Creating a Shell or Command Interperter Program
Contents
1. Shell Pipes The pipe is an IPC mechanism in uniprocessor Linux and other versions of UNIX 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 see Section 2 1 5 for details on invoking non blocking read operations Pipes are FIFO first in first out buffers designed with an API that resembles as closely as possible the file I O interface A pipe may contain a system defined maximum number of bytes at any given time usually 4KB As indicated in following figure 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 ee The kernel creates the pipe as a kemel FIFO data structure with two file identifiers In this example code pipeID O is a file pointer an index into the process s open file table to the read end of the pipe and pipeID I 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 contains 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 examp2. r dy edie E E E Introduction A shell or command line interpreter program is a mechanism wit 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 windows 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 window interface the window manager handles the shell tasks Once the shell has initialized its data structures and is ready to start work it clears the display and prints a prompt in the first few character positions Linux machines usually include the machine name For example ACC domanm2 aspen Then the shell waits for the user to type a command terminated by enter or return character in Linux this is represented internally by the NEWLINE character n When the user enters a command it is the shell s job to cause the OS to execute the command embedded in the command line 1 0 Redirection A process when created has three default file identifiers stdin stdout and stderr If it reads from stdin then the data that it receives will be directed from the keyboar
3. should contain enough detail for a beginner to UNIX to use the shell For example you should explain the concepts of the program environment 2 Methods and materials This section tells the reader how and with what the work was done a For this assignment the method would a list of the directories where you found the information and your code 3 Results While this is usually the meat of the report for us it is the output of your code Please explain the output Don t just cut and pasts the screen I want to know that you understand what is being presented 4 Discussion and conclusions For this lab this section is a bit contrived But you can talk about why this information is useful 5 References if used Submission Items Firstly your source code will be assessed and marked Commenting is definitely required in your source code The user manual can be presented in a format of your choice within the limitations of being displayable by a simple Text Editor Again the manual should contain enough detail for a beginner to UNIX to use the shell For example you should explain the concepts of i o redirection the program environment and pipes Grading Rubrics Marking Criteria 100 marks total Submission of required files only 5 marks Warning free compilation and linking of executable with proper name 5 marks Performance of internal commands and aliases 30 marks External command functionality 10 marks Readability
4. suitability amp maintainability of source code and instructions on compiling and executing makefile if needed 20 marks e User manual 30 marks o Description of lab and concepts 20 marks o Overall layout and display of understanding 10 marks
5. Creating a Shell or Command Interperter Program CSCI411 Lab Adapted from Linux Kernel Projects by Gary Nutt and Operating Systems by Tannenbaum Exercise Goal You will learn how to write a LINUX shell program This will give you the opportunity to learn how new child processes are created and how parent processes can follow up ona child process Contents Titrod ctON ereen rene r a E E ET EE E E OE EE R VO Redirection incinin iseina a aa e ai a STEA a EE EEIE a Inter process Communication Call IPC Pipes eeseseeseseseeseesesreeresserseesreesersrrsrresrrsseseeesee Problem Statement 1 Create your own shell eesesseseseeeeseeseeseesrrsrreserssesrresreseseresresseseresresseseest Problem Statement 2 Fil Redirect Onisisisssisisnsns inini EEE EE EE Problem Statement 3 Fork and Wait and Pipes ssseseesseeesesseseeseessreseesersesreesressreeresresseseresrenseese Attacking the Problem ssssisseisrisiiisescisiiiasiisasiissare isise kiisi aiiiar i iii e Siaa Ei aiei Create your own she aera nee ane ee na oe E Ste orn E E E eee rere ne ee Fil Redirection caesarean tunccccns eccthateen E E E eno nie eee Pipe System Cal goss tas accsunceaced aa cceneevaic dead tate ceawencea ced na EEEE aia aaas Ee aa aTi easda taa EdE RSRS Submission Req irements aise s0cneczaachdectadeeeeceaseuasgeuasauaacteaeasandeeeteacesbesueaacupuezaaciderascenverenauesquadans SUMS SUT EMSs r sake aes E E TEE A E E E E T Grading RUDTICS ieren a a uses
6. command line Note that cin will only read up to the first white space You Il need to use getline to ensure the read is not terminated until the NEWLINE n character is read 3 Parse the command It is acceptable to assume there is only one space between the command and any parameters If you read the command in as a string you can find the first blank space to delineate the command and parameter 4 Find the file to execute Prepare the arguments to send to the command argc argv arguments 6 Execute the command mi File Redirection ONE WAY 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 same keyboard and display The shell can change the child s file descriptors so that it reads and writes streams to files rather than to the keyboard and display You can use e the _dup or _dup2 system calls e cout rdbuf function to redirect the cout buffer ANOTHER WAY In repeat command you can just write to the file with standard IO Pipe System Call e pipe is a system call that facilitates inter process communication It opens a pipe which is an area of main memory that is treated as a virtual file The pipe can be used by the creating process as well as al
7. d to the stdin file descriptor Similarly data received from stdout and stderr 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 file name with a left angular brace 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 kiowa gt WC lt main c gt program stats will create a child process to execute the wc command Before it launches the command however it will redirect stdin so that it reads the input stream from the file main c and redirect stdout so that it writes the output stream to the file program stats Inter process Communication Call IPC Pipes Conceptually a pipe is a connection between two processes such that the standard output from one process becomes the standard input of the other process It is possible to have a series of processes arranged in a a pipeline with a pipe between each pair of processes in the series Implementation A pipe can be implemented as a 10k buffer in main memory with 2 pointers one for the FROM process and one for TO process One process cannot read from the buffer until another has written to it
