Brace expansion from a variable in Bash

问题

I would like to expand a variable in Bash. Here's my example:

variable="{1,2,3}"
echo $variable

Expected output:

1 2 3

Actual output:

{1,2,3}

回答1:

The expansion doesn't work because of the order in which bash performs command-line expansion. If you read the man page you'll see the order is:

1. Brace expansion
2. Tilde expansion
3. Parameter expansion
4. Command substitution
5. Arithmetic expansion
6. Process substitution
7. Word splitting
8. Pathname expansion
9. Quote removal

Parameter expansion happens after brace expansion, which means you can't put brace expansions inside variables like you're trying.

But never fear, there's an answer! To store a list of numbers in a variable, you can use an array.

variable=(1 2 3)
echo "${variable[@]}"

回答2:

This won't work first because double-quotes suppress brace expansion:

variable="{1,2,3}"

This still won't work because bash will not assign a list to a variable:

variable={1,2,3}

The solution is to assign your list to an array:

$ variable=( {1,2,3} )
$ echo "${variable[@]}"
1 2 3

来源：https://stackoverflow.com/questions/30064674/brace-expansion-from-a-variable-in-bash