Special Characters

Folder structure

Like in natural language, in UNIX we define as path the route to reach a specific element (a file, a folder). The path can be absolute or relative.

If the path is absolute its origin is called the root folder, and it is indicated as / at the very beginning.

$ pwd
/home/AD/flescai/tests/bash

In the example above the folder bash can be reached starting from the root down the tree through the home folder, inside the home through the AD folder, and inside of it the user folder, and so on. Note that each folder in a path is separated by a subsequent / character.

If the path is relative, its origin refers to the current folder where we are located with the terminal. It can be any sub-folder

./current/sub-folder/sub-sub-folder

Or it can be a sub-folder of the parent folder

../parent-sub-folder/file

Special Characters

There are many special characters in UNIX which serve different functions. Among the most common:

Character	Function
#	The character is used mostly in scripts to indicate a comment, i.e. a line that should not be executed
‘’	Strong quotes: disable all special characters between quotes, everything is considered a string
“”	Weak quotes: disable all special characters except $, i.e. allowing variables to be transformed in strings
``	Backquotes: command substitution, i.e. text inside is interpreted as a prompt command
>	Redirect output (for example, to write the shell output into a file)
>>	Append output. The difference with redirect, is that output is written at the end of a file without deleting previous content
<	Redirect input.
$	Used to indicate variables, by preceding the variable name.
|	Pipe: used to combine shell commands one after the other, while directing the output of the previous command into the next one
*	The star is considered a wild card, i.e. indicating any possible caracter or file or folder

Escape

The character \ is used to escape (i.e. disable) special characters and instruct UNIX to consider them like strings. It can be used in several useful cases, for example if we wanted to print quotes

$ echo "this is a single quote \""

Or if we wanted to write several options of a command in a new line, without starting the execution until we completed the code

$ ls \
-l \
-t