Default values
Most people know a variable can be referenced as ${FOO}, but you can put extra syntax inside the { } that can save you some if/else blocks.
If null, use default value
mingus:~ 07:47:10
$ export FOO=first
mingus:~ 07:48:58
$ echo "The ${FOO-second} choice"
The first choice
mingus:~ 07:51:49
$ unset FOO
mingus:~ 07:51:56
$ echo "The ${FOO-second} choice"
The second choice
mingus:~ 07:51:59
$ export FOO=
mingus:~ 07:52:03
$ echo "The ${FOO-second} choice"
The choice
If null or empty, use default value
mingus:~ 07:52:06
$ export FOO=first
mingus:~ 07:53:39
$ echo "The ${FOO:-second} choice"
The first choice
mingus:~ 07:53:44
$ unset FOO
mingus:~ 07:53:52
$ echo "The ${FOO:-second} choice"
The second choice
mingus:~ 07:53:56
$ export FOO=
mingus:~ 07:54:04
$ echo "The ${FOO:-second} choice"
The second choice
Default value can be a variable
mingus:~ 07:55:06
$ export FOO=first
mingus:~ 07:55:47
$ export BAR=second
mingus:~ 07:56:01
$ echo "The ${FOO:-$BAR} choice"
The first choice
mingus:~ 07:56:15
$ unset FOO
mingus:~ 07:56:51
$ echo "The ${FOO:-$BAR} choice"
The second choice
Default value can be a command
mingus:~ 08:08:02
$ export FOO="Friday"
mingus:~ 08:08:10
$ date +%A
Wednesday
mingus:~ 08:08:15
$ echo "Today is ${FOO:-$(date +%A)}"
Today is Friday
mingus:~ 08:08:19
$ unset FOO
mingus:~ 08:08:26
$ echo "Today is ${FOO:-$(date +%A)}"
Today is Wednesday
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