Bash assignment value to variable as command substitution and print value output

Question

I would like to achieve this in Bash: echo $(a=1)and print the value of variable a

I test eval, $$a,{}, $() but none of them work as most of the times either I got literally a=1 or in one case (I don't remember which) it tried to execute the value.

I known that I can do: a=1;echo $a but because I'm little fun one command per line (even if sometimes is getting little complicated) I was wondering if is possible to do this either with echo or with printf

Answer 1

If you know that $a is previously unset, you can do this using the following syntax:

echo ${a:=1}

This, and other types of parameter expansion, are defined in the POSIX shell command language specification.

If you want to assign a numeric value, another option, which doesn't depend on the value previously being unset, would be to use an arithmetic expansion:

echo $(( a = 1 ))

This assigns the value and echoes the number that has been assigned.

It's worth mentioning that what you're trying to do cannot be done in a subshell by design because a child process cannot modify the environment of its parent.