System Software COP3402

Midterm Review

• 13 Multiple Choice

• 6 Short Answer

• 1 Long Answer

• Homework questions, Lecture Notes, Coding Exercises

File Systems

• What makes UNIX File System hierarchical

  • They are in trees and you can go to subdirectories.

  • Directories are special files that store a table of names and i_node numbers

  • Without the table there would be no files by name

  • A subdirectory will also have its own table of names and i_node numbers

  • Hierarchy comes from the directories being special files with tables so they can exist in other directory files

  • Root does not have a parent directory, but all other files should

    • Root's parent directory is just itself

• Absolute vs Relative paths

  • Absolute starts at Root

• How are parent directories referenced in the file system

  • It uses ..

• Draw File System Tree

Navigations

• What is the working directory and how do you display it?

  • The current directory for process that is executed

  • Bash automatically shows the working directory when asking for a prompt

  • Command to display is pwd

    • Will return something like this in Eustis if you're in the root directory for your account.

    • /home/net/nid

• What is the unix standard command to rename a file?

  • mov

    • Move one file to move it to another named file, so it renames it

    • mov oldName.c newName.c

      • Will rename oldName.c to newName.c

    • Doesn't change the i_node number just the name

• What is tab-completion?

  • Will auto-fill a file or directory name that matches the already written characters

  • Auto-correct for the command line

• What unix standard will show you the text of a file?

  • cat

  • cat newName.c

    • Will show all the text in the file.

• What does grep do?

  • Searches within a file that matches the pattern that is passed through.

  • Pattern is the first input

  • File name is the second input

  • Is case-sensitive

  • grep textPattern fileName -> grep word newName.c

    • Will print out all the lines that have the substring "word" appear in them.

• How do you change the working directory to your home directory?

  • cd

  • cd will take you back to your root/home directory

  • cd .. will take you up one folder level

  • cd ./ will keep you in the same directory

• What is the unix command to delete a file?

  • rm

  • rm newFile.c will delete the file newFile.c

• How does the implementation of deleting a file work? Does it remove the file's contents from the storage medium?

  • Delete the directory entry

  • Free the space that is malloced so the system can use it again for other stuff

Processes and Advanced Processes

• What is a process?

  • A running program

• How does one CPU core run multiple programs at the seemingly same time?

  • Time sharing

• Command to List files

  • ls

  • ls will automatically list out the current working directory, but you can pass a path and it will list out the files in that directory

    • ls newDirectory will list out files in that directory

• Difference between cat and echo

  • echo takes command line arguments and writes them to stdout

  • cat takes stdin and writes them to stdout

• How do you redirect standard (out, in) of bash command to a file? for instance, i want to redirect grep's (out, in) to the file grep.txt what do i type?

  • grep hello > grep.out

    • Takes in input from the command line

    • Writes to a file called grep.out

  • grep word textFile.txt > grep.out

    • Will search through textFile.txt for lines with the substring "word" and write them in the file grep.out

• How do you redirect standard out from one command to another command's standard in? for instance, let's say i want to count the results of find with wc, what do i type?

  • Use a pipe

  • find /usr/include/ | grep stdio

  • find will get the file tree and pass it into the grep, and grep will search through each one to find where the substring "stdio" appears.

  • find ./ | grep new > grep.out

    • find will get the file tree (so all the files recursively as a path that are accessible)

    • The pipe "|" will pass the find output to grep as the input text

    • grep will search through the text to see if any lines contain the substring "new"

    • grep will write to grep.out with any lines it finds

• Find command

  • Will print out all the files throughout the file tree starting from the path, or if not path is passed in then at the current working directory.

  • find will return ./Directory, ./Directory/File and so on

Editor

• How do you edit files or emacs (pick one)?

  • emacs filename

• How do you quit the editor or emacs (pick one)?

  • CTRL X + CRTL C

• How do you save?

  • CRTL X + CRTL S

Build Automation

• What does the (target, recipe, prerequisite) of a makefile rule do.

  • Target is the file that is generated

  • Recipe is the sequence of commands that run

  • Prerequisites are the files that need to exist to run the recipe

  • main: main.c square.c exponent.c

  • gcc -o main main.c square.c exponent.c

• Do makefiles use spaces or tabs?

  • Makefiles use tabs as indentation to have proper syntax.

• By convention, what does the clean target do?

  • Deletes the generated file

  • clean:

  • rm -f main

• Here is a Makefile, add a clean target to remove the binaries.

• clean

• rm -f main

• Write a Makefile that will create a program called hello from two source files, main.c and hello.c, when make is run.

• hello: main.o hello.o

• gcc -o hello main.o hello.o

Version Control

• What git command copies commits from the local repository to the remote repository?

  • git push

• What git command copies commits from the remote repository to the local repository?

  • git pull

• What git command stages a new file?

  • git add

• What git command commits a commit?

  • git commit

• What git command creates a log of the change to a staged file to the local repository?

  • git log -> Shows you the history

  • git commit

File Syscalls

• Using the open syscall (man 2 open, not fopen) to open a path given in the string char *filepath variable.

  • int fd = open(filepath, O_RDONLY);

• How do you check for and terminate the program on an error with opening a file?

