Bash: arithmetic expansion in array indices - is the dollar sign needed?

Question

When I use arithmetic expansion in an array index in bash, like this:

declare -a FILES
declare -i INDEX=0

for FILE in ./*
do
    FILES[((INDEX++))]=\"$FILE\         


        

Answer 1

The dollar sign is required in some contexts but not the others:

$ bash --version
GNU bash, version 4.3.42(1)-release (x86_64-redhat-linux-gnu)
Copyright (C) 2013 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later 

$ echo ((1+2))
bash: syntax error near unexpected token `('

$ echo $((1+2))
3

$ for ((x=0; x<3;++x)); do echo $x; done
0
1
2

$ for $((x=0; x<3;++x)); do echo $x; done
bash: `$((x=0; x<3;++x))': not a valid identifier

---

After reading bash man page, the dollar sign is not required in compound commands:

Compound commands are the shell programming constructs. Each construct begins with a reserved word or control operator and is terminated by a corresponding reserved word or operator. Any redirections (see Redirections) associated with a compound command apply to all commands within that compound command unless explicitly overridden.

In most cases a list of commands in a compound command’s description may be separated from the rest of the command by one or more newlines, and may be followed by a newline in place of a semicolon.

Bash provides looping constructs, conditional commands, and mechanisms to group commands and execute them as a unit.

for (( expr1 ; expr2 ; expr3 )) is a compound command and hence the dollar sign is not required to enable arithmetic evaluation.

Whereas echo $((expr)) is not a compound command because it does not start with a reserved bash keyword, so it requires a dollar sign to enable arithmetic evaluation.