the stdin or stdout file descriptors by using the lt and gt characters as filename pre fixes For example MyShell gt main gt out txt MyShell gt cat out txt Hello World MyShell gt will place the output of the Hello World program into the file
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