  • if (fd == -1) -> Error value is -1

• Using the read syscall (man 2 read, not fopen), you already have an open file with the file descriptor stored in fd, read the first 200 bytes of the file and print it to stdout

  • read(fd, buf, BUFSIZE); -> BUFSIZE should be 200 for this question and buf should have enough memory

  • read is not guaranteed to give you all the data

  • Do it in a loop to make sure you get everything

• What syscall can you use to find the (size, number of hard-links) of a file?

  • stat(path) syscall

• What syscall can you use the find the name of a file? (Not on exam)

  • stat

  • Get i_node from stat then search the file system for a name that points to it.

• Write a code snippet that will print all files in a given directory, except do not print the "." and ".." names

  • fs project

• Write all the file open macros.

  • O_CREAT -> Creates file if it doesn't exist

  • O_TRUNC -> Deletes the file and 0's it out if it exists

  • O_RDONLY -> Opens the file for reading

  • O_WRONLY -> Opens the file for writing

Process, Pipe, Syscalls

• Write code that uses unix standard syscalls to create a new process that runs the ls command.

  • Fork and exec

• Does fork create a new program?

  • It duplicates the current program

• Write code that creates a new process, where the original process writes "parent" and the new process writes "child", both to stdout.

Example code for fork problem.

#include <stdio.h>
#include <unitstd.h>
int main(void) {
    pid_t pid = fork();
    if (pid < 0) {
        // fork() failed
        perror("fork failed");
        return 1;
    } else if (pid == 0) {
        // This is the child process
        printf("child\n");
    } else {
        // This is the parent process
        printf("parent\n");
    }

    return 0;
}

• Write code that replaces the current processes running program with the stat/ls command (not the stat syscall).

  • exec

Example Code for exec problem.

#include <unistd.h>
int main(void) {
    char *args[] = {"ls", "-l", NULL};
    execvp("ls", args);  // Searches PATH for "ls"
    return 1;            // Only reached if execvp() fails
}

• Write a program that opens a pipe and reads to and writes from it.

Example code for pipe problem.

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>

int main(void) {
    int fd[2];  // fd[0] = read end, fd[1] = write end
    pid_t pid;
    char message[] = "Hello from parent!";
    char buffer[100];

    // Create the pipe
    if (pipe(fd) == -1) {
        perror("pipe failed");
        exit(1);
    }

    // Create a new process
    pid = fork();
    if (pid < 0) {
        perror("fork failed");
        exit(1);
    }

    if (pid == 0) {
        // Child process
        close(fd[1]); // Close unused write end
        read(fd[0], buffer, sizeof(buffer));
        printf("Child received: %s\n", buffer);
        close(fd[0]);
    } else {
        // Parent process
        close(fd[0]); // Close unused read end
        write(fd[1], message, strlen(message) + 1);
        close(fd[1]);
    }

    return 0;
}

• Write a program that redirects the standard output to a file called "output.txt".

  • dup_2 to copy a file descriptor to file IO and redirect

Example code for dup_2 problem.

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>

int main(void) {
    int fd;

    // Open (or create) output.txt for writing
    fd = open("output.txt", O_WRONLY | O_CREAT | O_TRUNC, 0644);
    if (fd < 0) {
        perror("open failed");
        exit(1);
    }

    // Redirect stdout (file descriptor 1) to the file
    if (dup2(fd, STDOUT_FILENO) < 0) {
        perror("dup2 failed");
        close(fd);
        exit(1);
    }

    close(fd); // fd no longer needed after duplication

    // Now, anything printed with printf goes to output.txt
    printf("This text will be written to output.txt\n");
    printf("dup2() successfully redirected stdout!\n");

    return 0;
}

• Fork Stuff

  • Fork returns 0 for a new process

  • Fork returns > 0 for original process

Fork Template

enter int pid;
  switch (pid = fork()) {
case -1:
    perror("fork");
    exit(EXIT_FAILURE);
    break;
case 0: // child
    break;
default: // parent
    break;
}

Pipe Example

#include <stdio.h>
#include <stdlib.h>
#include <sys/wait.h>
#include <unistd.h>

int main(int argc, char **argv) {
    int pipefd[2];
    char buf;
    char *msg;
    pid_t pid;
    
    if (argc < 2) {
        printf("USAGE: %s message_to_send\n", argv[0]);
        exit(EXIT_FAILURE);
    }
    msg = argv[1];
    
    if (pipe(pipefd) == -1) {
        perror("pipe");
        exit(EXIT_FAILURE);
    }
    
    pid = fork();
    switch (pid) {
        case -1:
            perror("fork");
            exit(EXIT_FAILURE);
            break;
            
        case 0: // child
            close(pipefd[1]); // close write end (parent does writing)
            sleep(3);
            
            while (read(pipefd[0], &buf, 1) > 0) {
                write(STDOUT_FILENO, &buf, 1);
            }
            
            write(STDOUT_FILENO, "\n", 1);
            close(pipefd[0]);
            exit(EXIT_SUCCESS);
            break;
            
        default: // parent
            close(pipefd[0]); // close read end (child does reading)
            write(pipefd[1], argv[1], strlen(argv[1]));
            
                // it's very important to close the write end of the pipe so
                // that the read end will know to stop reading
            close(pipefd[1]);
            wait(NULL);
            
            sleep(3);
            exit(EXIT_SUCCESS);
            break;
    }
}