8. he program with a zero return value Use the standard exit function vii help display the user manual You will need to write this for the commands you support For those commands defaulting to the system commands you will need to build a command to call man for that command viii For all other command line inputs relay the command line to the parent shell for execution using the system function Problem Statement 2 File Redirection Create a command called repeat that works like an echo command repeat lt string gt use the echo command to repeat the string listed Repeat lt string gt redirect list the contents of directory lt directory gt If the user types a gt after the string then redirect the string to the file specified Problem Statement 3 Fork and Wait and Pipes External Commands All other command line input is interpreted as program invocation which should be done by the shell forking and execing the programs as its own child processes Create a command called hiMom This will result in a child process being forked The child will then send a message to the parent This message will be sent through pipes Once sent the child can exit The parent will listen for the message and print it out when it arrives When the child exits the parent will close the pipe Attacking the Problem Create your own shell Consider the following steps needed for a shell to accomplish its job 1 Print a prompt 2 Get the
9. l its child processes for reading and writing e One process can write to this virtual file or pipe and another related process can read from it e Ifa process tries to read before something is written to the pipe the process is suspended until something is written e The pipe system call finds the first two available positions in the process s open file table and allocates them for the read and write ends of the pipe Recall that the open system call allocates only one position in the open file table Syntax in a C C program include lt unistd h gt int pip 2 void pipe pip With error checking include lt unistd h gt int pip 2 int result result pipe pip if result 1 perror pipe exit 1 Programming notes e The pipe system call is passed a pointer to the beginning of an array of two integers e It appears in C C as if pipe is passed the name of the array without any brackets e The system call places two integers into this array These integers are the file descriptors of the first two available locations in the open file table e pip 0 the read end of the pipe is a file descriptor used to read from the pipe e pip 1 the write end of the pipe is a file descriptor used to write to the pipe The buffer size for the pipe is fixed Once it is full all further writes are blocked Pipes work on a first in first out basis Brief Example e no error checking no closing of pi
10. le suppose that a parent creates a pipe It then can create a child and communicate with it bv using a code fragment such as the following pipe pipeID if forkO 0 The child process read pipeID O childBuf len Process the message in childBuf else The parent process Send a message to the chi Id write pipeID 1 msgToChild len Pipes enable processes to copy information from one address space 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 Problem Statement 1 Create your own shell Write a small program that loops reading a line from standard input and checks the first word of the input line If the first word is one of the following internal commands or aliases perform the designated task Otherwise use the standard system function to execute the line through the default system shell 1 The command line prompt must be one of your design A suggestion is that it contain the pathname of the current directory The shell must support the following internal commands A built in command is one for which no new process is created but instead the functionality is build directly into the shell itself You should support the following built in commands myprocess Return the current process ID from the getpid function getpid allprocesses Return all current processes using the system function ps sy
11. pe include lt unistd h gt int main int pid pip 2 char instring 20 pipe pip pid fork if pid 0 child sends message to parent write pip 1 Hi Mom 7 write to the pipe else parent receives message from child read pip 0 instring 7 read from the pipe Submission Requirements e Submit your source code Shell script file and makefile if necessary e Submit a lab report This includes from http www columbia edu cu biology faculty mowshowitz howto_guide lab_report html It is extremely important that you understand the need for and format of a good report Scientific work of any sort is useless unless its results can be communicated to others Over the years a particular format or general outline has evolved for the preparation of scientific reports It is this format which you should get accustomed to using First of all a report should have a title In addition a scientific paper generally has five sections 1 Introduction Include a statement of the problem to be investigated why the work was carried out history and theoretical background of the problem a brief statement of the general method of approach to the problem and expected results a Terms if necessary i Shell ii Environment variables b A brief overview of the purpose and description of the lab or programming assignment It should be similar to a readme file The description
12. stem ps cd lt directory gt change the current default directory to lt directory gt If the lt directory gt argument is not present report the current directory If the directory does not exist an appropriate error should be reported This command should also change the PWD environment variable clr clear the screen system clear dir lt directory gt list the contents of directory lt directory gt List the current directory contents Is al lt directory gt you will need to provide some command line parsing capability to extract the target directory for listing Once you have built the replacement command line use the system function to execute it environ list all the environment strings List all the environment strings the environment strings can be accessed from within a program by specifying the POSIX compliant environment list extern char environ as a global variable environ is an array of pointers to the environment strings in the format name value terminated with a NULL pointer A preferred way of passing the environment is by a built in pointer extern char environ NULL terminated array of char main int argc char argv char env environ while env printf s n envt t example output GROUP staff HOME usr user LOGNAME user PATH bin usr bin usr user bin PWD usr user work SHELL bin tcsh USER user vi quit quit the shell Quit from t
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