Reference no: EM131522

Problem 1. Create a simple shell. Basically your shell should read the line from standard input, parse the line with command and arguments, and operate the command with arguments. You'll need the fork() and exec*() system calls.

o The shell should search the command in your current working directory first. If not found, it should search the directories in the shell's pathname list which will be explained in the part 2.

o You are not allowed to use system(), as it just invokes the system's /bin/sh shell to do all the work. You are not allowed to utilize execlp() or execvp() in the standard library, because our shell has its own path variable, explained in part 2.

o You can assume that arguments are separated by white spaces. You do not have to deal with special characters such as ', ", \, etc; thus, you need to handle the redirection operators (<, >, and 2>) and the pipeline operator (|).

o You will assume that the command line a user types is no longer than 4096 bytes. Therefore, you should not assume that there is a restriction on the number of arguments to a given command.

o The executable file for your shell could be named myshell, for the TAs' grading pleasure.

Problem 2. Implement the following built-in commands.

o  exit: users can exit from the shell with the exit command.

o  cd: cd is a command to change directories. You will need to invoke the chdir system call.

o path: path is a command not only to show the current pathname list, but also to append or remove several pathnames. In your shell implementation, you may keep an variable or data structure to deal with pathname list (referred to as "path" variable below). This list helps searching for executables when users enter specific commands.

-  path (without arguments) displays the pathnames currently set. It should show pathnames separated by colons. ex) "/bin:/usr/bin"

-  path + /foo/bar appends the pathname to the "path" variable. Only one pathname will be given to each invocation of the path command. It's okay to add a pathname to the "path" variable even if it already contains the same pathname, i.e., duplicates in the "path" variable are fine.

-  path - /foo/bar removes the pathname from the "path" variable. It should remove all duplicates of the given pathname. Only one pathname will be given to each invocation of the path command.

Problem 3. (Extend your shell with I/O redirection (<, >, and 2>), as described above.

o stdin maps to file descriptor 0, stdout to 1, and stderr to 2.

o You'll need the open(), close(), and dup2() system calls.

o You can assume that there is no space inside the stderr redirection operator "2>"

o You should handle cases when all three streams are redirected, such as cmd < in.txt > out.txt 2> err.txt (not necessarily in this order).

o You don't need to handle other forms of redirections. For example, you don't need to handle the following redirections : cmd 1> filename (another way to redirect standard output)), cmd 2>&1 (redirect standard error to standard output). (Most other shells do handle the above cases.)

o Your shell do not need to handle I/O redirection for built-in commands (cd, exit, path).

Problem 4. Extend your shell with pipeline (|), as described above.

o You'll need the pipe system call.

o There is no restriction on the depth of a pipeline. That is, your solution should handle arbitrary number of commands chained together with operator |

o You should handle combinations of redirection and pipeline when they can be combined. An example is: sort < file.txt | uniq. However, if there is a conflict between redirection and pipeline, i.e. one file descriptor gets associated with two things, you should report a syntax error. An example is: ls > 1.txt | grep FOO; we cannot redirect the stdout of ls to both 1.txt and grep at the same time.

o Your shell do not need to handle built-in commands (cd, exit, path) in pipeline.

o Your shell should not accept new user commands while the pipeline is still running.