or at least the ones i am not too tired to note down. i'm lazy and forgetful. this is not the full list.

summary-ish.md

that's it for the basics

or at least the ones i am not too tired to note down. i'm lazy and forgetful. this is not the full list.

• ssh login to a remote machine

  • password
  • ssh keys
    • generate: ssh-keygen -t ed25519 -C [meaningful comment]
    • connect: ssh -i [path/to/key] user@host

• *nix

  • UNIX an operating system of old
    • has philosophy regarding how computer systems should function
  • various branches based on UNIX follow some or all of the philosophy

  • some philosophy followed on all branches (not verbatim)

    • everything is a file
    • make every program do one thing; and do it well

• paths

  • indication of where a file or directory is on the filesystem
  • use command pwd to find path of current directory (aka working directory)
  • filesystem is tree-like hierarchy starting / - root
  • each level is separated by /
    • i.e /home/avengers = root » home » avengers

  • home dir

    • /home/<your username> = your home directory; holds personal files
    • ~ in a path is often synonymous to your home directory

  • path types

    • absolute paths

      • start at /
      • working dir doesn't matter while using paths of this type

    • relative paths

      • start at working directory
        • . means working directory; optional
        • .. means parent of .
      • working dir matters while using paths of this type

  • hidden files and directories

    • have names starting with . (not to be confused with working dir)
    • ./.secrets path to hidden file in .

  • bin dirs

    • hold applications; some common ones are
      • ~/.local/bin
      • ~/.bin
      • /usr/bin/
      • /usr/local/bin

  • src dirs

    • hold source package of apps installed from source; some options are
      • ~/.local/src
      • /usr/local/src

• stdout

  • special file that shows text output

• stderr

  • special file that shows text errors

• /dev/null

  • special file that makes everything written to it disappear

• shell is the interface

  • bash is common and has good features

  • the shell

    • treats everything as space-separated words
      • aka word-splitting
      • prevent using
        • " or ' around the entire phrase
          echo "hello world"
        • or escape () the spaces
          echo hello\ world
      • words with special meaning (variables, command substitution) in shell work inside "..." but not '...'
    • is mostly case-sensitive
      • date ≠ Date
    • supports variables
      • accessible using $var_name
      • set using var_name=value
      • echo $var_name to view; empty/unset var prints blank line
      • has special ones
        • $0 - current application
        • $_ - last arg of previous command
        • $? - exit code or previous command
        • $USER - username
        • $HOME - path to home directory of user
        • $1 - $9 - positional args of current command
          (useful in scripting using bash)
        • $PATH - list of paths to directories holding executable files expected as commands. each path is separated by :
    • expects valid command names
      • a command is
        • an executable file in $PATH
        • absolute or relative path to an exectuable file
        • something built into the shell
    • substitutes commands wrapped in `` with output
      • e.g. echo today's date is : `date`
        • `date` will be subtituted by its output and passed as argument to echo
    • redirects stdout and stderr using >
      • lhs of > is source
        • 1 is stdout and 2 is stderr of the current command
        • no space between source and >
          • i.e one unsplit unit 1>; not 1 >
        • lack of source implies 1
        • BASH only &> indicates both 1 and 2
      • rhs of > is the destination
        • usually path to a file
        • can also be &1 or &2 to refer to stdout and stderr respectively
          • thus shell way of writing to stderr is echo hello world 1>&2
      • behaviour
        • > overwrites destination
        • >> appends to destination
      • NB shell sees > in any part of command as redirection, not as command argument, unless escaped with one of " ' \
    • uses patterns
      • * can be used to match anything in a path
        • ls /home/user/D* = list items in ~ starting with capital D
    • executes login scripts
      • ~/.profile
        • set variables (incl PATH) for current user
      • (for bash) ~/.bashrc
        • configure other bash options for current user
      • /etc/profile
        • set variables (incl PATH) for all users

• applications

  • types

    • compiled
      • are transformed to binary that can be executed directly
    • interpreted (scripts)
      • interpreted apps need to be called with the interpreter - python /path/to/script.py - instead the !#/path/to/interpreter shebang is used at the start of the script file
  • should have permission to execute
    • granted using chmod +x </path/to/app>
  • need to be in $PATH dirs to be executed directly from the shell
  • can also be executed using absolute or relative path

• common applications

  • pwd
  • cd [path]
  • ls [path]
    • ls -1 - show in one column
    • ls -a - incl. hidden files
    • ls -l - show more info
  • echo
  • mkdir [path]
    • mkdir -p - create required non-existent directory along the way
  • touch [path]
  • `cp [src paths...] [dest path]
    • cp -r - copy a directory
  • mv [src paths...] [dest path]
  • chmod [options] [path] - change permission
    • chmod +x path - make executable
  • file - show filetype
  • date
  • man [name of command] - manual for command
  • nano - text editor
  • git
    • git init - make . a git repo
    • git clone [url]
      • git clone [url] [path] - clone to directory named [path] (should be empty)
    • git add [paths] - stage files; prepare for commit
    • git commit -m "[meaningful message]" - commit staged changes
    • git remote add [remote name] [remote url] - remote name is usually origin
    • git fetch
    • git pull
      • git pull --set-upstream origin main - first time pull
    • git push
      • git push --set-upstream origin branch - first time push
    • git branch [name] - create branch [name] based on last commit
    • `git checkout [branch name] - switch to branch

• uncommon applications

  • finger - show logged in users
    • finger [username] - info on user
  • chsh - change login shell
  • bash
  • sh
  • gcc [C source file] - compile C code
    • gcc -o [name] - output name for compiled file (default: a.out)

Wow