The first step is to find where the Swift command is; even if you have used the xcode-select command, it is possible that the Swift command is not accessible from your PATH variable. So, you can localize your Swift command using find /Applications/Xcode.app -name swift -a -type f. In my case, I got /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/swift. However, current versions of Xcode have the Swift command at /usr/bin. If it is necessary, add this directory to your PATH variable with the command export PATH="$PATH:/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/". At this moment, we can enter into the Swift interpreter just typing swift.

If you want to use Swift from the command line, sometimes, it's a good idea to have this PATH variable set permanently. To make this, we need to add the previous command into our .profile file, such as echo 'export PATH="$PATH:/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/" ' >> $HOME/.profile && chmod +x $HOME/.profile. From now on, if you restart your computer, it won't be necessary to look for the Swift path and set the PATH environment variable again.

Now, let's enter into our Swift command line and type the following code:

var dividend = [3,2,1,0]
var divisor = 6

You will see a message showing the content of these variables after typing each of them. Now, type the following loop code:

for i in dividend { 
      println("\(divisor) / \(i) = \(divisor / i)")  
}

Now, you can see that we will receive the following result:

6 / 3 = 2
6 / 2 = 3
6 / 1 = 6
Execution interrupted. Enter Swift code to recover and continue.
Enter LLDB commands to investigate (type :help for assistance.)

As you can see, the last option failed because we can't divide by 0 and that's a fast way we can test some code, using the command line. Most of the time, we will test using a playground, but sometimes using the command line is much faster.