Project1: Unix Shell with History Feature

Project1: Unix Shell with History Feature • Goals • Descriptions • Methodology • Submission

Goals • Understand how a simple shell works. • Understand systems calls, such as fork, read, wait, execvp, and etc. • Understand signal handling mechanisms

Descriptions • Demo • command> ls • commnad> cat proj1.c • command> ctr-c • command> ctr-d • Input: commands from keyboard • Fork a child process to perform the command • Store the past commands in a buffer • Given a signal, display the most recent commands in the buffer • Ctrl-C terminates the shell

Methodology • How to get the command from the keyboard? • Use system call read() with STDIN_FILENO • Implement a setup() void setup(char inputBuffer[], char *args[], int *background) setup() reads in the next command line, separating it into distinct tokens using whitespace as delimiters. setup() sets the args parameter as a null-terminated string. Also set background =1 if & is met If “ctrl-d” is met, just simply call exit(0);

Methodology • How to execute the command? • while (1){ /* Program terminates normally inside setup */ • background = 0; • printf(" COMMAND->\n"); • setup(inputBuffer,args,&background); /* get next command */ • /* the steps are: • (1) fork a child process using fork() • (2) the child process will invoke execvp() • (3) if background == 1, the parent will wait, • otherwise returns to the setup() function. */ • }

Methodology • How to display recent commands? • Use signal handler: CTRL-C is the SIGINT signal • /* the signal handler function */ • void handle_SIGINT() { • write(STDOUT_FILENO,buffer,strlen(buffer)); • exit(0); • } • int main(int argc, char *argv[]) • { • /* set up the signal handler */ • struct sigaction handler; • handler.sa_handler = handle_SIGINT; • sigaction(SIGINT, &handler, NULL); • strcpy(buffer,"Caught <ctrl><c>\n"); • /* wait for <control> <C> */ • while (1); • return 0; • }

Methodology • How to keep track of past commands? • Limited-size buffer, why not use circular buffer? • Modify setup() to store the current command which may overwrite the oldest command in the buffer • Implement SININT signal handler to display the 10 most recent commands

Suggested Steps • Step 1: implement setup() • Step 2: execute the command from setup() • Step 3: add the history feature

Submission • Email to zhuy@seattleu.edu • All source files • A readme file that describes each file, how to compile the file(s), and how to run the file. If there is any problem running the file, please state it here as well. • Makefile is preferred!

Questions

Project1: Unix Shell with History Feature