SC2300 – ShellCheck Wiki

Parameter expansion can't be applied to command substitutions. Use temporary variables.

Problematic code:

echo "Building ${$(git rev-parse --show-toplevel)##*/}"

Correct code:

tmp=$(git rev-parse --show-toplevel)
echo "Building ${tmp##*/}"

Rationale:

ShellCheck found a parameter expansion that begins with a command substitution, such as $(..) or `..`. This is not valid. Parameter expansion only works on variables (normal or special).

In the example, the user hoped to apply the construct ${var##*/}, stripping the path, to the current git root directory as output by git rev-parse --show-toplevel. Since parameter expansion only works on variable, the command substitution must be assigned to a variable first like in the correct example.

If the goal was instead to dynamically generate a variable name to expand, see SC2082.

Exceptions:

None

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