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Command-line Environment

Job Control

  • Shell communicates with processes using signals, and when process receives them, they stop execution and deal with signal
    • Signals are called software interrupts for this reason
  • Signal examples
    • ctrl-c = SIGINT signal
    • ctrl-\ = SIGQUIT signal
    • ctrl-z = SIGSTOP signal
    • man signal shows all available
  • kill -SIGNAL %JOB_ID: sends signal to corresponding job/process
    • -TERM sends SIGTERM, which works well for ending background processes gracefully
  • & at the end of a command runs it in the background and hands the terminal back over
  • jobs: lists unfinished jobs in terminal session, which can be referred to by their PID or %JOB_ID, latter of which is shown by jobs
  • bg %JOB_ID: continue suspended job in background
  • fg %JOB_ID: bring background job, suspended or running, into current terminal and resume
  • nohup COMMAND: will not end process if sent a SIGHUP, which happens when terminal windows is closed

Terminal Multiplexers

  • Terminal multiplexer: allows you to use multiple terminal windows via tabs/panes, attach/detach, etc. tmux is a popular example
  • tmux is controlled via keyboard commands, which are triggered by a prefix key, which by default is ctrl-b, expressed as C-b in their notation
  • Sessions
    • tmux= starts a new session
    • tmux new -s NAME= new session with specified name
    • C-b + d = detach from current session
    • tmux ls= view active sessions
    • tmux attach -t NUM_OR_NAME = attach to session
    • tmux rename -t NUM NAME = rename session of index NUM to NAME
  • Panes
    • C-b + % = split left-right
    • C-b + “ = split top-bottom
    • C-b + ARROW = navigate panes
  • Windows (aka tabs)
    • C-b + c = new window
    • C-b + p = prev window
    • C-b + n = next window
    • C-b + NUM = go to window of index NUM
  • https://tmuxcheatsheet.com/
  • All commands can be viewed inside tmux by pressing C-b + ?
  • Create file ~/.tmux.conf to make it more usable/intuitive 
    # remap prefix from 'C-b' to 'C-a'
unbind C-b
set-option -g prefix C-a
bind-key C-a send-prefix

# split panes using | and -
bind | split-window -h
bind - split-window -v
unbind '"'
unbind %

# switch panes using Alt-arrow without prefix
bind -n M-Left select-pane -L
bind -n M-Right select-pane -R
bind -n M-Up select-pane -U
bind -n M-Down select-pane -D

# enable mouse mode (tmux 2.1 and above)
set -g mouse on

# don't rename windows automatically
set-option -g allow-rename off

Aliases

  • Aliases allow you to assign lengthier commands to shorter strings:
    alias ALIAS_NAME="COMMAND_STRING [ARGS] ..."
    • If a string is assigned as via alias, its raw string can be accessed by prepending with \ or running unalias ALIAS_NAME
    • alias ALIAS_NAME reveals definition
  • Aliases do not persist across sessions, so place in ~/.bashrc

Dotfiles

  • Files that start with ., hidden by ls by default
  • Examples
    • bash: ~/.bashrc, ~/.bash_profile
    • git: ~/.gitconfig
    • vim: ~/.vimrc and the ~/.vim directory
    • ssh: ~/.ssh/config
    • tmux: ~/.tmux.conf
  • For portability, place them all in a single directory with version control, symlinked to the correct places with a script
    • Symlink: ln -s FILE_OR_DIRECTORY_PATH LINK_PATH
  • If you want different programs to share configurations, put those in their own file, then source the file in each program’s dotfile

Remote Machines

  • Secure Shell (SSH) is used to access remote machines:
    ssh USER@IP_ADDRESS_OR_URL [COMMAND]
    • add a COMMAND at the end to just run it directly
  • SSH keys allow you to forego password use each time you log into remote
    • ssh-keygen -t ed25519 -C "your_email@example.com"
    • Generated in pairs, public and private, i.e. id_ed25519 and id_ed25519.pub - public is passed to remote, private is kept locally to as a method of authentication
    • Passphrase ensures that if private key file is obtained by someone else, they can’t use it
    • Remote machine
      • Copy to remote with ssh-copy-id -i .ssh/id_ed25519.pub USER@IP_ADDRESS_OR_URL
    • GitHub/GitLab
      • cat ~/.ssh/id_ed25519.pub, then copy output and paste it into text field on website
  • rsync -avP FILE_OR_DIRECTORY_PATH USER@IP_ADDRESS_OR_URL:DESTINATION syncs files and directories between two hosts or machines
  • List remote devices in ~/.ssh/config and you can connect by simply typing ssh HOST_NAME 
    Host HOST_NAME
	User USER
	Hostname IP_ADDRESS_OR_URL
	IdentityFile ~/.ssh/id_ed25519
	LocalFoward 9999 localhost:8888  # see port forwarding below
  • tmux works well on SSH connections, because detached sessions will persist even if connection falters, and can be picked back up
  • Other shell alternatives for accessing remote machines are Mosh and SSHFS

Port Forwarding

  • Computer handles many incoming/outgoing requests by placing them on ports and assigning a port number - software will listen to specific ports on a machine to receive info
  • If you want to reach a remote server without directly accessible ports (via network/internet, etc.), you can do local port forwarding, which connects an accessible local port to the remote port via SSH, and so when the local port is accessed, it “redirects” to the remote instead
    • If Jupyter Notebook is run on remote server which listens on port 8888, and we want to access it locally on port 9999, command is
      ssh -L 9999:localhost:8888 USER@IP_ADDRESS_OR_URL
  • Conversely, if you want a remote port to provide access to a port locally, you can do remote port forwarding
    • If remote port 8888 needs to reroute to local port 9999, command is
      ssh -R 8888:localhost:9999 USER@IP_ADDRESS_OR_URL
  • Helpful Stack Exchange post