Add the "Bash: Parameter expansion" page.

Author: paveloom

content/notes/Bash_Parameter_expansion.md

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+title = "Bash: Parameter expansion"
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+
+In Bash, the `$` character can be used to introduce parameter expansion.
+
+Here's a brief list of such parameter expansions:
+
+- Use default value: `${parameter:-word}`
+- Assign default value: `${parameter:=word}`
+- Display error if null or unset: `${parameter:?word}`
+- Use alternate value: `${parameter:+word}`
+- Substring expansion:
+  - `${parameter:offset}`
+  - `${parameter:offset:length}`
+- Names matching prefix:
+  - `${!prefix*}`
+  - `${!prefix@}`
+- List of array keys:
+  - `${!name[@]}`
+  - `${!name[*]}`
+- Parameter length: `${#parameter}`
+- Remove matching prefix pattern:
+  - `${parameter#word}`
+  - `${parameter##word}`
+- Remove matching suffix pattern
+  - `${parameter%word}`
+  - `${parameter%%word}`
+- Pattern substitutions:
+  - `${parameter/pattern/string}`
+  - `${parameter//pattern/string}`
+  - `${parameter/#pattern/string}`
+  - `${parameter/%pattern/string}`
+- Case modification:
+  - `${parameterˆpattern}`
+  - `${parameterˆˆpattern}`
+  - `${parameter,pattern}`
+  - `${parameter,,pattern}`
+- Parameter transformation: `${parameter@operator}`
+  - `U`: all uppercase
+  - `u`: first character uppercase
+  - `L`: all lowercase
+  - `Q`: quoted for input
+  - `E`: expand backslash sequences
+  - `P`: prompt string
+  - `A`: assignment statement
+  - `K`: quoted key-value pairs
+  - `a`: parameter's attributes
+  - `k`: like `K`, but keys and values are separate words
+
+See [`man bash`](https://www.man7.org/linux/man-pages/man1/bash.1.html) for more information.