sh -c: Unterminated quoted string error

Question

I've spent hours trying to understand the following error.

My Script

CMD="sh -c 'echo \"a\"'"
$CMD

Error:

"a"': 1: "a"': Syntax error: Unterminated quoted string

of course when I do echo $CMD and paste the result, it works as expected...

What is the cause of the error?

Edit:

I'm asking for an explanation not only a workaround. Therefore, I think the following link doesn't answer my question: Quoting in a function results in error

Solution 1

That cannot work. When your shell performs word splitting, you will have four arguments:

sh
-c
'echo
"a"'

To accomplish this, you must use a bash array:

cmd=( sh -c 'echo "a"' )
"${cmd[@]}"

Solution 2

The main error here is thinking that the single quotes around 'echo \"a\"' would stop "word splitting" performed by the shell and pass echo "a" as a single argument to sh.

In this case, the single quotes are actually treated as "literal" not "syntactical" because they are included within the outer double quotes!

The following link helped me the most: http://mywiki.wooledge.org/Arguments

Comments

[yeachan park]

Thanks, I didn't know aboud word splitting performed by the shell, I'm going to check this out!

[Angel Todorov]

See also I'm trying to put a command in a variable, but the complex cases always fail!

[Angel Todorov]

Also, spend some time with the bash manual: gnu.org/software/bash/manual/bashref.html#Shell-Expansions

[yeachan park]

see my answer below! thanks for pointing me in the right direction!