Student Information

Nick Grifasi | nickgrifasi@vt.edu Inhan Park | inhanp@vt.edu

How to execute the shell

run './cush' in the command line

Important notes

Description of Base Functionality

builtIns() provides support for 'exit', 'fg', 'jobs', 'stop', 'bg', and 'kill'. Each of these built-in commands have their own method to support the argument.

'jobs' - Loops through the list of jobs, printing the job as it iterates. 'fg' - Obtain current job from jid, sends SIGCONT to run any stopped jobs. Sends the job to the foreground. Gives terminal control to the job. Waits for further signals and returns terminal state to the shell. Otherwise, prints a fg continue error. 'bg' - Obtain current job from jid, sends SIGCONT to run any stopped jobs. Sends the job to the background. Otherwise, prints a bg continue error. 'stop' - Obtain current job from jid, and sends SIGSTOP to the process indicated by the job. Checks if the job has been adequately halted after waitpid(). Sets job->status to STOPPED. 'kill' - Obtain current job from jid, and sends SIGTERM to the process indicated by the job. 'exit' - Calls exit(0), which exits the shell. Or, calls exit(n) with the given signal, n.

Description of Extended functionality

Exclusive Access: handle_child_status(); This method is a utility function for handling child processes and their respective signals. Finds the job this pid is a part of using pid2job. WIF*() macros are used to determine status change of the child process. WIFSTOPPED is sent to the shell if the user stops the process, kill, stop, ctrl-z. Job status is sent to STOPPED and the Job is printed. WTERMSIG is sent to the shell if a process is terminated, either by user or itself. There are multiple different signals that can occur in this case, SIGABRT, SIGSEGV, SIGKILL, SIGTERM, AND SIGFPE. Each of these signals are all termination signals but they print different output messages but on the signal. (Floating point, seg fault, abort, etc). Last case, if status = 0, job struct decrements num_processes_alive.

I/O redirection (< > >>) < (Overwrites stdin): If iored_input is non-NULL, first command should read from file. Used O_RDONLY flag to be read. posix_spawn_file_actions_addopen(3) opens file after < as stdin. In this case, only the O_RDONLY is used in the addopen method so that the file is read.

and >> (Overwites stdout): If iored_output non-NULL, last command should write to file. If pipe->append_to_output == 1, then >> case in logic tree occurs. Redirection in this case appends to the file. posix_spawn_file_actions_addopen(3) opens the file after >> as stdout. oflags used: O_CREAT creates a file if needed. O_RDWR opens file in read/write so that the file can accept input from read side of >. O_APPEND appends to the file. Else, then > case in logic tree occurs. Redirection in this case overwites the file. First, local variable dup_sts is a pseudo-boolean that is set to cmd->dup_stderr_to_stdout. If dup_sts = 1, then stdout as well as stderr needs to be redirected in the file overwrite. posix_spawn_file_actions_addopen(3) opens file to the right of > as stdout and called again to open in stderr. The same flags are used, O_CREAT if file does not exist, O_RDWR to open file for writing, and O_APPEND. Else if dup_sts = 0, then only stdout needs to be redirected in the file overwrite. posix_spawn_file_actions_addopen(3) opens file to the right of > as stdout. The same flags are used as above except for O_TRUNC. O_TRUNC is used over O_APPEND to overwrite the previous contents in the file.

Piping Pipe is set up by the parent shell process before a child is forked. fd is a 2d array. General structure of fd is fd[command][read/write]. read = 0, or reading from stdin descriptor. write = 1, or write to stdout descriptor. posix_spawn_file_actions_adddup2() tells implementation to call 'open' for the given file during posix_spawnp. The logic for the pipe commands is determined based on which process in the ast_pipeline is being manipulated. If commands_order == 1, stdout is connected with write side of fd pipe. The logic then goes to commands_order == commands_size, in which addup2 is called again, this time stdin connected with read side of fd pipe. If commands_order > 1, we need to account for the first process, last process, and all of the processes inbetween in the Piping process. The first and last process case are the same as above. For the middle processes, stdin connected to read side of pipe while write side is connected to the next process. Finally, after posix execution and all commands in a job have finished, fd pipe array needs to be closed. In a for loop iterating i, both read fd[i][0] and write fd[i][1] need to be closed.

List of additional Builtins implemented

- cd built-in command. Utilizes <pwd.h> library to retrieve UID of calling process into passwd struct. This struct has ->pw_dir attribute which is the home directory. Calls chdir() which changes the current processes working directory to the given path. So, cd builtin changes the directory to the home directory. <my_prompt> user customizable prompt. Invoked with built-in 'my_prompt'. build_changed_prompt(char *prompt, int max_len) is a method that creates user customizable prompt and is invoked within the read/eval loop after a successful 'my_prompt' call. my_prompt has a few different options for user customizablility. \t displays time h:m:s \# displays number of commands \j displays the number of jobs \h displays host name \d displays the date in m/d/y

ex: my_prompt "\t"