Shell Usage for Dummies

Documentation based on materials from MIT The Missing Semester of Your CS Education course

1. Shell overview

What is shell?

• A program that takes commands from the keyboard and gives them to the operating system to perform

  What is bash?

• Bourne Again SHell, a very widely used shell

  Is there any other (cooler) shell?

• I use Z shell (zsh - an extended version of bash) with ohmyzsh configurations from Ming Rui
• Check this file for how to install zsh with Ming Rui’s configs

2. Basic Shell Usage

• When you open the server, you should see something like this:

  user.name@server-name-ip-11-222-333-444:~$

  • The part user.name@server-name-ip-11-222-333-444 indicates the machine you are on, the ~ symbol means that you are in the home directory (aka the path that you will reach when you type cd) and $ means that you are not the root user.
  • Root is the superuser account in Unix and Linux. This user will have access to all command and files, with powers that normal user cannot such as installing software or changing file ownership. If a user is provided root access, the user can use the sudo command (short for SuperUser DO).
  • However, with great power comes great responsibility. You might easily break something while using sudo. This will affect a lot of people if you are sharing a server with others. In short, do NOT sudo into a shared server if you’re not sure!

• Common shell commands can be found under this Bash Scripting Cheatsheet. It works for Z shell too!

• When you are uncertain, -h or --help flag will print out the short explanation for the command. Try ls -h and see.

• Common commands to note for dummies:

  • pwd: print your current working directory.
  • cd: take you to home directory.
    • cd + path: take to the specific path.
    • cd .: current directory.
    • cd ..: go up one level, i.e. parent directory.
    • cd -: go back one level.
  • ls: list files inside your current directory.
    • ls -lah -t: list all files, including hidden file (-a flag), in long format (-l flag), in human readable size (-h flag), ordered by recency (-t flag).
    • ls + path: list all files inside specific path
  • cat + filename: print file, i.e. shows the contents of the file in your terminal. Works for multiple files too.
  • vim + filename: open vim editor to view or edit the file. See section 4 for more information on vim editor.
  • mkdir: makes a new directory. E.g. mkdir ~/shell-usage/dummies
  • touch: creates a new file. E.g. touch filename.txt
  • cp: copy files. E.g. cp [source directory] [destination directory]. Works for multiple files too.
  • mv: move files or change file name. Same usage as cp.
  • rm: remove files
  • head -n [N] filename, tail -n [N] filename: prints first (head) or last (tail) N lines of a file
  • rsync: to sync your local files to the server and vice versa. See Rsync Cheat Sheet.

• How to keep your scripts running on the server even though you lost connection: use tmux (see Tmux Cheat Sheet) or screen (Screen Cheat Sheet).

• man: For each command there are certain flags, for example typing ls will give you the list of files inside your current directory. However to see all hidden files, you need to use the a flag, i.e. by typing ls -a. To see all the details about the flags, use the man command. E.g. man ls.

3. STDIN, STDOUT, and basic piping

• STDIN is a stream where a program receives and reads the input data
• STDOUT is a stream that delivers the text output from the command to the shell
• One simple way to rewire these streams is by > (rewire input) and < (rewire output).
• For example:
  ```shell
  root@machine:~/playground/demo

  23" class="reference-link">echo ‘hello’ > file1 20-09-03 - 823

root@machine:~/playground/demo

cat file1 20-09-03 - 852

hello

* As you see from the above, output of `echo 'hello'`, which is the text 'hello', became the input to be written in file1. You can also use `>>` to append a file.
* Another method is to use pipes with the `|` operator. The output of the command before `|` will be the input of the command after `|`. For example:
```shell
root@machine:~/playground/demo
# cat file1 | head -n1                                                                             20-09-03 - 8:25:54
hello

4. Vim editor

• There are many text editors to write code or edit text files, including vim.
• To learn more about vim commands, run vimtutor in your terminal (that has already have vim inside) and follow the instructions
• To start editing a file, use vim filename
• Basic usage inside vim:
  • esc: Escape key to go back to normal mode
  • i: insert mode (to insert words)
  • v: visual mode (for highlighting)
  • hjkl: navigating, left, down, up, right respectively
  • :q!: force quite, discard changes
  • :x or :wq: write and quit
• The above list is non-exhaustive. Look through vimtutor for more commands.

5. Some additional stuff

• If you want to have some modifications to your shell commands, e.g. you want a shorter way of typing ssh -N -f -L [port][port] username@servername every time you want to tunnel to server, you can edit it in your ~/.zshrc file.
• Start editing with vim ~/.zshrc
• At the end of the file you can add:

  function tunnel() {
  ssh -N -f -L localhost:$1\$1 $2@$3
  }

• After this, whenever you want to tunnel just type tunnel [port] username servername
• This is called alias, you can Google it to customize your bash, zsh, and even vim commands!

Important things to note while using a shared server

• Do NOT sudo into a server that you have root access. This will affect the whole server and other people who are using it.
• Use a virtual environment or Docker to do your projects and install dependencies. Check out the following:
  • Docker Documentation
  • How to create and use Virtual Environments (Python)
  • How to create and use Virtual Environments (Conda)