CS 214 Spring 2023
c++代做 | Algorithm | oop代做 | 代做project | assignment代做 | 代写os – 该题目是一个常规的os通过c++编写shell的练习题目代写, 涵盖了report/Algorithm/oop/ios等程序代做方面, 该题目是值得借鉴的assignment代写的题目
project II: My shell
For this project, you and your partner will design and implement a simple command-line shell, similar tobashorzsh. Your program,mysh, will provideinteractiveandbatchmodes, both of which will read and interpret a sequence of commands. This project will provide experience of
- Posix (unbuffered) stream IO
- Reading and changing the working directory
- Spawning child processes and obtaining their exit status
- Use ofdup2()andpipe()
- Reading the contents of a directory
Extensions In addition to the requirements that all projects must satisfy, this assignment includes
fiveextensions(see section 3). For full credit,your project must implement two of these
extensions.
1 Overview
Your programmyshtakes up to one argument. When given one argument, it will run inbatch mode. When given no arguments, it will run ininteractive mode. For full credit, your program must have one input l oop and command parsing Algorithm that works for both modes.
Batch mode When called with an argument,myshwill open the specified file and interpret its
contents as sequence of commands (see section 2). The command are given by lines of text, separated
by newlines.myshwill execute each command as soon as it is complete, before proceeding to execute
the next command. myshterminates once it reaches the end of the input file or encounters the
commandexit.
Interactive mode When called with no arguments,myshwill read commands from standard
input. Before reading a command,myshwill write apromptto standard output. After executing
the command,myshwill print a new prompt and read the next command. To ensure that prompts
are printed appropriately,myshmust be careful not to callread()again if a newline character has
already been entered.
myshterminates once it reaches the end of the input file or encounters the commandexit.
In this mode,myshshould print a greeting before the first prompt and a message when terminating
normally. The format of these are left to you.
Prompt format When running in interactive mode,myshwill print a prompt to indicate that it is ready to read input. The prompt is normally the string mysh> (note the trailing space). If the last command failed (meaning its exit status was non-zero), the prompt is the string !mysh> (that is, it is preceded by an exclamation point).
Usage Batch mode:
$ cat myscript.sh
echo hello
$ ./mysh myscript.sh
hello
$
Interactive mode:
$ ./mysh Welcome to my shell! mysh> cd subdir mysh> echo hello hello mysh> cd subsubdir mysh> pwd /current/path/subdir/subsubdir mysh> cd directory_that_does_not_exist cd: No such file or directory !mysh> cd ../.. /current/path$ exit mysh: exiting $
Note the exclamation point after a failed command.
2 Command format
Amyshcommand comprises one or moretokens. Tokens are sequences of non-whitespace characters, and are usually separated by whitespace. The exceptions are|,<, and>, which are considered tokens by themselves. Thus, a stringfoo bar<bazconsists of four tokens: foo, bar, <, and baz.
The vertical bar (|) is used to combine sub-commands (see section 2.4). The first token of a command (or sub-command) is thecommand name, which usually identifies a program to execute (see section 2.1). The < and > tokens introduce file redirection (see section 2.3). Tokens containing an asterisk (*) stand for a sequence of file names matching the given pattern (see section 2.2). The commands tokens, excluding file redirection sequences and with wildcards replaced, become the arguments to the command (usually a subprocesses). For example, this command
foo bar < baz | quux *.txt > spam
executed in a directory containing filesbacon.txtandeggs.txtwould result in the execution of
two programs,fooandquux, where:
- fooreceives the arguments foo and bar
- quuxreceives the arguments quux, bacon.txt, and eggs.txt
- foohas standard input set to the file baz
- quuxhas standard output set to the file spam
- foohas standard output sent to a pipe andquuxhas standard input set to the other end of the same pipe
There is no intrinsic limit to the length of a command.
2.1 Command name
The first token of a command (or sub-command) indicates the program or built-in operation to execute.
Built-in commands The commandscdandpwdare provided bymyshdirectly. Their purposes
are to change and print the current working directory, respectively. The standard library includes
functionschdir()andgetcwd()for these purposes.
cdtakes a single argument, a path indicating the new working directory.
pwdtakes no arguments.
In the event of an error (such as changing to a non-existant or inaccessible directory),mysh
should print an error message (preferably usingperror()orstrerror()) to standard error and set
the last exit status to 1.
Path names If the first token contains a slash (/), it represents a path to an executable program.
myshshould usefork()to create a subprocess,execv()to execute the specified program (providing
arguments), andwait()to obtain the programs exit status.
If the specified program cannot be executing,myshshould print an error message and set the
last exit status to 1.
Bare names If the command name is not a built-in and is not a path,myshwill check the following
directories for a file with the specified name:
1./usr/local/sbin
2./usr/local/bin
3./usr/sbin
4./usr/bin
5./sbin
6./bin
The directories must be searched in the order shown above. It is recommended to usestat()to check for the existence of a file, rather than traversing the directory itself. If no file is found, or if a file is found but cannot be executed,myshshould print an error message and set the last exit status to 1. (If a file cannot be executed, do NOT continue looking in the remaining directories.)
