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Uni Notes

Materials Notes

CSE 391

Table of contents
  1. Week 1
    1. Introduction to Linux
    2. Flags, Arguments, and Editors
  2. Week 2
    1. More Unix Commands
    2. Input Output Redirection
    3. Pipes
  3. Week 3
    1. I/O Redirection, xargs
  4. Week 4
    1. Introduction to Git
    2. Four Phases of Git
    3. Git Practice
  5. Week 5
    1. Git Branching
    2. Git Merge
    3. Git Branching: Working with Remote
    4. Git Merge Requests
  6. Week 6
    1. Introduction to Regular Expressions
    2. Or and Repeated Characters
    3. Character Sets and Backreferences
  7. Week 7
    1. Intro to sed
  8. Week 8
    1. Introduction to users, groups, adn permissions
    2. bashrc, bash_profile, and $PATH
    3. File and Directory Permissions

Week 1

Introduction to Linux

  • Linux - another operating system, another way to interact with the computer.
  • CS department has servers which can be remotely accessed with ssh.
  • Virtual machine - put an OS within your OS.

Flags, Arguments, and Editors

  • ls -l: contains a flag or an option l which provides arguments. Long-listing format.
  • Command: the command name, flags, then arguments. Flags provide optional specifications.
  • man: pull up the manual.
    • Forward slash: can search for a word, e.g. /long - press n to cycle through the matches.
  • -a command: do not ignore hidden files .*.
  • Every directory has . and ..: references to the current directory and the previous parent directory.
  • cd brings you back to your home directory.

Week 2

More Unix Commands

  • cat - print out contents to the file. It’s limited to a buffer size, though
  • less filename opens up a scrollable interface with very similar keybindings to vim.
  • more filename - similar to less
  • head - see the beginning of the file. Specify -n flag. tail - see the end.
  • wc - word count
  • sort - outputs a sorted version of the file argument
  • grep re_exp filea fileb - performs regular expression matching on files
    • You can also specify a directory in lieu of files: grep re_exp project/*.
  • How to run Java in the command line? Compile with javac file.java, then run with java file. java file.java will compile and run simultaneously.

Input Output Redirection

  • Three streams (abstracted locations): stdin, stdout, stderr.
  • A program accepts stdin and outputs to stdout and stderr.
  • You can redirect stdout into other areas.
  • You can direct stdin into programs.
  • Ise 2> to redirect both stdout and stderr.
  • Distinguish between stdin and parameters.
  • If no file is provided, programs will read from stdin.
  • To append to a file, use the double wakka: >>.

Pipes

  • Pipes allow you to chain together commands |. It takes stdout from the left argument and puts it as stdin into the right argument.
  • Unix philosophy - do one thing and do it well.
  • You can impose repeated conditions by chaining.
  • uniq - prints the number of repeated lines
  • sort - sorts a file by lines
  • Log file - stores progress on a file.

Week 3

I/O Redirection, xargs

  • && can be used to chain together commands in which commands are executed only if the previous command does not raise an error.
  • || will only execute if the previous command fails.
  • ; - chain together commands regardless of failure or correctness.
  • Some commands don’t accept standard input - you can use xargs to take standard input and convert it into arguments. For instance, xargs javac < ls *.java.
  • find - recursively searches all files in the current folder or with the current folder as parent.
  • $(...) - command substitution. Command line evaluates the command first.
  • 2>&1 - have standard error go to wherever standard output is.
  • tee - prints to the console and saves to the file as well.
    • e.g. java printer.java | tee output.txt
  • echo - prints to the terminal.
  • cut - helps with indexing, e.g. -c 4 gets the fourth character in the input.
    • -c4-6, -c1,3,5
    • Cut is usually used to break input strings by delimiters. -d, -f2
  • Pipe operators let you get pretty far pretty quick.

Week 4

Introduction to Git

  • Git - version control software.
  • How to work with partners?
  • Repository - a location which stores a copy of all files. What should be in a file? Source code files, build files, images, general resource files - but not executables and object files.
  • Everyone working on the project has a complete version of the repository.
  • Remote repository - a central repository hosted on services like GitHub
  • README.md is the standard documentation.
  • .git subdirectory - the ‘actual’ repository, manages the state of the repository for us.
  • git status - updates us about the state of changes.
  • git stage - get ready to take a snapshot of the repository.
  • git commit - create a snapshot of the repository.
  • git push - push to remote repository

Four Phases of Git

  • Phases
    1. Working directory
    2. Staging Area / Index
    3. Local repository
    4. Remote repository
  • git stage or git add - move into the staging area.
  • git commit - move from the staging area to the local repository.
  • git push - move from the local repository to the remote repository.
  • git status - tells us about the state of the working directory.
  • git log - tells us about the interaction between the staging area and the local repository.
  • Each commit has a unique identifier.
  • What is the point of a staging area? Why use an intermediate step? Git makes us be deliberate about the changes we want to make, we have to manage the history.
  • Only changes which are staged get stored into the local repository during the commit. This allows you to group many different changes and push them all at once.

