1 The Var Shell (vsh)

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1 CS 470G Project 1 The Var Shell Due Date: February 7, 2011 In this assignment, you will write a shell that allows the user to interactively execute Unix programs. Your shell, called the Var Shell (vsh), will read commands typed by the user and then execute them. 1 The Var Shell (vsh) You will write a new Unix shell, similar to existing shells such as csh or ksh. However, it will have a slightly different command set and a slightly different syntax for specifying commands. It will lack some of the functionality found in ksh, bash, csh, tcsh, etc. Your shell will accept two types of commands: Built-in commands and User-Progam commands Built-in Commands: cd directory name This command changes the current directory to directory name. You need not handle cd with no arguments (to take you back to the home directory). See getwd(3) and chdir(2) system calls. setvar variable name string This command sets the variable specified by variable name to string. Variable names start with a $. Variable names are case sensitive. There are two special/reserved variable names. The $PATH variable is used to store the search path for executables. The $ShowParseOutput variable can be set to 1 to print out the results of parsing the command line, or 0 to not print the results of parsing. exit Gracefully exit the shell. User-Program Commands: cmd [arg]* cmd is the filename of the program the user wants to execute. The special shell variable PATH contains a list of directories where you should look to find the file to be executed. The filename may also be specified as a full pathname. cmd is followed by 0 or more arguments (args). Arguments may be variable names (i.e., prefixed with a $). The variable names in commands should be replaced by their values set via the setvar command before being executed. Example Commands: setvar $PATH "/bin:/usr/bin" setvar $Myfile "/home/me/file_a" 1

2 setvar $Filebase "paper" setvar $ShowParseOutput 1 cd /usr/bin cd./bin cd../misc/oldstuff cd /usr cd src/proj1 /usr/bin/wc -l $Myfile wc -l f1 f2 f3 cat $Filebase.txt /bin/ls ls -l -F -g -s /tmp exit 2 Scanning and Parsing The first step is to read and analyze the command line typed by the user. For this purpose you will need to write a scanner routine and a parser routine. 2.1 The Scanner Your shell will read an input line and break it into a command name and its arguments. Assume that inputlines are less than 256 characters long. In order to read the input line, you should write a scanner routine. A scanner divides the stream of input characters into a sequence of tokens, where a token refers to a sequence of characters that are treated as a logical unit. The scanner does not have any knowledge about the meaning of individual tokens, it only knows how to group characters into tokens. Specifically, the scanner should return the following tokens: word any sequence of characters (excluding meta-characters - see below) containing no white space (where white space is any combination of tabs or space). variable name a word that starts with $. string a sequence of one or more characters enclosed within double quotes. The use of double quotes is the only way in which white space or meta characters can be included as part of a token. The double quotes are not part of the token and should be removed during the scanning phase. For simplicity, you may assume that there will be no quotes within quotes. end-of-line an indication that the current line has ended (e.g., a newline character was encountered). exit the exit token is just the word exit setvar the setvar token is just the word setvar cd the cd token is just the word cd 2

3 meta characters characters that have special meaning to the shell. Each meta character is treated as a single token. Meta characters can only appear inside of another token when enclosed within double quotes. Your scanner should recognize the following meta characters:, %, >, <, and &. While you need to recognize these characters as having a special meaning, you do not need to do anything special with them in this project. When it comes to parsing them, you can treat them as if they were word tokens. Meta characters behave like white space in that they separate other tokens, and they do not need to be separated from words or other meta characters by white space. For example, the (nonsense) line: /bin/fgrep "Student Chapter" /etc/passwd cat sort>out1&wc</etc/passwd > out2 would be scanned into the tokens: Token Type = word Token = /bin/fgrep Token Type = string Token = Student Chapter Token Type = word Token = /etc/passwd Token Type = metachar Token = Token Type = word Token = cat Token Type = metachar Token = Token Type = word Token = sort Token Type = metachar Token = > Token Type = word Token = out1 Token Type = metachar Token = & Token Type = word Token = wc Token Type = metachar Token = < Token Type = word Token = /etc/passwd Token Type = metachar Token = > Token Type = word Token = out2 Token Type = end-of-line Token = EOL 2.2 The Parser A second module, called a parser, will take the sequence of tokens produced by the scanner and determine how each token is being used. For example, it might determine that a word token is being used as a command name (cmd), or that a string is being used as an argument to a command. Once the parser sees how the tokens are being used, it can determine whether the input line has a legal syntax. The legal syntax for commands was given in section 1. Note that in the syntax specification, a cmd: must be a word token, an arg: can be a word token, a variable name token, or a string token, and a directory name: must be a word token For built-in commands like setvar $PATH "/bin:/usr/bin", the output from the scanning and parsing routines (if ShowParseOutput = 1) would be: 3

