Mac OS X Leopard - ARCAism

Command Substitution
Another way to assign a variable is through command substitution. This allows you to work
with the result of any shell command by enclosing the command with backticks (the ` character
below the Esc key on most Mac keyboards). This allows you to take advantage of any shell command,
making up for most of the shell’s natural shortcomings. For example, the Bourne shell
doesn’t have built-in capabilities for simple arithmetic; however, using command substitution,
you can take advantage of command-line executables that do this for you anyway, like this:
#!/bin/sh
x=2
y=3
z=`expr $x + $y`
echo $z
This little script uses the expr executable to do the math for you and then prints 5, which is
what you’d expect.
NOTE The Bash shell actually does have built-in arithmetic capability, so if you were so
inclined, you could replace the z=`expr $x + $y` line with let z=$x+$y and get the same
results.
Controlling the Flow
The ability to control the flow of a script makes it much more adaptable and useful. There are
two primary ways to control the flow of a script: conditionals, which will execute commands if
certain conditions are met, and loops, which can repeat commands over and over a predetermined
number of times.
Conditional Statements
There are two common conditional statements available in shell scripts: if statements and case
statements. An if statement checks to see whether a condition is true or false. If the condition is
true, then a block of code is run; if it is false, an else block is run. The whole thing looks something
like this:
if [ condition ]
then
condition is met block
else
condition not met block
fi
The fi at the end signals the end of the if statement.
One other thing that can be added to respond to multiple conditions is the elif condition.
This allows you to create logic like this:
if [ condition1 ]
then
condition1 block
elif [ condition2 ]
condition2 block
elif [ condition3 ]
...
else
no conditions met block
fi
CHAPTER 19 EXTENDING THE POWER OF DARWIN 333