Mac OS X Leopard - ARCAism

One final common conditional is to utilize the exit status of a command. Every command
you run will provide an exit status; that’s usually a 0 if everything went smoothly and some other
number if something went wrong. if knows about this, so you can use statements like this:
if command
then
if command exits normally do this
else
if command resulted in an error do this
fi
Another way of dealing with multiple potential conditions is the use of the case statement.
A case statement looks at a variable and responds differently depending on its value.
A case statement is generally much easier to use then an if statement; however, it’s a viable
alternate only if you are dealing with multiple similar outcomes that may be stored in a singe
variable. A case statement looks like this:
Loops
case "$caseVar" in
1) command(s) to run if the value of $caseVar is "1" ;;
2) command(s) to run if the value of $caseVar is "2" ;;
dog) command(s) to run if the value of $caseVar is "dog" ;;
*) command(s) to run if none of the previous cases matched, "*" is a wildcard ;;
esac
Now that you’ve learned a bit about how to selectively run or not run a command based on a
condition, we will cover how you can run a command over and over with loops. There are three
main types of loops for shell scripting: while, until, and for loops.
The while and until loops are similar in idea, but they do the opposite in practice. Each of
them takes a condition (similar to the if statement); however, where the while loop will run a
block of code as long as the condition is true, the until loop will run as long as the condition is
false.
CAUTION Poorly written loops can have the adverse effect of running forever. If you are running
a script from the command line, it’s easy enough to stop such a runaway script with
Ctrl+C, but if this is part of a background or start-up script, things can get more complicated.
Therefore, it’s a good idea to test your scripts from the command line before you place them
in start-up files and such.
A simple while loop could look like this:
#!/bin/sh
x=1
while [ $x -le 10 ]
do
echo $x
x=`expr $x + 1`
done
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This script will simply go through the while loop, printing the value of $x and then increasing
the value of x by 1 as long as the value of $x is equal to or less than 10. If we switched while
to until, the loop would skip entirely since our declared value of x (1) would immediately be less
than or equal to 10.
NOTE Like if statements, while and until loops can also evaluate multiple conditions using
logical and (&&) and or (||) statements.