The Shell or Command Line Interpreter is the fundamental User interface to an Operating System. Your task is to write a simple shell - myshell - that has the following properties: Tasks: The shell must support the following internal commands: cd - Change the current default directory to . If the 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. cls - Clear the screen. iii. dir - List the contents of directory page by page. xcopy to - Copy the folder to . print - Display on the display followed by a new line (multiple spaces/tabs may be reduced to a single space). md - Create the folder . vii. rd - Remove the folder if the folder is empty, should display error message if it is not. quit - Quit the shell. The shell environment should contain shell=/myshell where /myshell is the full path for the shell executable (not a hardwired path back to your directory, but the one from which it was executed). Notes: 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. The programs should be executed with an environment that contains the entry: parent=/myshell where /myshell is as described in 1.ix. above. The shell must be able to take its command line input from a file. That is, if the shell is invoked with a command line argument myshell batchfile then batchfile is assumed to contain a set of command lines for the shell to process. When the end-of-file is reached, the shell should exit. Obviously, if the shell is invoked without a command line argument, it solicits input from the user via a prompt on the display. The shell must support i/o-redirection on either or both stdin and/or stdout. That is, the command line programname arg1 arg2 < inputfile > outputfile will execute the program programname with arguments arg1 and arg2, the stdin FILE stream replaced by inputfile and the stdout FILE stream replaced by outputfile. stdout redirection should also be possible for the internal commands dir, environ, echo, & help. With output redirection, if the redirection character is > then the outputfile is created if it does not exist and truncated if it does. If the redirection token is >> then outputfile is created if it does not exist and appended to if it does. The shell must support background execution of programs. An ampersand (&) at the end of the command line indicates that the shell should return to the command line prompt immediately after launching that program The command line prompt must contain the pathname of the current directory. Note: You can assume that all command line arguments (including the redirection symbols, <, > & >> and the background execution symbol, &) will be delimited from other command line arguments by white space - one or more spaces and/or tabs (see the command line in 3. above).
The Shell or Command Line Interpreter is the fundamental User interface to an
Tasks:
The shell must support the following internal commands:
- cd <directory> - Change the current default directory to
<directory>. If the <directory> 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.
- cls - Clear the screen.
iii. dir <directory> - List the contents of directory <directory> page
by page.
- xcopy <source> to <destination> - Copy the <source> folder to <destination>.
- print <comment> - Display <comment> on the display followed by a new line (multiple spaces/tabs may be reduced to a single space).
- md <directory> - Create the folder <directory>.
vii. rd <directory> - Remove the folder <directory> if the folder is
empty, should display error message if it is not.
- quit - Quit the shell.
- The shell environment should contain shell=<pathname>/myshell where <pathname>/myshell is the full path for the shell executable (not a hardwired path back to your directory, but the one from which it was executed).
Notes:
- 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. The programs should be executed with an environment that contains the entry: parent=<pathname>/myshell where <pathname>/myshell is as described in 1.ix. above.
- The shell must be able to take its command line input from a file. That
is, if the shell is invoked with a command line argument
myshell batchfile
then batchfile is assumed to contain a set of command lines for the shell to process. When the end-of-file is reached, the shell should exit. Obviously, if the shell is invoked without a command line argument, it solicits input from the user via a prompt on the display.
- The shell must support i/o-redirection on either or both stdin and/or
stdout. That is, the command line
programname arg1 arg2 < inputfile > outputfile
will execute the program programname with arguments arg1 and arg2, the stdin FILE stream replaced by inputfile and the stdout FILE stream replaced by outputfile.
stdout redirection should also be possible for the internal commands
dir, environ, echo, & help.
With output redirection, if the redirection character is > then the outputfile is created if it does not exist and truncated if it does. If the redirection token is >> then outputfile is created if it does not exist and appended to if it does.
- The shell must support background execution of programs. An ampersand (&) at
the end of the command line indicates that the shell should return to the
command line prompt immediately after launching that program
- The command line prompt must contain the pathname of the current directory.
Note: You can assume that all command line arguments (including the redirection symbols, <, > & >> and the background execution symbol, &) will be delimited from other command line arguments by white space - one or more spaces and/or tabs (see the command line in 3. above).
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