# Time Travel Shell # ## 0. INTRODUCTION ## Files read-command.c, execute-command.c, main.c, command.h, command-internals.h were modified. All parts had been finished and passed all test cases with no errors. ## 1. ALGORITHM / FEATURES: ## **==========================Part A==========================** The algorithm for making command stream is described below: First, get the file stream and read every single character. When a newline character is hit, then put the entire line into a buffer, then assuming the line contains non-simple command (e.g. AND_COMMAND, OR_COMMAND), command(s) would be created by calling an auxiliary function which recursively separates the command with the special tokens in the middle (i.e. && and ||) using left-associativity, until the command contains no more && and ||. Of course, syntax error will be checked for during that process. After that, another function would be called to separate | or ; in the middle recursively, until the command is simple, then the auxiliary function at the top level would link all of the commands together, creating a command stream. For reading command, each command stream node has a member variable which keeps track of whether the command stream node has been read. Until all nodes are read, then there would be a function that deallocates all dynamically-allocated memory used for the command stream, and the strings, comamnds inside of the stream. **==========================Part B==========================** This part was pretty simple, for simple command, the program forks a new process and calls execvp() directly. And for compound commands such as OR, AND, PIPE and SUBSHELL, the program executes their subcommands recursively. **==========================Part C==========================** The hard part of this part is to determine file dependencies. The program does that by extracting the arguments and I/O redirections of each command, and then compare them to see if they have at least one identical item. If so, then one command depends on other one. There will also be an array determining whether a certain command can be executed now. Refer to page 5 of the following link for the algorithm in executing the commands in parallel. http://lasr.cs.ucla.edu/vahab/cs111/labs/lab1/lab1c-discussion-slides.pdf ## 2. LIMITATION: ## Currently, the program does not have any limitations, and the program passed all test cases provided by the skeleton, and the test cases made on our own which includes mix of subshell commands and compound commands with I/O redirections. However, for time travel implementation, since the program checks for arguments other than I/O redirections upon building the dependency graph, so if a command such as 'echo the command executed successfully' are being executed for few times, then they will not be executed in parallel because the program treats them being dependent to each other even if they are not. But for correctness of executing commands, checking for arguments should be kept since commands such as 'diff' and 'rm' that have no I/O redirections may have file dependencies.