• View file contents with cat and open
• Print text with echo
• Redirect text
• Set PATH and environment variables
• Look up Bash documentation with man

Sometimes when we interact with computers via the graphical user interface (GUI), it takes several clicks or deep dives into the controls to accomplish what we want. We've started to see in the earlier lessons that interacting via the command line interface (CLI) can make us faster!

As we begin to build applications, it will become important to also be familiar with tasks related to displaying files, editing them with a code editor, or getting information from the operating system.

We can print the contents of a file by using the cat command. Entering $ cat filename reads a file and prints the content to your command line. (In the previous command filename is just a placeholder for where you would write out a valid file name).

ASIDE: cat comes from catenate a middle-English word that means "make like a chain". Unix thinks of a file as a "chain of bytes" that it feeds to the screen.

Note: Any time you see the $ character, you shouldn't type it in. This is just a standard way to represent a bash prompt. Yours may or may not be a $.

The open command is interesting because it will trigger the default action associated with the file type. "Default actions" are defined by the operating system. This command is only available on Mac OSX.

$ open . will popup a Finder window with the current directory in finder (recall: . is an alias to the current directory). Entering $ open hello_world.rb will run the default program for files of a "Ruby" type — usually a code editor. Some other platforms have adopted an open program because they liked OSX's idea. Try it out on your computer and see if it's supported.

Developers will often find themselves needing to write text in the terminal. The echo command has a number of uses — probably the most common is to have a program display a message or instructions, such as "Please enter Y or N" in a dialog with users.

The echo command takes a string and prints it to the screen.

$ echo "Hi world"
Hi world

You can "redirect" echo text into a file:

$ echo "I'm printing to the screen" >> my_file.txt

This is called redirection. We're "redirecting" what we see on the monitor into a file. The >> symbol will append content into the file while > means overwrite.

BE CAREFUL Using > when you mean >> can make you real sad because > "clobbers" or "overwrites" the file. Some files on your system are very important and "clobbering" them could hurt your machine!

An environment variable is a variable which can be configured to change the way the shell works and used by multiple applications or processes. You might tell the shell, via an environment variable "use colors whenever you can" or "never use colors in output". Discussing how to set these up is beyond our scope right now. But it's important to see that environment variables, like directory path shortcuts, or those mysterious hidden files configure and adjust our CLI experience.

For example, PATH is a very important environment variable. In Unix, programs are often installed in many different directories. Directories whose names are listed in the PATH variable can have their programs run without having to cd to the directory where they are to run them (OR provide a long absolute path).

Paths are assigned to the PATH and separated by :.

Let's try viewing the current PATH.

$ echo $PATH
/Users/kellyegreene/.rvm/gems/ruby-2.7.4/bin:/Users/kellyegreene/.rvm/gems/ruby-2.7.4@global/bin:/Users/kellyegreene/.rvm/rubies/ruby-2.7.4/bin:/Users/kellyegreene/.nvm/versions/node/v8.9.4/bin:/Applications/Postgres.app/Contents/Versions/9.4/bin:/usr/local/share/npm/lib/node_modules/grunt-cli/bin:/usr/local:/usr/local/bin:/usr/local/sbin:/usr/bin:/usr/local/bin:/usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin:/Users/kellyegreene/Library/Android/sdk/tools:/Users/kellyegreene/Library/Android/sdk/platform-tools:/Users/kellyegreene/Library/Android/sdk/tools:/Users/kellyegreene/Library/Android/sdk/platform-tools:/Users/kellyegreene/.rvm/bin

Any program in these directories can be run by simply typing the program name. When kellyegreene types ruby the shell starts by looking at the first directory in the PATH to see if it finds a program that matches.

In this case it would find the ruby program in /Users/kellyegreene/.rvm/rubies/ruby-2.7.4/bin and run it. The first match in the PATH variable wins so the order of the PATH is important.

If this directory were not in the PATH, kellyegreene would have to do one of the following to run ruby -v — a command that shows the program's version.

$ cd /Users/kellyegreene/.rvm/rubies/ruby-2.7.4/bin
$ ./ruby -v
ruby 2.7.4p191 (2021-07-07 revision a21a3b7d23) [x86_64-darwin20]

or

$ /Users/kellyegreene/.rvm/rubies/ruby-2.7.4/bin/ruby -v
ruby 2.7.4p191 (2021-07-07 revision a21a3b7d23) [x86_64-darwin20]

In the shell, all programs that are not in the path have to be run with an absolute path. In both of these examples, you should see how we send the shell a full absolute path (remember what . means!).

Top Tip: If you want to find out where the program being run is located when you type a command at the command line, use the which command; entering $ which ruby will tell you where the Ruby binary is located. Whenever you type a command the path is going to be searched in order until it finds an executable that matches name

There are a lot of environment variables and, as you grow as a developer, you will encounter more of them. Others you might see include:

• Execution mode (e.g., production, development, staging, etc.)
• Domain names
• API URL/URIs
• GIT_AUTHOR
• GIT_AUTHOR_EMAIL

Eventually, you'll learn to configure your shell environment to help you do work more efficiently. You'll define variables like BIG_PROJECT_DIRECTORY or write custom programs that you can run from the CLI that make you more efficient.

The man command is the key to command line wisdom. It brings up a manual page for almost any command. It's the equivalent of a help system for the command line. When Unix was first invented, its manuals were so big and heavy and expensive to print the developers had the idea to write the manual in the operating system itself. This is an idea that's been popular with programmers and environmentalists and mail-carriers ever since.

For example, if you're curious what the options on ps mean, you can go to your terminal and enter:

$ man ps

In the terminal output, read about what the a and u options do. Notice that the x option is a suffix on the a option. The man command reveals very useful reference documentation on the various bash commands. You'll notice that your command prompt has disappeared. Don't panic! You're just inside the documentation. Enter $ q ("quit") to return to your command prompt.

Software developers still rely heavily on command-line interfaces to perform tasks more efficiently, configure their machine, or access programs and program features that are not available through a graphical interface. File interactions and process management may be difficult for new CLI users to grasp because the tools used are different from their graphical counterparts. However, with a little practice, these new ways of working will become your ally.

• Using The Terminal Command cat To View The Contents Of Files