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Sometimes, you want to pass arithmetic results to a command. There are two forms that you can use to expand an arithmetic expression and pass it to the shell: $[expression] or $(expression). The following is an example:

 $ echo "I am $[2020 - 1957] years old." I am 63 years old.

The shell interprets the arithmetic expression first (2020 - 1957) and then passes that information to the echo command. The echo command displays the text with the results of the arithmetic (63) inserted.

Here's an example of the other form:

 $ echo "There are $(ls | wc -w) files in this directory." There are 14 files in this directory.

This lists the contents of the current directory (ls) and runs the word count command to count the number of files found (wc -w). The resulting number (14, in this case) is echoed back with the rest of the sentence shown.