Lesson 2 - Shell, Directory Structure and STDIO

Shell

What is a shell?

command line interface to the operating system
a programm that take commands from the standard input and executes them in the operating system
replaced by Graphical User Interfaces for most end users
most common shells are bash (Bourne Again Shell) and zsh (Z Shell)

Shell commands

user types a line of text
shell reads the line of text and executes the command
shell waits for the command to finish
shell prints the output of the command
then waits for the next command

structure is command [options] [arguments]
eg ls -l /home

arguments are input to the command
- it can be a file, a directory, a string, etc
options are flags that modify the behaviour of the command
- they are usually preceded by a - or --
the core utilities are documented in the man pages
the binary is usually located in /bin or /usr/bin
the shell indexes these folders because they are in the $PATH variable

stdio

standard input, standard output, standard error are the three default streams that are opened when a program is executed
they are abstract devices that are mapped to the terminal

Manipulating stdio

> redirects the output of a command to a file
>> appends the output of a command to a file
< redirects the input of a command to a file
2> redirects the error output of a command to a file
&> redirects the output and error output of a command to a file
| pipes the output of a command to the input of another command

Filesystem

the filesystem is a tree structure
everything is a file!
- files are files
- directories are files that contain other files
- devices are files that represent hardware
- sockets are files that represent network connections
- pipes are files that represent interprocess communication
- processes are files that represent running programs