Parameter expansion in Bash is a powerful feature that 
allows for the manipulation and transformation of shell 
variables and other parameters. It utilizes a special 
syntax, typically involving curly braces and various 
operators, to perform operations such as extracting 
substrings, replacing parts of a string, setting default 
values, or modifying case.
Here are some common types of parameter expansion:

Simple Expansion: ${parameter} substitutes the value of 
the parameter. This is equivalent to $parameter but is 
often preferred for clarity and to avoid ambiguity when 
concatenating with other strings. 

Código

my_var="hello"
echo "${my_var} world" # Output: hello world

Default Values:
${parameter:-word}: If parameter is unset or null, it expands to word. Otherwise, it expands to the value of parameter.
${parameter:=word}: If parameter is unset or null, it expands to word and parameter is set to word. Otherwise, it expands to the value of parameter. 

Código

unset my_var
echo "${my_var:-default}" # Output: default
my_var="value"
echo "${my_var:-default}" # Output: value

Error Handling: ${parameter:?word}: If parameter is unset or null, it prints word to standard error and exits the shell. Otherwise, it expands to the value of parameter. 

Código

unset my_var
echo "${my_var:?Error: my_var is not set}" # Exits with error message

Substring Extraction:
${parameter:offset}: Extracts a substring starting from offset.
${parameter:offset:length}: Extracts a substring of length characters starting from offset. 

Código

string="abcdefg"
echo "${string:2}"    # Output: cdefg
echo "${string:2:3}"  # Output: cde

Removing Prefixes/Suffixes:
${parameter#pattern}: Removes the shortest matching prefix pattern.
${parameter##pattern}: Removes the longest matching prefix pattern.
${parameter%pattern}: Removes the shortest matching suffix pattern.
${parameter%%pattern}: Removes the longest matching suffix pattern. 

Código

filename="image.tar.gz"
echo "${filename%.gz}"   # Output: image.tar
echo "${filename##*.}"   # Output: gz

String Length: ${#parameter} expands to the length of the value of parameter. 

Código

name="Bash"
echo "${#name}" # Output: 4

These are just some of the common uses of parameter expansion in Bash, offering a flexible and efficient way to work with variables in shell scripts.