Bash equivalent of Ruby and JavaScript OR when initializing variables

Question

Sometimes I see in JavaScript:

a||=1

Which means - as far as I know - that if "a" is not defined, or null then it was initialized with value 1, otherwise nothing happens. I do the same in Ruby scripts, for example when a command line argument wasn't passed:

gamma=ARGV[0]||"1.0"

Then variable gamma gets its value from ARGV[0] assuming it is not nil, a value was passed, otherwise it will be "1.0".

It is a great shorthand for:

if ARGV[0]==nil then
  gamma="1.0"
else 
  gamma=ARGV[0]
end

and even:

gamma=ARGV[0]==nil ? "1.0" : ARGV[0]

I would like to use a similar cinstruction in a ruby script, but it doesn't work as expected, because a nil or null value doesn't exist, so expression:

$0||"1.0"

always gives the value of $0, even if it is an empty string "". Is it possible to use something similar shorthand syntax in bash scripts too?

Note: in JavaScript you do `foo = foo || bar`, there's no `||=` operator (yet). — gcampbell, Jun 04 '16 at 11:51
And in Ruby everything except nil and false is considered true, even empty strings. — gcampbell, Jun 04 '16 at 11:52

score 2 · Accepted Answer · answered Jun 04 '16 at 13:42

In bash you can use the Assign Default Values parameter expansion operator.

: ${a:=1.0}

${parameter:=word}
If parameter is unset or null, the expansion of word is assigned to parameter.

The : command is a no-op, it's used just so we can perform the parameter expansion in its argument list.

Bash equivalent of Ruby and JavaScript OR when initializing variables

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