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Getting Results from Other Programs
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There are three simple ways to run another program from within Ruby: the system method (defined in the Kernel module), backtick syntax (``), and delimited input literals (%x{}).Using system is ideal when you want to run another program and aren t concerned with its output, whereas you should use backticks when you want the output of the remote program returned. These lines demonstrate two ways of running the system s date program:
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x = system("date") x = `date`
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CHAPTER 11 ADVANCED RUBY FEATURES
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For the first line, x equals true, whereas on the second line x contains the output of the date command. Which method you use depends on what you re trying to achieve. If you don t want the output of the other program to show on the same screen as that of your Ruby script, then use backticks (or a literal, %x{}).
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Note
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%x{} is functionally equivalent to using backticks; for example, %x{date}.
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Transferring Execution to Another Program
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Sometimes it s desirable to jump immediately to another program and cease execution of the current program. This is useful if you have a multistep process and have written an application for each. To end the current program and invoke another, simply use the exec method in place of system. For example:
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exec "ruby another_script.rb" puts "This will never be displayed"
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In this example, execution is transferred to a different program, and the current program ceases immediately the second line is never executed.
Running Two Programs at the Same Time
Forking is where an instance of a program (a process) duplicates itself, resulting in two processes of that program running concurrently. You can run other programs from this second process by using exec, and the first (parent) process will continue running the original program. fork is a method provided by the Kernel module that creates a fork of the current process. It returns the child process s process ID in the parent, but nil in the child process you can use this to determine which process a script is in. The following example forks the current process into two processes, and only executes the exec command within the child process (the process generated by the fork):
if fork.nil exec "ruby some_other_file.rb" end puts "This Ruby script now runs alongside some_other_file.rb"
CHAPTER 11 ADVANCED RUBY FEATURES
If the other program (being run by exec) is expected to finish at some point, and you want to wait for it to finish executing before doing something in the parent program, you can use Process.wait to wait for all child processes to finish before continuing. Here s an example:
child = fork do sleep 3 puts "Child says 'hi'!" end puts "Waiting for the child process..." Process.wait child puts "All done!"
Waiting for the child process... <3 second delay> Child says 'hi'! All done!
Note Forking is not possible with the Windows version of Ruby, as POSIX-style forking is not natively
supported on that platform. However, threads, covered later in this chapter, provide a good alternative.
Interacting with Another Program
The previous methods are fine for simple situations where you just want to get basic results from a remote program and don t need to interact directly with it in any way while it s running. However, sometimes you might want to pass data back and forth between two separate programs. Ruby s IO module has a popen method that allows you to run another program and have an I/O stream between it and the current program. The I/O stream between programs works like the other types of I/O streams we looked at in 9, but instead of reading and writing to a file, you re reading and writing to another program. Obviously, this technique only works successfully with programs that accept direct input and produce direct output at a command-prompt level (so not GUI applications).
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