Git Practice

  • Git doesn’t work on files, it works on diffs - changes from one file to another.
  • Commits are groups of diffs
  • Just typing git commit will open up vim to provide a message - can provide a message and a longer body, too.
  • git pull - automatically merges repositories such that the repository can be pushed.
  • Git histories often aren’t linear: need to be repeatedly separated and merged. These are recorded all by git.
    • git log --decorate --all --graph

Week 5

Git Branching

  • Continuous integration - any time you push changes, you need to do tests before you commit to the master.
  • Master - single source of truth, the history of the project. SUpposed to be stable.
  • To make modifications, we need to work on a separate branch.
  • Commit history
  • HEAD - ‘current pointer’, the pointer which belongs to us, the user. We can move HEAD around from branch to branch.
  • Head - pointer / reference to the current git branch.
  • git branch feature - creates a new branch called feature. But it does not automatically switch branches.
  • Type git checkout feature to switch to a branch.
  • git log can look differently depending on where MAIN is pointing.
  • How to reconcile different histories?

Git Merge

  • git merge feature - merges feature into the current branch.
  • Git can’t fix things for you - you need to go into the files and fix the conflicts.
  • After we finish editing, we need to stage / add the files with the resolved changes.
  • We need to make a commit with the conflicts resolved.
  • git log --graph --oneline will show you the history in a visual format.

Git Branching: Working with Remote

  • How to use branches with remote repositories?
  • Origin is a way of referring to the remote repository.
  • If you try to git push when the remote has been changed since when you last updated it, you’re going to get an error.
  • git fetch - does not change your local repository, but the origin master and origin head will be missing. Downloads copy of remote repository to a new branch.
  • You can try to git merge origin/... and resolve the issues.
  • Git is built on simple foundations.
  • git pull is git fetch + git merge.

Git Merge Requests

  • In practice, we often don’t merge branches locally and push to remote
  • GitHub / GitLab can help us handle these.
  • Steps:
    1. Create a local branch and make commits
    2. Push commits to remote
    3. Open a pull/merge request on GitLab
    4. Collaborate and fix conflicts
    5. Merge into master or another branch
  • We need to be very deliberate about what we put on master
  • git checkpoint -b feature
  • Merge requests - can be handled on Gitlab.
  • git branch -d feature removes feature branch from your local repository.

Week 6

Introduction to Regular Expressions

  • Regex - description of a pattern of text
  • Many applications of regular expressions
  • Almost all programming languages use regular expressions
  • grep is case sensitive by default
  • . will match any character.
  • Use -E for extended regex.
  • $ - end of line
  • ^ - start of line
  • Spaces are interpreted literally.
  • Why doesn’t .... match 4 letter characters?
  • \< - beginning of word; \> - end of word

Or and Repeated Characters

  • Or can be done with |.
  • We can group together an expression with parentheses: (e|a)t is equivalent to et|at.
  • * - ‘zero or more of’
  • ? - ‘zero or one of’
  • + - ‘one or more of’

Character Sets and Backreferences

  • Enumerating a lot of characters indiivudally is a pain. We can use a character set with square brackets: [abcde] is equivalent to (a|b|c|d|e).
  • [A-Z], [a-z], [0-9], or combine: [a-zA-Z0-9].
  • Curly brackets: {n}, matches exactly 3 instances from the set. {a,b}, anything from a to b (inclusive).
  • ^ negates the entire character set - anything except for what is in the set. Interprets signs literally.
  • (..)\1 will match abab, acac, all repeated. \1 is whatever was the frist saved one.

Week 7

Intro to sed

  • grep -E "CSE[^3][0-9]{2}" classes.txt
  • sed -r 's/REGEX/TEXT/' - normal sed usage
  • By default sed does not change the contents of the file
  • You can use sed -ri.bkp to make changes in-place.
  • By default, sed replaces the first occurrence on each line. To change all matches, we need 's/REGEX/TEXT/g'.
  • You can escape special characters for the search criteria, but not for the replace string.
  • You can replace with backwards references.

Week 8

Introduction to users, groups, adn permissions

  • whoami - tells you your username
  • users - see other users on the server
  • pinky - see user IP addresses, names, etc.
  • groups
  • ps - see processes
  • ps -u username`
  • top / htop - updated processes on the system by intensity
  • kill pid - kill process ID, only accessible to admins. Give people only as much power as they need.
  • Virtual machine
  • sudo - super user do
  • sudo - gives you higher permissions
  • Short-hand: !! refers to the last command that you ran, e.g. sudo !!

bashrc, bash_profile, and $PATH

  • Customizability of linux systems - customize your shell, giving you a starting point for your UX on Linux.
  • .bash_profile and .bashrc
  • .bash_profile, goes through line by line and runs various commands. Every time you log into your system, bash runs the contents of this file.
  • You can set up an alias on .bashrc - alias name="cmd".
  • source .bashrc - reload/rerun the file.
  • $PATH variable, string that bash does to look for commands in.
  • Use PATH=$PATH:/... to redirect the path variable to the current path.
    • To make this persistent, add it to your .bashrc.

File and Directory Permissions

  • File permissions:
    • Is it a directory - d
    • Owner: rwx
    • Group: rwx
    • Others: rwx
  • If you own a file, you can always change the permissions.
  • chmod u+x: give execute permissions to the owner (u).
  • Octal codes: +4, +2, +1. 444 - everyone gets write.
  • umask: sets default permissions for newly created files.