It's an unfortunate fact of life that our applications have to deal
with the big, bad world. In this chapter, we'll look at how Ruby
interacts with its environment. Microsoft Windows users will probably
also want to look at platform-specific information beginning
on page 163.
``In the Beginning was the Command Line.''[Title of a
marvelous essay by Neal Stephenson
(available online at
http://www.spack.org/essays/commandline.html).]
Regardless of the system in which Ruby is deployed, whether it be a
super high-end scientific graphics workstation or an embedded PDA
device, you've got to start the Ruby interpreter somehow, and that
gives us the opportunity to pass in command-line arguments.
A Ruby command line consists of three parts: options to the Ruby
interpreter, optionally the name of a program to run, and optionally a
set of arguments for that program.
ruby [options][--][programfile][arguments]
The Ruby options are terminated by the first word on the command line
that doesn't start with a hyphen, or by the special flag ``--''
(two hyphens).
If no filename is present on the command line, or if the filename is
a single hyphen (-), Ruby reads the program source from standard
input.
Arguments for the program itself follow the program name. For example:
% ruby -w - "Hello World"
will enable warnings, read a program from standard input, and pass it
the quoted string "Hello World" as an argument.
'
The number ``0'' flag specifies the record
separator character (\0, if no digit follows). -00 indicates
paragraph mode: records are separated by two successive default
record separator characters. -0777 reads the entire file at once (as it
is an illegal character). Sets $/.
-a
'
Auto split mode when used with -n or -p;
equivalent to executing
{$F at the top of each loop iteration.
-C directory
'
Changes working directory to directory before executing.
-c
'
Checks syntax only; does not execute the program.
--copyright
Prints the copyright notice and exits.
-d, --debug
'
Sets $DEBUG to true.
This can be used by your programs to
enable additional tracing.
-e 'command'
'
Executes command as one line of Ruby source. Several -e's
are allowed, and the commands are treated as multiple lines in the
same program. If programfile is omitted when -e is
present, execution stops after the -e commands have been run.
-F pattern
'
Specifies the input field separator ($;) used as the default for
split() (affects -a).
-h, --help
'
Displays a short help screen.
-I directories
'
Specifies directories to be prepended to $LOAD_PATH
($:). Multiple -I options may be present, and
multiple directories may appear following each -I. Directories
are separated by a ``:'' on Unix-like systems and by a ``;'' on
DOS/Windows systems.
-i [extension}
'
Edits ARGV files in place.
For each
file named in ARGV, anything you write to standard output will
be saved back as the contents of that file.
A backup copy of the file will be made if
extension is supplied.
% ruby -pi.bak -e "gsub(/Perl/, 'Ruby')" *.txt
-K kcode
'
Specifies the code set to be used. This
option is useful mainly when Ruby is used for Japanese-language
processing. kcode may be one of: e, E for EUC; s, S for SJIS;
u, U for UTF-8; or a, A, n, N for ASCII.
-l
'
Enables automatic line-ending processing;
sets $\ to the value of $/ and chops every input line
automatically.
-n
'
Assumes ``while gets; ...; end'' loop around your program.
For example, a simple grep command might be implemented as:
% ruby -n -e "print if /wombat/" *.txt
-p
'
Places your program code within the loop
``while gets; ...; print; end.''
% ruby -p -e "$_.downcase!" *.txt
-r library
'
requires the named library before executing.
-S
'
Looks for the program file using RUBYPATH
or PATH environment
variable.
-s
' Any command line switches found after the program filename, but
before any filename arguments or before a --, are removed
from ARGV and set to a global variable named for the switch. In
the following example, the effect of this would be to set the variable
$opt to ``electric''.
% ruby -s prog -opt=electric ./mydata
-T[level}
'
Sets the safe level, which among other things enables tainting
checks
(see page 253). Sets $SAFE.
-v, --verbose
'
Enables verbose mode and print the version number.
In verbose mode, compilation warnings are
printed. If no program filename appears on the command line, Ruby exits.
--version
Displays the Ruby version number and exits.
-w
'
Enables verbose mode. Unlike -v, reads program from
standard input if no program files are present on the command line.
We recommend running your Ruby programs with -w.
-X directory
'
Changes working directory to directory before executing.
Same as -Cdirectory.
-x [directory}
'
Strips off text before #!ruby line and changes working
directory to directory if given.
-y, --yydebug
'
Enables yacc debugging in the parser (waaay too much
information).
Any command-line arguments after the program filename
are available to your Ruby program in the global array
ARGV.
