Bash: Error in assigning default value to a variable

Question

Inside my bash script:

This works:

CWD="${1:-${PWD}}"

But, if I replace it with:

CWD="${1:=${PWD}}"

I get the following error

line #: 1ドル: cannot assign in this way

Why I can't assign to ${1}?

Answer 1

From bash's manpage:

Positional Parameters
 A positional parameter is a parameter denoted by one or more digits,
 other than the single digit 0. Positional parameters are assigned from
 the shell's arguments when it is invoked, and may be reassigned using
 the set builtin command. Positional parameters may not be assigned to
 with assignment statements. The positional parameters are temporarily
 replaced when a shell function is executed (see FUNCTIONS below).

and later, under Parameter Expansion

${parameter:=word}
 Assign Default Values. If parameter is unset or null, the
 expansion of word is assigned to parameter. The value of param‐
 eter is then substituted. Positional parameters and special
 parameters may not be assigned to in this way.

If you want to assign a default value to the positional parameter 1ドル like in your question, you can use

if [ -n "1ドル" ]
then
 CWD="1ドル"
else
 shift 1
 set -- default "$@"
 CWD=default
fi

Here I've used a combination of shift and set. I've just come up with this and I'm not sure if it's the proper way to change a single positional parameter.

• Kusalananda: Additionally, to assign something to 1ドル, you would use e.g. set -- "$PWD" (this would clear the other positional parameters though, unless you did set -- "$PWD" "$@", which would "push" $PWD to the front of $@).
• Stefan Hamcke: @Kusalananda: I wonder if you can change a single pos. parameter. I've now tried a combination of set and shift, see my edit.
• Stefan Hamcke: Oh, and I probably forgot the --, just in case some of the arguments start with a hyphen. Thanks @Kusalananda
• Kusalananda: Simplified: if [ -z "1ドル" ]; then shift; set -- default "$@"; fi; CWD="1ドル"