Day 2: Thursday, January 12, 2017 - Introducing the Command Line

Why would we ever give up our nice point-and-click OS for a 1970s interface?


Doesn’t require the command-line.

In-class work

Today we’ll be doing work in-class on the iMacs. It’s a little difficult (and unfair) to assign a bunch of command-line work until next week, when we will all have the same machine to work on.

The main takeaways for today should be:

  • Understanding that the command-line is just an interface to your system, instead of clicking things, you have to memorize a bunch of commands and their options.
  • The command-line is a REPL: it reads your command (upon hitting Enter), executes it, prints the results, and then goes back to waiting for the next command.
  • Keyboard shortcuts are nice. Use the Up key at the command-line to cycle through past commands in the history.
  • The Tab key is essential. Use Command-Tab (Ctrl-Tab on Windows) to cycle between open applications, i.e. Chrome and the Terminal window. At the Nix command line, Tab serves as auto-complete
  • Use pwd, whoami, and hostname to figure out which computer you’re on, and where.
  • How simple each command is. cat prints out files. curl just downloads them. They don’t make assumptions about where you want to print things, or save files, etc.

Most of the basics we cover in class are reviewed/compiled here. This class isn’t heavily focused on the Nix command-line – i.e. we won’t be writing real programs with it – it’s just necessary for moving around computer systems in general, while getting some insight into programmatic concepts.

Hands-on exercises (which we did not get to)

Creating directories/moving around

Learn how to use cd to change directories, pwd to figure out where you are, and mkdir to make directories: Introduction to the Command Line Interface


$ pwd

$ cd /tmp

$ pwd

$ mkdir hello

$ cd hello

$ pwd

Hello world and say

Open up your terminal, use cd to change the working directory to the Desktop:

$ cd ~/Desktop

Start off by using echo to say “hello world” to standard output, i.e., your screen.:

$ echo hello world

Now use the OSX-specific say command:

$ say hello world

This is a command that does not print to standard output. Use man say to get a list of commands, and learn how to change voices and save an output file:

# get a list of voices
$ say --voice ?

# say hello world in Vicki's voice
# consider how the arguments are parsed
$ say --voice Vicki hello world

# send the voice output to a file named `hello.aiff`
$ say -o hello.aiff hello world

Using youtube-dl

Let’s start with something more high-level: getting video files off of the Internet.

youtube-dl is a command-line program for downloading videos.



youtube-dl is installed on the McClatchy computers. You can try installing it on your own via these instructions – ask me if you need help, we won’t be using youtube-dl for actual assignments.

Using your web browser, go to a Youtube video page that you’d like to download – I recommend picking something from The White House.

Open up your Terminal. Use cd to change to the Desktop.

$ cd ~/Desktop

And then run this command (notice how the files get downloaded to the Desktop):

$ youtube-dl

That should begin the process of downloading a file.

Run youtube-dl with the --help flag to get a full list of options.

One useful flag is --write-sub, which will download a subtitle file if it exists.

$ youtube-dl --write-sub

To open the movie file with the OSX-specific application, use open:

$ open whatevermoviefilenameis

Using > to redirect output to a file

Youtube-dl and say are examples of program that do not send things to standard output.

echo, however, is. And if we don’t want to just print to the screen, we use > – the redirection operator – to say, “send the arguments/input to a file”:

$ echo hello world > hello.txt



Like most things in Nix command-line world, these commands are destructive and they don’t second-guess you. If hello.txt already exists, the redirection operator will wipe out what existed there before writing to it.

Using >> to redirect and append to a file

$ echo hello world again >> hello.txt

Using cat to “concatenate” and print contents of a file

$ cat hello.txt

As with echo, we can redirect the output of cat to a new file:

$ cat hello.txt > hello2.txt

cat takes multiple arguments, e.g. multiple files, as needed

$ cat hello.txt hello2.txt > hellomore.txt

Using curl

Download a webpage from the command-line and print it to screen:

$ curl

Download a webpage into a file:

$ curl > somefile.txt

Healthcare practitioner payments

via ProPublica’s Dollars for Docs

Download this zip file containing PDFs of drug companies payments:

$ curl >

Unzip it with the unzip command:

$ unzip

It will create a new directory named pharma-hcps. We want to navigate to the sub-directory, pharma-hcps/txts (the pdfs directory contains the original files, which were converted using the Poppler command-line tool).

$ cd pharma-hcps/txts

Then use ack to look for patterns:

$ ack 'BOSTON' eli-lilly-2009.txt

$ ack 'BOSTON' *.txt

$ ack '\d{3},\d{3}' eli-lilly-2009.txt

Do one thing and do it well

The Unix Philosophy:

This is the Unix philosophy: Write programs that do one thing and do it well. Write programs to work together. Write programs to handle text streams, because that is a universal interface.

Brian Kernighan explains how to “glue” together Unix programs (via AT&T archives):


One of the most well-known Unix utilities, grep is as much a verb as it is a noun, e.g. “grep those files”.

While you can use grep to look for literal word patterns, like your typical Find command in a word-processor.

An excerpt of its man description in 6th edition of Unix:

 grep — search a file for a pattern

 Grep searches the input files (standard input default) for
 lines matching the regular expression. Normally, each line
 found is copied to the standard output. If the -v flag is
 used, all lines but those matching are printed. If the -c
 flag is used, only a count of matching lines is printed.

grep-likes: ack and ag

Note that for this class, we’ll be using either of these grep-likes:

  • ack - this is installed on the McClatchy Hall iMacs and will be installed on the Amazon cloud servers.
  • ag - this is installed on the Stanford Farmshare machines, i.e.

You can try installing them on your own, but the main reason we use them is because the version of grep installed on OSX is not as fully-featured as we need. Mainly, the difference is that ack and ag support the full range of regular expressions we want (commonly referred to as Perl-compatible regular expressions)

Otherwise, the tools share common flags, such as -o for print matching output and -v for print non-matching lines.