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Introduction to the shell

I've recently joined Eva Dee from include JS on a study group of the Missing Semester lessons on youtube. The first lesson is all about the shell, this post contains my notes about the first lesson- it's a good, introduction to the shell.

✏️ Official Notes

Introduction

The command echo prints out the given argument to the terminal.

example

echo hello

will print

hello

Note: Arguments are separated by white space

  • By default running a command it will try to run on the current directory

Installed programs

  • Shell uses environment variables to check for programs installed. This variables are set and don't change with each restart of the shell
  • The environment variable responsible to keep a list of all programs installed is called $PATH
    • Path is a list of program paths separated by a colon :
  • We can check the programs path by running the command which
    • Running which echo will give us /usr/bin/echo on a unix machine
  • Absolute paths are paths that fully determine the location of the file
  • Relative Path is the path that is relative to the place where we are.
    • If you are inside your documents folder and have a file caled text.txt the relative path of that file is ./text.txt

Since arguments are separated with white space, you need to deal with them in two ways:

  • escape the space with \
    • example: echo Hello\ World
  • put the name in quotes
    • example: echo "Hello World"

Useful Commands

  • mv allows you to rename a file or move a file if you give a path to the second argument.
    • mv text.txt test.txt will rename the file text.txt to test.txt
    • mv test.txt /documents/text.txt will move the file into the documents folder and rename it back to text.txt
  • cp allows you to copy a file it takes two arguments - the path and file to copy from and the path to copy to
    • cp text.txt /documents/text.txt will copy the file text.txt to the documents folder
  • rm allows you to remove a file
  • rmdir allows you to remove a directory if it's empty
  • mkdir allows you to create a directory
  • man allows you to pass a program name as argument and read the manual about that program
    • man ls will give the manual pages of the ls command
      • press q to quit the manual pages
  • clear or shortcut ctrl+L clears all terminal text and moves prompt back to the top
  • cat prints the contents of a file
  • tail prints the last n lines of a file
    • We can run tail -n1 to print the last line of a file
  • grep let's you search for an input string for a given keyword
  • tee takes a command and writes it to a file but also to stdout (standandard output - the terminal)
  • Command pwd - print working directory - shows full path of the directory we are in
  • Command cd - change current directory - change to a directory
    • example: cd home

cd - Change Directory

Useful cd arguments

  • . means current directory
  • .. means parent directory
    • using cd .. moves working directory to the parent folder - one folder up
  • ~ means home directory
  • - move to the directory that you were previously in

Example: Moving directories

Let's assume you have the following directory:

home
  documents
    pictures

If our pwd is documents, we can move to the home by running the command:

cd ..

Now running pwd will give us

/home

We can then use the relative path to go back to documents

cd ./documents

We can think of these two as .. move a directory up and . as move a directory down.

Let's assume you are inside the pictures folder, you can go straight to the home folder by running this command

cd ../../

You can see that it moved the current directory first to documents then to home.

Since we moved to the home directory with the previous command, we can run cd - to move back to the pictures directory. If we run cd - again we will be moved back to the home directory.

ls- Listing Files in Current Directory

  • ls list files in the current directory

We can pass flags or options to commands by using a - followed by that flag/option argument.

  • flag anything that doesn't take a value
  • option anything that takes a value

Example

Running ls -l will run the ls command with the -l flag - this command will run the command with a long listing format (it shows permission, owner, group, size, last changed)

Let's assume you have the following after running ls -l

drwxr-xr-x 1 FabioRosado FabioRosado 24 Jun 13 12:42 _data
  • The initial d means that a file is a directory
  • The rest of the letters - rwxr-xr-x - are the permissions
    • The first group of letters -rwxr- - are the permissions for the group FabioRosado
    • The second group of letters - xr- are permissions for anyone else that is not in the group.
    • The last one is everyone else
  • The number after - 1 - means the permissions of the group
    • In this case 1 means that all files are owned by the FabioRosado group
  • FabioRosado FabioRosado is the user and group
  • The date is the last time the file was modified
  • The final bit, contains the file/folder name

Permissions

Permissions are shown on the shell like the previous example: rwxr-xr-x

Note: they will always follow the pattern rwx.

  • r means reading permission
  • w means writing permission
  • x means executing permission
  • - means you don't have that permission

For directories things are a bit diferent:

  • r means if you are allowed to see which files are inside the directory
  • w means if you are allowed to rename, create or remove within the directory
  • x means if you are allowed to enter the directory
    • to cd into the directory you must have the x permission set on all parent directories.

We can change the permissions of a file with the command chmod.

  • chmod u+x file Give the [u]ser who owns a file the right to e[x]ecute it

From IncludeJS - Eva Dee Notes

Streams: Combining Programs

In shell you can combine programs by chaining commands, interacting with files, etc.

  • Every program has at least two separated streams
    • Input stream
    • output stream
  • > rewire the output of this program to the file on the right
  • < rewire the output of the file on the right into the left program
  • >> append output of the program to the right file
  • | takes the output of the program to the left and make it the input of the program on the right

Example: Using the rewire operators

Running this command will create a file with the contents of the command echo hello

echo hello > hello.txt

Note: Nothing will be printed because the output of the command echo hello is being added to the hello.txt file

We can check that the contents of hello.txt do contain the word hello by running the command

$ cat hello.txt
hello

We could combine the two operators (> and < ) to copy the contents of a file 

cat < hello.txt > hey.txt

This means that we are

  • Opening hello.txt and pass it to the cat command, which will output the text hello
  • Use that hello text to input it back to a file called hey.txt

Note: The cat command doesn't know about the redirection to the hey.txt file - the shell handles everything

Now running the command cat hey.txt we can see that we get back hello

$ cat hey.txt
hello

If we want to append another hello to our file we can use the append operator

$ echo hello >> hey.txt
$ cat hey.txt
hello
hello

Example: Using the pipe operator

We could use the pipe operator to get the last line of the command ls -l by running the command:

ls -l | tail -n1

We can also write the output of that last line into a file by combining both operators

ls -l | tail -n1 > ls.txt