`cd` is the command in the UNIX operating system, and later its open-source offspring, Linux and its many distributions. It's used to change the current working directory. The "cd" stands for "**c**hange **d**irectory". It is one of the most frequently used commands in the Linux terminal.

`$SPARK_HOME` is the environment variable for Spark. This needs to be set so that OS knows where to find Spark.
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