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UWM FA2023 CS537 – Assignment 3 9.19.2022
[email protected] FA2023 CS537 Assignment 3 1 prompt> ./wis-grep foo bar.txt this line has foo in it so does this foolish line; do you see where? even this line, having barfood in it, will be printed. Unix Utilities In Assignment 3 you will build a set of Unix utilities to execute on your qemu-gdb hosted xv6 VM. These versions will be simpler versions of common commands like ls, cat, grep, etc. They’re simpler because the Unix originals are considerably more functional and complicated. We will use different names to avoid confusion: namely wis-grep, wis-tar, wis-untar. Learning Objectives: To re-familiarize yourself with the C programming language, with a Unix shell / terminal / command-line, and to learn how UNIX utilities are implemented. Summary of what gets turned in via Canvas: • Each should compile successfully when compiled with the -Wall and -Werror flags. • Each should (hopefully) pass tests we’ll supply. • Include a single README.md for all the files describing the implementation. wis-grep The first utility you will build is called wis-grep, a variant of the UNIX grep tool. This tool looks through a file, line by line, trying to find a user-specified search term in the line. If a line has the word within it, the line is printed out, otherwise it is not. Here is how a user would look for the term foo in the file bar.txt: Details Your program wis-grep is always passed a search term and zero or more files to grep through (thus, more than one is possible). It should go through each line and see if the search term is in it; if so, the line should be printed, and if not, the line should be skipped. The matching is case sensitive. Thus, if searching for foo, lines with Foo will not match. Lines can be arbitrarily long (that is, you may see many characters before you encounter a newline character (\n). wis-grep should work as expected even with very long strings. For this, you might want to look into the getline() library call. If wis-grep is passed no command-line arguments, it should print “wis-grep: searchterm [file …] (followed by a newline) and exit with status 1. If wis-grep encounters a file that it cannot open, it should print “wis-grep: cannot open file” (followed by a newline) and exit with status 1. If a search term, but no file, is specified, wis-grep should work, but instead of reading from a file, it should read from standard input. Doing so is easy, because the file stream stdin is already open; you can use fgets() (or similar routines) to read from it. For simplicity, if passed the empty string as a search string, wis-grep can either match NO lines or match ALL lines, both are acceptable. wis-tar & wis-untar The next two utilities to build will also be simpler versions of Unix tar and untar, commonly used utilities to combine (or expand) a collection of files into one file (one file into a collection of files). This functionality is useful in several scenarios UWM FA2023 CS537 – Assignment 3 9.19.2022
[email protected] FA2023 CS537 Assignment 3 2 prompt> echo abcd > a.txt # creates the file a.txt prompt> echo efgh > b.txt # creates the file b.txt prompt> ./wis-tar e.g., offering a single file to download for software. If you’ve heard the phrase tarball, that comes from using tar! The input to your wis-tar program will be the name of the tar file followed by a list of files that need to be archived (Fun Fact: The name tar comes from tape archives!). Example: wis-tar format For this assignment, we will use a simple file format for the tar file: Here are a few points to consider: 1. You can assume that the files provided as an input exist in the directory where the program is run from (no need to handle ways to store pesky path names). 2. You can also assume the files provided as inputs only contain ASCII characters. Details If fewer than two arguments are supplied to your program, you should print “wis-tar: tar-file file […]” (followed by a newline) and exit with status 1. If any of the input files that should be a part of the tar file are not found, you should print “wis-tar: cannot open file” (followed by a newline) and with exit status 1. If a tar-file of the same name already exists, you can overwrite it with the new contents specified. If any of the input file names are longer than 100 characters, you can only use the first 100 characters as the filename to store in the tar format. If any of the input file names are shorter than 100 characters, you can pad the filename such that it uses 100 bytes. For example, if the file name was “a.txt”, that only has 5 characters. In this case you can append 95 NULL i.e., \0 characters to make the name use 100 bytes in the tar file. (Remember '\0' is not the same as '0'. The first one represents NULL while the second one represents the character 0!) Here’s a complete example to make sure we understand the format and how to understand the contents of a valid tar file. In the following example we first create a text file which contains the string hey. Its size is therefore 3. We run wis-tar to create a.tar as shown below. Finally, we print the contents of a.tar using hexdump, a utility to print the contents of a binary file. The comments to the right explain its output. The bytes are represented in hexadecimal format, so that a table like this (https://www.ascii-code.com/) helps you look up the ASCII values for strings. Try to decode the contents based on the comments on the right! As you can see below, the fields in the tar file are packed together with no new lines or other separators between them. file1 name [100 bytes in ASCII] file1 size [8 bytes as binary] file1 contents [in ASCII] file2 name [100 bytes] file2 size [8 bytes] file2 contents [in ASCII] ... https://www.ascii-code.com/ UWM FA2023 CS537 – Assignment 3 9.19.2022
[email protected] FA2023 CS537 Assignment 3 3 ➜ p1a cat a.txt hey% ➜ p1a ./wis-tar a.tar a.txt ➜ p1a hexdump -v a.tar Note that this is not in ASCII! ----------------------------------------------------> The last three bytes contain the string hey, the cont ents of the file prompt> ./wis-untar test.tar prompt> ls # should contain a.txt and b.txt 0000000 2e61 7874 0074 0000 0000 0000 0000 0000 --> The first five bytes here contain the file name a.txt 0000010 0000 0000 0000 0000 0000 0000 0000 0000 0000020 0000 0000 0000 0000 0000 0000 0000 0000 0000030 0000 0000 0000 0000 0000 0000 0000 0000 0000040 0000 0000 0000 0000 0000 0000 0000 0000 0000050 0000 0000 0000 0000 0000 0000 0000 0000 ---> We have padded using \0 till we hit 100 bytes 0000060 0000 0000 0003 0000 0000 0000 6568 0079 ---> The byte containing 03 indicates the file size is 3. wis-untar As the name suggests, the wis-untar program will do the reverse of the wis-tar program. Here you will take in the name of an archive created using wis-tar and the program will create files corresponding to those in the archive in the same directory where the program is run from. For example Details If no arguments are provided, then you should print “wis-untar: tar-file” (followed by a newline) and exit with status 1. If the tar filename provided on the command line does not exist, you should print “wis-untar: cannot open file” (followed by a newline) and exit with status 1. To simplify this assignment, you can assume that if a tar file is provided and if the file exists, it matches the wis-tar format described above. While extracting files, if there already exist files with the same name as those in the archive, you can overwrite them. Note For this and all subsequent C programming exercises you are to document your source code using standard comment blocks delineated by // or /* … */. The beginning of each program should contain a “header” containing, at minimum // program name // program author // program origination date // program last revision date In line accompanying the c source code should be sufficient comments for another programmer to understand the programs flow, any expected “side effects” and key assumptions. Unix Utilities wis-grep wis-tar & wis-untar wis-tar format wis-untar