Many, many bash commands are built around and meant to be used with a fundamental feature of the bash shell (actually, most shells), called piping. Put simply, piping takes the output of one command and provides it as input to the next. Here's a simple example of running ls and filtering the result with grep to find all .py files in the current directory.

$ ls # just run ls 
file1.md  file2.md  file3.md  script1.py  script2.py
$ ls | grep '\.py$'
script1.py
script2.py

To be precise, the | (pipe) operator takes the output from the command on the left, and provides it as input to the command on the right. Pipes can be chained practically as much as you'd like. For example, if we want to get amount of .py files in the current directory, we can pipe the output from grep to the wc (word count) command, with the -l option to count lines only.

$ ls | grep '\.py$' | wc -l
2

wc counts two lines, which is precisely the amount of .py files that we found. Let's move on to I/O redirection. Piping allows you to easily compose powerful programs from simple commands, and is a very intuitive way to work. Next week, I'll cover I/O redirection, which is another super useful feature of bash that's a bit more complicated.