Bash
└─ Parameter expansion

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 for more information.