The shell expands special characters (globs/wildcards) before passing arguments to commands. This is called pathname expansion (or globbing).

Core wildcards:

• * - zero or more characters. Example: ls *.jpg - all .jpg files in the current directory
• ? - exactly one character. Example: cp report?.pdf /backup/ - report1.pdf, reportA.pdf, etc.

For example, following commands prints a list of files in the current directory:

$ echo *

What's happening:

1. Globbing (before echo runs) - The shell — not echo — expands * by looking at the current directory and replacing * with a space-separated list of matching filenames.

Disabling expansion

• If you don’t want the shell to expand a glob in a command, enclose the glob in single quotes (' '). For example, the command echo '*' prints a star.
• This might be useful when running commands like find, to pass patterns containing wildcards
• The reason why find works sometimes is because shell expansion only happens if matches exist in the current directory

Here document is just another form of I/O redirection where we embed a body of text into our script, the format is:

command << token
text
token

Example:

cat << _EOF_
hello
here
_EOF_

If we change the redirection operator from « to «- the sell will ignore leading tabs.

Command substitution allows us to use the output of the command as expansion (substitution).

Basically it allows the output of the command to replace the command itself.

$ CURRENT_DATE=$(date)   # CURRENT_DATE will be assigned the output of date command

stat file.txt            # shows additional details of the file (when created/modified etc)