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Once one is inside $(...), quoting starts all over from scratch.

In other words, "..." and $(...) can nest within each other. Process substitution, $(...), can contain one or more complete double-quoted strings. Also, double-quoted strings may contain one or more complete process substitutions. But, they do not interlace. Thus, a double-quoted string that starts inside a process substitution will never extend outside of it or vice versa.

So, consider:

DIR="$( cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd )"

Inside the inner $(...) is:

dirname "${BASH_SOURCE[0]}"

In the above ${BASH_SOURCE[0]} is double-quoted. Any quotes, double or single, outside of the $(...) are irrelevant when determining that ${BASH_SOURCE[0]} is double-quoted.

The outer $(...) contains:

cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd

Here, the expression $( dirname "${BASH_SOURCE[0]}" ) is double-quoted. The fact that there are quotes outside of the outer $(...) is irrelevant when considering what is inside it. The fact that there are quotes inside the inner $(...) is also irrelevant.

Here is how the double-quotes match up:

Once one is inside $(...), quoting starts all over from scratch.

In other words, "..." and $(...) can nest within each other. Command substitution, $(...), can contain one or more complete double-quoted strings. Also, double-quoted strings may contain one or more complete command substitutions. But, they do not interlace. Thus, a double-quoted string that starts inside a command substitution will never extend outside of it or vice versa.

So, consider:

DIR="$( cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd )"

Inside the inner $(...) is:

dirname "${BASH_SOURCE[0]}"

In the above ${BASH_SOURCE[0]} is double-quoted. Any quotes, double or single, outside of the $(...) are irrelevant when determining that ${BASH_SOURCE[0]} is double-quoted.

The outer $(...) contains:

cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd

Here, the expression $( dirname "${BASH_SOURCE[0]}" ) is double-quoted. The fact that there are quotes outside of the outer $(...) is irrelevant when considering what is inside it. The fact that there are quotes inside the inner $(...) is also irrelevant.

Here is how the double-quotes match up:

$(...) binds tighter than "...".

Another way of thinking about it is, once one is inside $(...), quoting starts all over from scratch.

So, consider:

DIR="$( cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd )"

Inside the inner $(...) is:

dirname "${BASH_SOURCE[0]}"

In the above ${BASH_SOURCE[0]} is double-quoted. Any quotes, double or single, outside of the $(...) are irrelevant when determining that ${BASH_SOURCE[0]} is double-quoted.

The outer $(...) contains:

cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd

Here, the expression $( dirname "${BASH_SOURCE[0]}" ) is double-quoted. The fact that there are quotes outside of the outer $(...) is irrelevant when considering what is inside it. The fact that there are quotes inside the inner $(...) is also irrelevant.

Here is how the double-quotes match up:

Once one is inside $(...), quoting starts all over from scratch.

In other words, "..." and $(...) can nest within each other. Process substitution, $(...), can contain one or more complete double-quoted strings. Also, double-quoted strings may contain one or more complete process substitutions. But, they do not interlace. Thus, a double-quoted string that starts inside a process substitution will never extend outside of it or vice versa.

So, consider:

DIR="$( cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd )"

Inside the inner $(...) is:

dirname "${BASH_SOURCE[0]}"

In the above ${BASH_SOURCE[0]} is double-quoted. Any quotes, double or single, outside of the $(...) are irrelevant when determining that ${BASH_SOURCE[0]} is double-quoted.

The outer $(...) contains:

cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd

Here, the expression $( dirname "${BASH_SOURCE[0]}" ) is double-quoted. The fact that there are quotes outside of the outer $(...) is irrelevant when considering what is inside it. The fact that there are quotes inside the inner $(...) is also irrelevant.

Here is how the double-quotes match up:

$(...) binds tighter than "...".

Another way of thinking about it is, once one is inside $(...), quoting starts all from scratch.

So, consider:

DIR="$( cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd )"

Inside the inner $(...) is:

dirname "${BASH_SOURCE[0]}"

In the above ${BASH_SOURCE[0]} is double-quoted. Any quotes, double or single, outside of the $(...) are irrelevant when determining that ${BASH_SOURCE[0]} is double-quoted.

The outer $(...) contains:

cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd

Here, the expression $( dirname "${BASH_SOURCE[0]}" ) is double-quoted. The fact that there are quotes outside of the outer $(...) are irrelevant when considering what is inside it. The fact that there are quotes inside the inner $(...) is also irrelevant.

$(...) binds tighter than "...".

Another way of thinking about it is, once one is inside $(...), quoting starts all over from scratch.

So, consider:

DIR="$( cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd )"

Inside the inner $(...) is:

dirname "${BASH_SOURCE[0]}"

In the above ${BASH_SOURCE[0]} is double-quoted. Any quotes, double or single, outside of the $(...) are irrelevant when determining that ${BASH_SOURCE[0]} is double-quoted.

The outer $(...) contains:

cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd

Here, the expression $( dirname "${BASH_SOURCE[0]}" ) is double-quoted. The fact that there are quotes outside of the outer $(...) is irrelevant when considering what is inside it. The fact that there are quotes inside the inner $(...) is also irrelevant.

Here is how the double-quotes match up: