Bourne shell: trailing `-` operator in parameter substitution

Question

I'm reading a shell script for adding a progress bar to certain processes (found here). I'm having trouble understanding this (optional) line:

#BAR_EXT=${BAR_EXT-}

The comment says that this will add an extension to each file, and maybe I just need to read further, but I'm not familiar with that use of the - operator.

I know about this kind of substitution, as found in the Bash Reference Manual:

${parameter:-word}

I also know that the above will replace a null value for parameter with word, whereas ${parameter-word} will not. (At least, I think I know that.)

But with nothing specified after the - here, I'm not sure what's going on. Will this simply replace parameter with a null value? Generally, I would accept that as a working guess and just keep reading, but with the comment mentioning adding extensions to files.

Answer 1

Does the script contain the command set -u?  That means

Treat unset variables and parameters other than the special parameters "@" and "*" as an error when performing parameter expansion.  If expansion is attempted on an unset variable or parameter, the shell prints an error message, and, if not interactive, exits with a non-zero status.

In other words, if $BAR_EXT is not set, something like

BAR_FILE="$BAR_FILE$BAR_EXT"

would fail.  The command

BAR_EXT=${BAR_EXT-}

will explicitly set $BAR_EXT to an empty string if it is not defined at all, thereby avoiding such an error.

Answer 2

${VARIABLE-} expands to the value of VARIABLE if it's set, and to the empty string otherwise. This is usually what the straight $VARIABLE does. However, when the option set -u (set -o nounset) is on, $VARIABLE triggers an error when the variable is unset, whereas ${VARIABLE-} still produces an empty string. So ${VARIABLE-} means “I intended this to result in an empty string even if the variable is unset”, whereas $VARIABLE could mean that or could mean “I intended the variable to be set”.

Answer 3

Check the man page for the recent Bourne Shell:

http://schillix.sourceforge.net/man/man1/bosh.1.html

and look for the section "Parameter Substitution", this is currently on page 8. Two pages downwards, there is a table that explains all possible expansions.

The Bourne Shell supports much more than just '-' in parameter substitution.