Assignment 00a
Linux CLI

The usual way to access the Linux CLI is through a program called a terminal emulator, or just terminal. To get a terminal, you can browse through the programs to find one called xterm and click on it, or a slightly faster way is to type the Windows key on the keyboard, then start typing "t" "e" "r" "m" to search for programs on the machine. When you see xterm you can select it with the arrow keys and hit Enter to run it.

The main point of the terminal is to display a grid of text characters, and to accept input from the keyboard. The program that does most of the work of deciding what characters get displayed is called the Shell. Think of it as a "shell" of interactive functionality around the "core" or Kernel of the operating system. Most modern operating systems have a kernel with one or more "shells". The shells can be text-based (as in this case) or graphical (think MacOS or Windows). The default shell used by the terminal in SICE computers and many other Linux-based operating systems is called bash.

xterm

a terminal emulator that provides a place for keyboard input and text output

bash

the default shell program

There are other terminal programs, and other shell programs. The differences are typically minor. Stick with the defaults if you're starting out.

After launching the terminal, you should see a small amount of text and a (possibly blinking) cursor. You can type or paste in text-only commands, and typing the Enter key (or providing a newline character in your pasted text) will execute everything typed before the newline/Enter.

Examples:

ashroyer@~ ls
ashroyer@~ ls -la
ashroyer@~ ls -la /tmp
ashroyer@~ pwd
ashroyer@~ man -k python
ashroyer@~ man -k python  # comments start with "#" and go to the end of the line

Notice the syntax: each line begins with a prompt which is read-only (ashroyer@~), followed by a command. Spaces separate different parts of commands. Some only need the name of the command, such as ls. Others add flags or options to modify the command, such as ls -la. Still others take arguments as in ls -la /tmp (the argument here is /tmp).

man pages can take some getting used to. For quick examples, check out the tldr pages. They don't replace the man pages, but are a good supplement: https://tldr.ostera.io/

In addition, there is built-in help and user manuals in Linux, but because it's text-only you have to know how to find it.

man

the "manual" pages. man ls sends you to the manual for ls. man -k python shows a list of manual pages related to python. Pick one and use it as your next argument to man.

help

for functionality built-in to the shell. Often overlaps with man.

Running man python launches a special program called a pager which is a read-only view of a file. The default pager program is usually one called less. You can scroll down and up inside of less using the j and k keys, respectively. You can quit the pager program to go back to the shell by typing q, and you can search within the document by typing / followed by the word you want to find, then typing Enter.

The echo command takes whatever you type after the first space and repeats it back to you.

ashroyer@~ echo stop copying me
stop copying me

This will actually come in handy later.

Printing output to the terminal screen is useful, but sometimes we want to save the output of a command to a file. The bash shell provides a single character command for this: >, example:

ashroyer@~ ls > myfile.txt

This saves the output of ls to a file called myfile.txt. If there was already a file with the same name, it is now overwritten. There is no "are you sure?" prompt, so be sure when you type. To append to a file you can use >> instead:

ashroyer@~ ls >> myfile.txt

The output text of one command can be used as input for another command. For example, you can run ls to see a list of folders and files, but if you want to know how many items total, you can send the output of ls into the program wc (word count). For example:

ashroyer@~ ls | wc  # newline count, word count, character count
     26      26     276
ashroyer@~ ls | wc -l  # lines only
26

This can be confusing when you're new to the CLI. For the most part, "words" including commands, options, and arguments, are separated from each other by spaces. But what if you want an argument that contains spaces? This can often be solved by wrapping the multi-word phrase in double quotes:

ashroyer@~ ls -lA | wc         # a command (ls -lA) piped into another command (wc)
     67     596    4031
ashroyer@~ echo ls -lA | wc    # Oops! This counted the output of echo ls -lA ("ls -lA" ) which was only 7 characters
      1       2       7
ashroyer@~ echo "ls -lA | wc"  # What I wanted instead was to echo the text of the command, rather than its output
ls -lA | wc

Most operating systems have the concept of directories (or folders) and files. Files are the data, and folders are places to put data or other folders. Linux is no different, but in a text-only environment we have text-based ways to find files and run commands in different folders.

A command called pwd, short for "print working directory" lists the full path from the root of the filesystem to your shell's current execution context.

Example (with output):

ashroyer@~ pwd
/u/ashroyer

The first line contains the prompt and the command; the second line contains the output of pwd. Here, the working directory is /u/ashroyer. The first slash character (/) indicates the root of the filesystem. The next part (u) is a folder called u. Next comes another slash, which serves to visually separate folders, and finally ashroyer which is another folder name.

This becomes clear if you change directory and use pwd again. The command cd changes directories. It takes as argument the path to the directory you want to go to.

ashroyer@~ cd /tmp
ashroyer@~ pwd
/tmp
ashroyer@~ cd
ashroyer@~ pwd
/u/ashroyer

Above, I first changed my current directory to /tmp, then I ran cd again (this time with no argument) and it sent me to /u/ashroyer again, because that's the home directory for my account. The home directory is often abbreviated with a tilde: ~, as in cd ~/myfolder (which is short for cd /u/ashroyer/myfolder). Another nice feature of cd is when you want to change back to the previous directory, you can use cd -. This can save a lot of typing:

ashroyer@~ cd /some/very/long/directory/path
ashroyer@~ cd /some/other/very/long/path
ashroyer@~ cd -
ashroyer@~ pwd
/some/very/long/directory/path

You can cd .. to move up one directory, or cd ../../ to move up two levels, etc:

ashroyer@~ mkdir -p some/very/long/directory/path  # mkdir means "make directory", the -p flag means "even if it doesn't already exist"
ashroyer@~ cd some/very/long/directory/path
ashroyer@~ pwd
/u/ashroyer/some/very/long/directory/path
ashroyer@~ cd ../
ashroyer@~ pwd
/u/ashroyer/some/very/long/directory
ashroyer@~ cd ../../../
ashroyer@~ pwd
/u/ashroyer/some

Finally, you can cd up and down a different path in a single command:

ashroyer@~ cd some/very/long/directory/path
ashroyer@~ cd ../../../other/long/path
ashroyer@~ pwd
/u/ashroyer/some/other/long/path