mjhjmtu

When a default value is provided for a variable expansion (e.g. $varname-default), how are the characters that make up default meant to be interpreted?

Naively, I would have guessed that variable substitution (e.g. $b), and expansion of quote-like characters ', ", would occur in this context ... but it appears that expansion of quote-like characters does not always occur.

For example,

> echo x$a-''y
xy

but

> echo x"$a-''"y 
x''y

I expected xy in both cases, since construction that expand variables like $a-$PATH and ( BRACE='}'; echo $a-$BRACE ), work fine.

However GNU bash, version 4.4.19(1)-release and ... the latest version of mksh both have the same behavior where '' is included literally in the output for the second example.

The $parameter:-[word] expansion is subject to several expansions:

From man bash:

In each of the cases below, word is subject to tilde expansion, parameter expansion, command substitution, and arithmetic expansion.

From POSIX spec:

word shall be subjected to tilde expansion, parameter expansion, command substitution, and arithmetic expansion.

Both report the same, and both do not include "quote removal".

The quotes get removed after the expansion of the whole variable has already been done. However, it is difficult to show exactly how that is done:

$ sh
$ unset b
$ set -x
+ set -x
$ echo 1 $b-e
+ echo 1 e
1 e
$ echo 2 $b-"e"
+ echo 2 e
2 e
$ echo 3 $b-'e'
+ echo 3 e
3 e
$ echo 4 "$b-e"
+ echo 4 e
4 e
$ echo 5 "$b-"e""
+ echo 5 e
5 e
$ echo 6 "$b-'e'"
+ echo 6 'e'
6 'e'
$ set +x
+ set +x

Or:

$ $ set -x; echo 1 x$b-ey; echo 2 x$b-"e"y; echo 3 x$b-'e'y; echo 4 "x$b-ey"; echo 5 "x$b-"e"y"; echo 6 "x$b-'e'y"; set +x
+ echo 1 xey
1 xey
+ echo 2 xey
2 xey
+ echo 3 xey
3 xey
+ echo 4 xey
4 xey
+ echo 5 xey
5 xey
+ echo 6 x'e'y
6 x'e'y
+ set +x
$