====== BASH ======

==== WILDCARD METACHARACTERS & EXPANSION ====

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:
<code bash>
$ echo *
</code>

What's happening:
  - 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 DOCUMENTS ====

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

Example:
<code bash>
cat << _EOF_
hello
here
_EOF_
</code>

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

==== COMMAND SUBSTITUTION ====

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.
<code bash>
$ CURRENT_DATE=$(date)   # CURRENT_DATE will be assigned the output of date command
</code>

==== ENVIRONMENT VARIABLEs ====

Environment variables are named values stored by the shell/OS that processes can read to configure their behavior. Think of them as a key-value store available to every running program.

Environment variables are **not** global system state. They are per-process (and inherited downward only).

Setting an env variable for the current session:
<code bash>
export STUFF=blah
</code>

To make env variable permanent, you need to store it in a file. Which file - depends on the Shell type:

Type of Shells:

^ Shell Type ^ When it starts ^ Example ^ Reads file ^
| Login Shell | You authenticate (SSH, TTY login, su -, bash -l) | SSH into a server | ~/.profile |
| Interactive non-login | You open a new terminal in an existing session | New tab in your terminal  emulator | ~/.bashrc |

  * **~/.profile** — runs once per login session. Good for:
    * export PATH=..., export EDITOR=vim
  * **~/.bashrc** — runs every time you open a new interactive bash shell. Good for:
    * aliases, shell functions, prompt


==== REDIRECTS vs PIPES ====

The core difference: redirects connect a stream to a file (a redirect always has a file on one side). Pipes connect one process to another process.


Pipes: process → process:
<code bash>
cmd1 | cmd2           # Takes cmd1's stdout and feeds it directly into cmd2's stdin. No file involved
ls | grep ".txt"      # ls output becomes grep's input
</code>
==== STANDARD INPUT and OUTPUT ====

Unix programs have 1 input and 2 outputs.

When you run a command from a terminal, they all go to/from the terminal by default, e.g.:
<code bash>
$ cat
hello   # Stdin is connected to the terminal, you can type there.
hello   # Stdout - cat prints it right away after you pressed enter.
</code>


** < redirects Stdin**
<code bash>
cat < foo.txt
bar
</code>