Macintosh Terminal Pocket Guide

The Shell

In order to use commands on a Macintosh, you’ll need a program that reads and executes them. That program is called the shell, which runs inside the Terminal and is OS X’s command-line user interface.^[3] You type a command and press Enter, and the shell runs whatever program (or programs) you’ve requested. For example, to list the files in your Documents folder, one per line, you could execute the ls -1 command in a shell:

➜ ls -1 ~/Documents
data.pdf
letter.txt
song.mp3

A single command can also invoke several programs at the same time, and even connect programs together so they interact. Here’s a command that redirects the output of the preceding ls command to become the input of the wc program, which counts lines of text in a file; the result is the number of lines printed by ls:

➜ ls -1 ~/Documents | wc -l
3

telling you how many files are in your Documents folder. The vertical bar, called a pipe, makes the connection between ls and wc.

A shell is actually a program itself, and OS X has several different ones: the Bourne shell, the Korn shell, the C shell, and others. This book focuses on a popular shell called bash, the Bourne-Again Shell, located in /bin/bash, which is the default for user accounts. However, all these shells have similar basic functions.

^[3] The same shell is found on Linux systems, and on Windows PCs that run Cygwin.