Re: Beginner Ruby Question

Calvin wrote:
> On May 18, 11:36 am, Jeff Schwab <[e-mail]> wrote:
>> Calvin wrote:
>>> I just intstalled Ruby 1.9 on my mac with macports. The installation
>>> was successful but when I type in ruby -v at the command prompt, Ruby
>>> 1.8.6 shows up still. The latest Pickaxe book says I have to have /opt/
>>> local/bin in my path to add it... but don't really get how that's
>>> supposed to work.

> when I type in: echo 'export PATH="/opt/local/bin:$PATH"' >>
> ~/.profile
> nothing happens

You're not supposed to see anything happen, except that the shell should 
prompt you for the next command.

I'm assuming you're using "bash" (Bourne Again SHell), the default 
command-line shell on Mac OS X (and most Linux distros).  Since "hash 
-r" is not working, you may not be using bash, after all.  The next most 
likely candidate is called tcsh.  What do you see when you run this?

     echo $SHELL

It's also traditional for different shells to use different prompts. 
Bash uses $, csh uses %, and tcsh uses % or >.  As a special case, the 
root user's prompt is always set to the comment character (#), to avoid 
accidental copy/paste mishaps.

When bash sees

     echo 'export PATH="/opt/local/bin:$PATH" >> ~/.profile

it does a several things.  One is to expand the tilde (~) to the path of 
your home directory.  Another is to recognize "echo" as a built-in 
command, and execute it.  The command's standard output is redirected 
(>>) to the end of the .profile, and echo's command line arguments are 
exactly two:

     echo
     export PATH="/opt/local/bin:$PATH"

Be careful not to mix up >> with a single >, which would redirect echo's 
standard output to overwrite the file entirely.

The bash command:

     export PATH="/opt/local/bin:$PATH"

prefixes "/opt/local/bin:" to the value of the PATH environment 
variable, and marks that variable as "exported" so its value will be 
inherited by subprocesses.  The equivalent command in tcsh is:

     setenv PATH "/opt/local/bin:$PATH"

The PATH variable holds a colon-separated list of directories to check 
for the programs you invoke.  For example, when you type ruby, the shell 
should find /opt/local/bin/ruby.  By contrast, if you were to type 
xeyes, the shell would check for /opt/local/bin/xeyes, but then have to 
keep looking through PATH (since xeyes lives in /usr/X11/bin).

The point of setting PATH in ~/.profile is that bash reads .profile when 
you "log in."  Logging in, in this context, just means starting a new 
terminal.  You also want to set it for the current shell, so you end up 
enter the export command twice:  Once as an argument to echo 
 >>~/.profile, and again at the current shell's prompt.

When bash finds the location of a program, it may cache that location, 
to make future look-ups faster.  If you change PATH, it's a good idea to 
make bash throw away its cache, and look up program locations anew.  The 
command to make bash discard its cache is

     hash -r

The equivalent for tcsh is

     rehash

> if I type in: echo 'export PATH="/opt/local/bin:$PATH"'
> 
> i get: export PATH="/opt/local/bin:$PATH"  on the terminal screen

To the shell, double quotes (") mean "group these things into a single 
command-line argument, but expand any environment variables."  Single 
quotes (') perform the grouping, but also prevent expansion.  You need 
single quotes for the echo to .profile, because you want $PATH to be 
expanded when the shell

For example, given the following commands:

     hello='What is up'
     echo $hello  world
     echo "$hello  world"
     echo '$hello  world'

echo will be invoked three times, each with a different set of 
arguments.  In the first case, the two arguments will be 'What is up' 
and 'world' (without the quotes).  The double space will be lost, and 
echo will just print a single space between its arguments.  In the 
second case, echo will receive the single argument 'What is up  world' 
(without quotes), with the variable expanded and the double space 
preserved.  In the third case, echo will receive the literal text 
'$hello  world' (again, no quotes), exactly as it was entered on the 
command line.

> and when i type in: hash -r
> the terminal says -r isn't valid

That's a little odd.  What happens when you type rehash?