The shell - missing semester notes

#todayilearned #beginners #computerscience #programming

Lecture 1: The Shell

Simply put, the shell is a program that takes commands from the keyboard and gives them to the operating system to perform.

shell prompt: where you type commands, in my case:
environment variable: variables set usually during the installation; they contain info necessary for the system shell when executing commands; one example: $PATH stores a list of paths on our machine where bash will be looking for a program to execute
absolute paths: full paths determining the location of the file in relation to the root directory (in MacOS it's /)
relative paths: paths in relation to a given file
arguments: what you write after a command with a space, for instance: in echo Hello, echo is a command and World is an argument
options: are optional arguments you can add to a command, they are are often preceded by a single dash, e.g. ls will list (ls command) all files and ls -a will also include those whose names begin with a dot (.)
flags: boolean-type options given to commands, they are invoked with two dashes --

pwd — print working directory
cd — change directory
ls- list files
mv - move files, takes two paths as arguments (old path and new path), which also allows you to rename the file if you just change the name or to move a file to another directory
cp - copy, also takes two paths (from and to)
rm - remove
rmdir - remove directory if it’s empty
mkdir — creates a new directory; it takes an argument of the directory name; note mkdir My Photos will create two directories ("My" and "Photos") so you'd need to call it mkdir My\Photos, mkdir “My Photos” or mkdir my-photos man -> for manual pass an arg of the program
cat - print out the contents of the file, takes an arg of a file path
man - print manual page for the given command; it takes an argument of the command, e.g. man echo
tail -n1 - prints the last n lines of the input
tee test.md - echoes the input but also saves it to the file
echo hello > hello.txt - overwrite the hello.txt with the hello input
cat < hello.txt - take hello.txt and populate cat method with it
cat < hello.txt > hello2.txt - take hello.txt and overwrite it into cat and then populate it into hello2.txt
>> - append, not overwrite
| - take the output of the program to the left and make it an input to the program to the right
sudo — "do as a su (superuser)", meaning execute a command as the root user (see below)
chmod - change file modes or Access Control Lists
grep - search for a substring in files

special user -- it can access (read, write, execute) any file
its id is 0
if you're operating as su, your prompt will start with # and not $
you can execute commands as root by running the sudo command
since has access to all the files, if you are using sudo, you’re acting as the operating system
generally not the best idea to use sudo too often cause you can mess up your computer

core of your computer
in Linux, you access it through ls/sys, which gives you an output of all the kernels in your computer browsable as files so you can use the programs at hand to manipulate them (cool demonstration)

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