2.2 Wildcards
A token containing an asterisk () is awildcard(orglob), representing a set of files whose names match a pattern. We allow a single asterisk in a file name or in the last section of a path name. Any file in the specified directory whose name begins with the characters before the asterisk and ends with the characters after the asterisk is considered to match. In other words, any name where we can replace a sequence of zero or more characters with an asterisk to obtain the pattern is considered a match. Thus,foobarmatches file names in the working directory that begin with foo and end with bar. Similarly,baz/foo*barmatches file names in the subdirectory baz that begin with foo and end with bar. When a command includes a wildcard token, it will be replaced in the argument list by the list of names matching the pattern. If no names match the pattern,myshshould pass the token to the command unchanged.
Hidden files Patterns that begin with an asterisk, such as*.txt, will not match names that
begin with a period.
2.3 Redirection
The tokens < and > are used to specify files for a program to use as standard input and standard output, respectively. The token immediately following the < or > is understood as a path to the file, and is not included in the argument list for the program. Normally, a child process will use the same standard input and output as its parent. When using file redirection,myshshould open the specified file in the appropriate mode and usedup2()in the child process to redefine file 0 or 1 before callingexecv().
When redirecting output, the file should be created if it does not exist or truncated if it does
exist. Use mode 0640 (S_IRUSR|S_IWUSR|S_IRGRP) when creating.
Ifmyshis unable to open the file in the requested mode, it should report an error and set the
last exit status to 1.
2.4 Pipes
A pipe connects standard input from one program to the standard output from another, allowing data to flow from one program to the next.myshallows for a single pipe connecting two processes. Before starting the child processes, usepipe()to create a pipe, and then usedup2()to set standard output of the first process to the write end of the pipe and standard input of the second process to the read end of the pipe. If the pipe cannot be created,myshshould print an error message and set the last exit status to
- Otherwise, the exit status of the command is the exit status of the last sub-command.
3 Extensions
To receive full credit, your project must implement at least two (2) of the extensions described below. In your README, indicate which extensions you are including.
3.1 Escape sequences
Extend the command syntax to allowescapingof special characters. The backslash character (\)
removes the special handling of the character following it, allowing it to be treated as a regular
token character.
For example, the command
foo bar\ baz\<qu\ux\\>spam
contains four tokens: foo, bar baz<quux\, >, and spam.
Notably, a backslash followed by a space is not considered a token boundary and a backslash
followed by another backslash does not affect the next character.
The backslash can be used to create tokens containing spaces, <, >, |, *, and \ characters.
A backslash followed by a newline disables the newline, but does not insert a newline character
into the token. Instead, both characters are removed.
3.2 Home directory
The environment variable HOME contains the users home directory. This is obtained using getenv("HOME"). Whencdis called with no arguments, it changes the working directory to the home directory. Tokens beginning with~/are interpreted as relative to the home directory. The~is replaced with the home directory.
3.3 Directory wildcards
Asterisks may occur in any segment of a path. For example,/.creferences files ending with.c in any subdirectory of the working directory (excluding files and subdirectories that begin with a period). You may allow more than one asterisk within a path segment, but this is not required.
3.4 Multiple pipes
Three or more sub-commands can be linked using pipes. For example,foo | bar | bazsends the output of foo to bar, and the output of bar to baz. There is no intrinsic limit to the number of sub-commands in a pipeline.
3.5 Combining with&&and||
The character sequences&&and||are treated as special tokens used to combine sub-commands, similar to |. Unlike a pipe, these tokens run commands sequentially. When using&&, the second command is executed when the first command succeeds (returns exit status 0). When using||, the second command is executed when the first command fails (returns exit status other than 0). For example, this command gcc foo.c && echo Success
will executegccfirst, and then executeechoifgccsucceeds. The pipe has higher precedence than&&and||, which have equal precedence. The exit status of the command is the exit status of the last sub-command to execute. Note thatfoo && bar || bazwill execute bar if foo succeeds, and bar if either foo or bar fails. In contrast,foo || bar && bazwill execute bar if foo fails, and baz if either foo or bar succeeds.
4 Submission
A system will be provided for you to declare your partnership prior to submission, details forthcoming. Determine in advance which partner will be responsible for submitting the completed assignment. Communication and coordination is key to a successful project! Submit a Tar archive containing:
- Your source code file(s), including testing code
- Your make file
- Your README and test plan
- Any test inputs used by your testing process Your README should be a plain text document containing the names and NetIDs of both partners and an indication of which extensions you implemented. Your test plan should be part of the README or a separate PDF document describing how your testing strategy and test cases. Describe the scenar ios you considered it important to check and how you performed those checks. Note that having a good test suite is an excellent way to detect errors that have been introduced or reintroduced into your code.