Macintosh Terminal Pocket Guide

Booleans and Return Codes

Before we can describe conditionals and loops, we need to explain the concept of a Boolean (true/false) test. A Boolean is an entity that can have the value true or false. A Boolean test simply checks a value to see if it’s true or false. To the shell, the value 0 means true or success, and anything else means false or failure. (Think of zero as “no error” and other values as error codes.)^[36]

Every command returns an integer value, called a return code or exit status, to the shell when the command exits. You can see this value in the special variable $?:

➜ cat myfile
My name is Sandy Smith and
I really like OS X Lion
➜ grep Smith myfile
My name is Sandy Smith and      A match was found...
➜ echo $?
0                               ...so return code is “success”
➜ grep aardvark myfile
➜ echo $?                       No match was found...
1                               ...so return code is “failure”

The return codes of a command are usually documented on its manpage.

test and “[”

The shell’s test command evaluates simple Boolean expressions involving numbers and strings, setting its exit status to 0 (true) or 1 (false):

➜ test 10 -lt 5       Is 10 less than 5?
➜ echo $?
1                     No, it isn’t
➜ test -n "hello"     Does the string “hello” have nonzero length?
➜ echo $?
0                     Yes, it does

Here are common test arguments for checking properties of integers, strings, and files:

File tests
-d name	File name is a directory
-f name	File name is a regular file
-L name	File name is a symbolic link
-r name	File name exists and is readable
-w name	File name exists and is writable
-x name	File ...