1	#!/bin/bash
2
3	### Lazy Evaluation of Alternative
4	i=0
5	x=x
6	echo ${x:-$((i++))}
7	echo $i
8	echo ${undefined:-$((i++))}
9	echo $i # i is one because the alternative was only evaluated once
10	# status: 0
11	# stdout-json: "x\n0\n0\n1\n"
12	# N-I dash status: 2
13	# N-I dash stdout-json: "x\n0\n"
14
15	### Default value when empty
16	empty=''
17	echo ${empty:-is empty}
18	# stdout: is empty
19
20	### Default value when unset
21	echo ${unset-is unset}
22	# stdout: is unset
23
24	### Assign default value when empty
25	empty=''
26	${empty:=is empty}
27	echo $empty
28	# stdout: is empty
29
30	### Assign default value when unset
31	${unset=is unset}
32	echo $unset
33	# stdout: is unset
34
35	### Alternative value when empty
36	v=foo
37	empty=''
38	echo ${v:+v is not empty} ${empty:+is not empty}
39	# stdout: v is not empty
40
41	### Alternative value when unset
42	v=foo
43	echo ${v+v is not unset} ${unset:+is not unset}
44	# stdout: v is not unset
45
46	### Error when empty
47	empty=''
48	${empty:?is empty}
49	# status: 1
50	# OK dash status: 2
51
52	### Error when unset
53	${unset?is empty}
54	# status: 1
55	# OK dash status: 2
56
57	### Error when unset
58	v=foo
59	echo ${v+v is not unset} ${unset:+is not unset}
60	# stdout: v is not unset