Bash: declare in depth: Part 4: the oddballs

We've covered almost all of declare's options, but there are two that stand out for being of questionable usefulness: -l and -u.

These flags change the case of a variable to lowercase (-l) and uppercase (-u) respectively when they are assigned.

In total, I counted three ways for changing casing in Bash:

• declare with -u or -l
• parameter expansion with ^^ and ,,: ${to_upper^^} and ${to_lower,,}
• using an @ parameter during expansion: ${to_upper@U} and {to_lower@L}

Interaction with namerefs

So how does declare -u work together with namerefs?

Does it upcase the name of the variable referred to by the nameref?

Does it upcase the assigned value?

Let's find out!

var=1
VAR=2
declare -nu ref=var
ref=3
printf "var=%d, VAR=%d\n" "$var" "$VAR"
# prints var=1 VAR=3

Applied to a nameref, the name of the variable referred to is changed to uppercase.

Application: metaprogramming and enforcing naming conventions

The only application of this that I can think of is enforcing naming conventions of variables when metaprogramming, e.g. global variables should always be in uppercase.

Here's defconst, a function which defines a global variable, whose value cannot be changed and whose name is always going to be in uppercase.

defconst() {
  local -nu ref="$1"
  ref="$2"
  readonly "${!ref}"
  printf "%s" "${!ref}" # the name the reference points to
}

defconst pi 3.14 # prints PI
printf "PI=%s\n" "$PI" # prints 3.14
PI=bar # error: PI: readonly variable