CompTIA Linux+ XK0-006 (V8) Practice Question

The requirement is satisfied by combining two separate tests with a logical OR (||):

• -z returns true when the length of CONFIG_FILE is zero, covering the cases where the variable is unset or empty.
• -f returns true when the given pathname is an existing regular file, so ! -f is true when the file is missing or not a regular file.

Using these together as [[ -z "$CONFIG_FILE" || ! -f "$CONFIG_FILE" ]] causes the condition to be true whenever either prerequisite is violated, allowing the script to abort as intended. The other expressions fail for one or more reasons:

• [[ -n "$CONFIG_FILE" && -f "$CONFIG_FILE" ]] is true only when the variable is non-empty and the file exists-the opposite of what is required.
• [[ -z "$CONFIG_FILE" && -f "$CONFIG_FILE" ]] can never be true because a zero-length variable cannot simultaneously reference an existing file.
• [[ -n "$CONFIG_FILE" || ! -d "$CONFIG_FILE" ]] uses the directory test (-d) instead of the regular-file test, so it would pass when the pathname is a directory and miss missing regular files.