How to create nonexistent subdirectories recursively using Bash?

You can use the -p parameter, which is documented as:

-p, --parents

no error if existing, make parent directories as needed

So:

mkdir -p "$BACKUP_DIR/$client/$year/$month/$day"

$ mkdir -p "$BACKUP_DIR/$client/$year/$month/$day"

While existing answers definitely solve the purpose, if your'e looking to replicate nested directory structure under two different subdirectories, then you can do this

mkdir -p {main,test}/{resources,scala/com/company}

It will create following directory structure under the directory from where it is invoked

├── main
│   ├── resources
│   └── scala
│       └── com
│           └── company
└── test
    ├── resources
    └── scala
        └── com
            └── company

The example was taken from this link for creating SBT directory structure