4 Token Type = word Token = setvar Usage = setvar Token Type = variable Token = $PATH Usage = variable_name Token Type = string Token = /bin:/usr/bin Usage = string Token Type = end-of-line Token = EOL Usage = EOL For user-program commands like wc -l f1 f2 f3 the output from the scanning and parsing routines (if ShowParseOutput = 1) would be: Token Type = word Token = wc Usage = cmd Token Type = word Token = -l Usage = arg 1 Token Type = word Token = f1 Usage = arg 2 Token Type = word Token = f2 Usage = arg 3 Token Type = word Token = f3 Usage = arg 4 Token Type = end-of-line Token = EOL Usage = EOL For the nonsensical example given earlier, the output would be: Token Type = word Token = /bin/fgrep Usage = cmd Token Type = string Token = Student Chapter Usage = arg 1 Token Type = word Token = /etc/passwd Usage = arg 2 Token Type = metachar Token = Usage = arg 3 Token Type = word Token = cat Usage = arg 4 Token Type = metachar Token = Usage = arg 5 Token Type = word Token = sort Usage = arg 6 Token Type = metachar Token = > Usage = arg 7 Token Type = word Token = out1 Usage = arg 8 Token Type = metachar Token = & Usage = arg 9 Token Type = word Token = wc Usage = arg 10 Token Type = metachar Token = < Usage = arg 11 Token Type = word Token = /etc/passwd Usage = arg 12 Token Type = metachar Token = > Usage = arg 13 Token Type = word Token = out2 Usage = arg 14 3 Details You should write procedures to read from standard input and scan/parse each line into tokens. After you have parsed the command line you should output the results of your parsing (if Show- ParseOutput = 1). If the syntax of the line is correct, your shell should fork and exec a process to execute the command given on the command line. It will then wait for the child to exit, report any errors detected, and repeat the process for the next command line. Begin the project by creating a scanner and a (simple) parser. The parser routine calls the scanner repeatedly, until the scanner returns an end-of-line indication. Upon end-of-line, the parser takes the sequence of tokens and executes the corresponding command. Here are some suggestions: Have the scanner return two items: a pointer to the actual token, and a type field that indicates whether the token is a word, a meta character, end-of-line, etc. As the parser calls the scanner, 4

5 it builds a list of null-terminated tokens, with the first item in the list corresponding the the first token, etc. When the scanner returns a end-of-line indication, the parser will find the name of the command to execute in the first token in the list, while the remaining tokens are the command s arguments. If the command specified is a built-in command, your shell program will execute it directly. For other commands, the shell should use the fork system call to create a child process that in turn calls execv or execl to execute the given command. The shell should simply wait for the child to finish. When the command completes, the shell fetches the next command line and the cycle repeats. Naturally, your shell should handle errors gracefully. For instance, it should print an informative message if the command line is malformed or if the command does not exist. In addition, the shell should not die, crash or exit (except with a exit command). If an error is detected, a message should be printed, the current command ignored, and the prompt displayed for the next command. To make your implementation easier, you should also accept control-d (end-of-file) at the input to be same as the command exit. That is, if a user types in cntl-d at the input, your shell will do exactly the same thing it does when it receives the built-in command exit. 4 Can I use Lex and Yacc? IfyouknowLexandYacc, youareencouragedtousethemtowritethescannerandparser. Forthose of you who are unfamiliar with Lex and Yacc, Lex is a program that will write a scanner function for you automatically. Yacc is a program that will write a parser function for you automatically. However, to use these programs requires an understanding of regular expressions and grammars. If you have not use Lex and Yacc before, you are probably better off not using them. In Linux, lex is called flex, and yacc is called bison. 5 Testing Your Program Clearly, you can test your program by typing commands interactively at the prompt. However, an easier way to test your program is to have it read a file of commands piped in from standard input. For example, if you have a file named testfile that contained: cd directory_1 /bin/cat file1 file2 file3 /bin/ls /bin/wc file1 you could test all of these commands simply by typing: vsh < testfile Your program must be able to execute commands fed to it this way. This is how we will test your program. 6 Helpful Manual Pages The following unix routines may be helpful in the assignment: fork(2), execv(2), execve(2), chdir(2), getpwnam(3c), malloc(3c), perror(3c), exit(2), wait(2), kill(2), getcwd(3c), and string(3c) You can 5

6 view these manual pages with the UNIX man command. Use the -s option to specify the section of the UNIX manual (shown in paranthesis above). For example, man fork or man -s 2 chdir. 7 What to Submit Your code must compile and run on the multilab machines or you will not receive credit. You will submit all your code plus a documentation file. You should not submit.o files or other binary files. To create your submission, tar and compress all files that you are submitting (e.g., tar czf proj1.tgz projectdirectory). You should submit the following: README file - listing all the files you think you are submitting Documentation file - brief description of your projet including the algorithms you used. Your external documentation should also include a description of any special features or limitations of your shell. This should be a simple text file. Do not submit MS Word, postscript, or PDF files. Makefile - we will type make and it better compile all your C/C++ files all your header files Once you create the tar file, go to to upload your code. Once logged in, you will see a Student menu item which you should click to see a list of your classes that are using the submission system. Select the CS470 class. Then click on the Assignment header to see a list of the assignments that are due. You can then click under the Submit Assignment header to submit your PDF file. Browse to, or type in, the name of your compressed tar file and click the Submit button to upload your.tgz file for grading. If the upload succeeds you will be given a confirmation number. Note, you may upload your.tgz file as many times as you like. Each submission will overwrite the previous submission, so we will only have a copy of your last submission. Also note that the system timestamps your submission with the date/time of the last submission. The system does allow late submissions (which will be charged a late penalty). 6

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