For instance, invoking Ruby as
% ruby -w ptest "Hello World" a1 1.6180
yields an ARGV array containing ["Hello World", a1,
1.6180]. There's a gotcha here for all you C
programmers---ARGV[0] is the first argument to the program, not
the program name.
The name of the current program is
available in the global variable $0.
The method Kernel#exit terminates your program, returning a
status value to the operating system. However, unlike some languages,
exit doesn't just terminate the program immediately.
Kernel#exit first raises a
SystemExit exception, which you may catch, and then performs a
number of cleanup actions, including running any registered
at_exit methods and object finalizers. See the reference for
Kernel#exit beginning on page 415 for details.
You can access operating system environment variables using
the predefined variable ENV. It responds to the same
methods as Hash.[ENV is not actually a hash, but
if you need to, you can convert it into a Hash using
ENV#to_hash.]
The values of some environment variables are read by Ruby
when it first starts. These variables modify the behavior of the interpreter,
as shown in Table 13.1 on page 139.
Environment variables used by Ruby
Variable Name
Description
RUBYOPT
Additional command-line options to Ruby; examined after
real command-line options are parsed ($SAFE
must be 0).
RUBYLIB
Additional search path for Ruby programs
($SAFE must be 0).
RUBYPATH
With -S option, search path for Ruby
programs (defaults to PATH).
RUBYSHELL
Shell to use when spawning a process; if not
set, will also check SHELL or COMSPEC.
DLN_LIBRARY_PATH
Search path for dynamically loaded
modules.
RUBYLIB_PREFIX
(Windows only) Mangle the RUBYLIB search path by
adding this prefix to each component.
A Ruby program may write to the ENV object, which on most systems
changes the values of the corresponding environment variables. However,
this change is local to the process that makes it and to any
subsequently spawned child processes.
This inheritance of environment variables is illustrated in the code
that follows. A subprocess changes an environment variable and this
change is seen in a process that it then starts. However, the change
is not visible to the original parent. (This just goes to prove that
parents never really know what their children are doing.)
puts "In parent, term = #{ENV['TERM']}"
fork do
puts "Start of child 1, term = #{ENV['TERM']}"
ENV['TERM'] = "ansi"
fork do
puts "Start of child 2, term = #{ENV['TERM']}"
end
Process.wait
puts "End of child 1, term = #{ENV['TERM']}"
end
Process.wait
puts "Back in parent, term = #{ENV['TERM']}"
produces:
In parent, term = xterm
Start of child 1, term = xterm
Start of child 2, term = ansi
End of child 1, term = ansi
Back in parent, term = xterm
You use require or load to bring a library module into your
Ruby program. Some of these modules are supplied with Ruby, some you
installed off the Ruby Application Archive, and some you wrote
yourself. How does Ruby find them?
When Ruby is built for your particular machine, it predefines a set
of standard directories to hold library stuff. Where these are
depends on the machine in question. You can
determine this from the command line with something like:
% ruby -e 'puts $:'
On a typical Linux box, you'll probably find something such as:
The site_ruby directories are intended to hold modules and extensions that
you've added.
The architecture-dependent directories (i686-linux
in this case) hold executables and other things specific to this
particular machine. All these directories are automatically included
in Ruby's search for modules.
Sometimes this isn't enough. Perhaps you're working on a large project
written in Ruby, and you and your colleagues have built a substantial
library of Ruby code. You want everyone on the team to have access to
all of this code. You have a couple of options to accomplish this. If
your program runs at a safe level of zero (see
Chapter 20 beginning on page 253),
you can set the environment variable
RUBYLIB to a list of one or more directories to be
searched.[The separator between entries depends on your
platform. For Windows, it's a semicolon; for Unix, a colon.] If your
program is not setuid,
you can use the command-line parameter -I
to do the same thing.
Finally,
the Ruby variable $: is an array of places to search
for loaded files. This variable is initialized to the list of standard
directories, plus any additional ones you specified using RUBYLIB
and -I. You can always add additional directories to this array
from within your running program.
When Ruby is compiled for a particular architecture, all of the
relevant settings used to build it (including the architecture of the
machine on which it was compiled, compiler options, source code
directory, and so on) are written to the module Config within the
library file ``rbconfig.rb''.
After installation, any Ruby program can use this module to get
details on how Ruby was compiled.
require "rbconfig.rb"
include Config
CONFIG["host"]
»
"i686-pc-linux"
CONFIG["LDFLAGS"]
»
"-rdynamic"
Extension libraries use this configuration file in order to compile
and link properly on any given architecture. See Chapter
17 beginning on page 169, and the reference
for mkmf beginning on page 451 for details.
