Mac OS X Leopard - ARCAism

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CHAPTER 19 EXTENDING THE POWER OF DARWIN
NOTE The possible names a variable can have are virtually endless, but there are some rules.
First, although the name can contain letters, numbers, or underscores, it’s best that they begin
with a letter (and they can’t begin with a number, since that is reserved for command-line argument
variables). Second, variables obviously can’t share a name with a reserved word, which is
one of the many words that have special meaning to the shell. Finally, by convention, environmental
variables are often written with all capital letters, and shell variables are traditionally
written in all lowercase letters, but of late many people use camel notation, which starts out
lowercase but uses a capital letter where a new word begins. An example of camel notation is
aVeryLongVariable.
It is important not to include spaces around the = when declaring your variables; if you do,
you will get an error.
If you want to make your variable an environmental variable, then you must export it using
the export command. Usually this is done immediately after the variable is declared, which can
be done all on one line like this:
ENVVAR=Value; export ENVVAR
After the shell exports the variable, it will be immediately available for any program or shell
script until the parent shell process exits. (In other words, every time you launch a new shell, you
will need to redeclare and export the variable to use it again.) If you are planning on using an
environmental variable over and over, it may be best to declare it in your .bash_profile file so it
gets declared each time you launch your terminal or a shell.
Argument Variables
Some of the most commonly used variables are ones you don’t need to declare at all. These are
variables passed to the script as arguments from the command line when the script is called. For
example, if you were to run an executable script called ascript, you could pass arguments to the
script when you call it, like this:
leopard:~ scott$ ascript arg1 arg2 arg3
Here the arg1, arg2, and arg3 values are automatically assigned to variables $1, $2, and $3 so
they can be used as needed in the script.
NOTE In many computer languages, counting begins with 0, not 1. Command-line arguments
in scripts are no exception. In scripts, $0 is always the script itself and will return the complete
path name of the executed script.
These special variables don’t have to be passed in from the command line, though. Occasionally
it may be advantageous to create and control these arguments from within the script.
This can be done by using the set command (as we did in our example shell script earlier). The
set command will replace all command-line arguments with the arguments provided.
NOTE set is a fairly complex command in Bash with lots of different capabilities. If you use
set alone on the command line, it will list every environmental variable and function available
to you in a nice, readable format. Used with options, set allows you to toggle many shell
attributes. If you are interested, the gory details of all the possibilities for this command begin
on page 51 of the Bash main page and continue until about three-quarters of the way through
page 53. Also of note, set behaves differently in other shells; for example, in csh you would
use set in place of export to create environmental variables.