Description
Your next project will tie together many of the Operating System/Unix concepts that we have studied so far this quarter. For the project, you will eventually write a Unix program called the Betterthan Average Shell, bash, which is a simple Unix command interpreter. To simplify the design of your shell, this first half of the project only deals with the input parsing aspects of the project. This semester, we’re going to use the standard Unix tools Flex and Bison to build the input parser for the shell. Flex generates a lexical analyzer that breaks an input stream into tokens. Bison understands language grammars. Once you’ve specified the grammar for your bash, Bison will do most of the parsing for you. We’ll talk about both of these programs extensively in class. Shell Grammar To use Lex and Bison, you need to understand the grammar that they need to read. For your shell, the command input must be in the following format: cmd (arg)* (<infile)? (2>(>)?errfile)?)* (| cmd (arg)* (2>(>)?errfile)?)* (>(>)? outfile)? Input Tokens Your shell understands the following input tokens: | The PIPE symbol, forward stdout to next cmd < stdin redirection > stdout redirection (create) >> stdout redirection (append) 2> stderr redirection (create) 2>> stderr redirection (append) WORD Any sequence of characters not already defined as tokens (separated by white space). Alternately, a WORD can be any sequence of characters between double quotes Example In the github repository for this assignment, under ”examples”, there’s an example lex/yacc/C program called arith that understands a grammar very similar to what you’ll need for your shell project. We’ll cover that example in great detail in class. Headstart In the github repository for this assignment, under ”headstart”, I’ve placed a skeleton bash “headstart” version with a Makefile stub header files, and etc. You should start from that. It should compile and ”run” a little, but you’ll need to finish a lot of it before it will do much. Requirements Your program must work for any reasonable number of arguments and must give meaningful error messages. Your shell should terminate neatly and quietly when it reaches the end of file. You must use Flex for lexical analysis and Bison for parsing. Turnin You must use the class github archive to submit all of your source files, along with a Makefile that generates a program called bash that can read input from standard input. Example Input and Output For example, if the input to your program looks like this: echo Hello World /bin/cat < /etc/motd /bin/cat > outfile echo “this is a test” | cat gcc -c -Wall file.c 2>> ERRS cat < infile | tr “a-z” “A-Z” | cat 2>caterrs | wc -l > out then the output to your program should look something like this: ========== line 1 ================= Command name: ’echo’ argv[0]: ’echo’ argv[1]: ’Hello’ argv[2]: ’World’ stdin: ’’ stdout: ’’ stderr: ’’ ========== line 2 ================= Command name: ’/bin/cat’ argv[0]: ’/bin/cat’ stdin: ’/etc/motd’ stdout: ’’ stderr: ’’ ========== line 3 ================= Command name: ’/bin/cat’ argv[0]: ’/bin/cat’ stdin: ’’ stdout: ’outfile’ stderr: ’’ ========== line 4 ================= Command name: ’echo’ argv[0]: ’echo’ argv[1]: ’this is a test’ stdin: ’’ stdout: PIPE1 stderr: ’’ Command name: ’cat’ argv[0]: ’cat’ stdin: PIPE1 stdout: ’’ stderr: ’’ ========== line 5 ================= Command name: ’gcc’ argv[0]: ’gcc’ argv[1]: ’-c’ argv[2]: ’-Wall’ argv[3]: ’file.c’ stdin: ’’ stdout: ’’ stderr: ’ERRS’ (append) ========== line 6 ================= Command name: ’cat’ argv[0]: ’cat’ stdin: ’infile’ stdout: PIPE1 stderr: ’’ Command name: ’tr’ argv[0]: ’tr’ argv[1]: ’a-z’ argv[2]: ’A-Z’ stdin: PIPE1 stdout: PIPE2 stderr: ’’ Command name: ’cat’ argv[0]: ’cat’ stdin: PIPE2 stdout: PIPE3 stderr: ’caterrs’ Command name: ’wc’ argv[0]: ’wc’ argv[1]: ’-l’ stdin: PIPE3 stdout: ’out’ stderr: ’’
