Parameter Expansion (Mutli-issue thread)

[jeff-hykin]

Right now, these are supported:

• $PARAMETER
• ${PARAMETER}

These are not yet supported:

• Indirection
  • ${!PARAMETER}
• Case modification
  • ${PARAMETER^}
  • ${PARAMETER^^}
  • ${PARAMETER,}
  • ${PARAMETER,,}
  • ${PARAMETER~}
  • ${PARAMETER~~}
• Variable name expansion
  • ${!PREFIX*}
  • ${!PREFIX@}
• Substring removal (also for filename manipulation!)
  • ${PARAMETER#PATTERN}
  • ${PARAMETER##PATTERN}
  • ${PARAMETER%PATTERN}
  • ${PARAMETER%%PATTERN}
• Search and replace
  • ${PARAMETER/PATTERN/STRING}
  • ${PARAMETER//PATTERN/STRING}
  • ${PARAMETER/PATTERN}
  • ${PARAMETER//PATTERN}
• String length
  • ${#PARAMETER}
• Substring expansion
  • ${PARAMETER:OFFSET}
  • ${PARAMETER:OFFSET:LENGTH}
• Use a default value
  • ${PARAMETER:-WORD}
  • ${PARAMETER-WORD}
• Assign a default value
  • ${PARAMETER:=WORD}
  • ${PARAMETER=WORD}
• Use an alternate value
  • ${PARAMETER:+WORD}
  • ${PARAMETER+WORD}
• Display error if null or unset
  • ${PARAMETER:?WORD}
  • ${PARAMETER?WORD}

Here's some documentation for future me, courtsey of akpircher

Quick Reference Guides

• Bash scripting cheatsheet: Super quick reference of available syntax and tests
• Bash Hackers Wiki: A more descriptive cheat sheet

GNU Bash Documentation

• Shell Parameter Expansion has a complete list of all parameter expansions syntax with examples, which also apply to arrays. Scroll past the paragraphs.

  • Detail hidden in paragraphs: the first 4 examples (the parameter value test expansions, like :-) do NOT need the colon. The colon means "unset or empty" and without the colon the tests just means "unset"
  • Documented ...somewhere?: when using a (numeric) variable for the key of an indexed array, or a variable for the offset/length, you can reference the variable without the expansion identifier ($)
    • This applies to any situation where shell arithmetic can be performed

• Shell Arithmetic has a complete list of shell arithmetic operators

  • Detail hidden in first paragraph: the syntax is valid in the following:
    • let expression[s]
    • declare -i expression[s]
    • local -i expression[s]
    • ((expression[s])) and $((expression[s]))
  • Detail hidden later: bash supports the base syntax of
    base#value, which looks like it's covered.

Builtin references pages

• Bash Builtins can direct you to where to find builtin commands/functions.
  • Pages you may care about are:
    • Bourne Shell Builtins and Bash Builtin Commands
    • The links in the 3rd paragraph
  • Ignore:
    • Modifying shell, it's a TOC for the set and shopt commands (not actually listed elsewhere)
    • "Special Builtins", describes behavior of previously listed builtins in POSIX mode