Monday, January 20, 2014

Executing Commands in Ruby

Ruby allows many different ways to execute commands or sub-processes. Here I will list few of them
1) Backtick
2) system
3) exec
4) sh
5) popen3

Backtick ( `cmd` ) :

backtick returns the output of the cmd  in a subshell.

output = `ls` puts "output is #{output}"

Result of the above code is 

$ ruby test.rb output is amit.txt test.rb

Here the backtick operation forks the master process and the operation is executed in a new process. However this is a blocking operation. The main application waits until the result of backtick operation completes.
If there is an exception in the sub-process then that exception is given to the main process and the main process might terminate if exception is not handled.
In the following case I am executing xxxxx which is not a valid executable name.
outut = `xxxxxxx` puts "output is #{output}"
Result of above code is : 
$ ruby test.rb test.rb:1:in ``': No such file or directory - xxxxxxx (Errno::ENOENT) from test.rb:1:in `<main>'
Notice that puts was never executed because the backtick operation raised exception. To check the status of the backtick operation you can execute $?.success?
output = `ls` puts "output is #{output}" puts $?.success?

Notice that the last line of the result contains true because the backtick operation was a success.

$ ruby main.rb output is lab.rb main.rb true

system : 

system behaves a bit like backtick operation. However there are some differences.
First let’s look at similarities.
Just like backticksystem is a blocking operation. You can get the result of the operation using $?.success? . systemoperations are also executed in a subshell.
Now the differneces between backtick and system .

system eats up all the exceptions. So the main operation never needs to worry about capturing an exception raised from the child process.
output = system('xxxxxxx') puts "output is #{output}"
Result of the above operation is given below. Notice that even when exception is raised the main program completes and the output is printed. The value of output is nil because the child process raised an exception.
$ ruby test.rb output is
Another difference is that system returns true if the command was successfully performed ( exit status zero ) . It returns false for non zero exit status. Returns nil if command execution fails.
exec : 
exec replaces the current process by running the external command. Let’s see an example.
Here I am in irb and I am going to execute exec('ls').
$ irb e1.9.3-p194 :001 > exec('ls') amit.rb test.rb amitk ~/dev/lab 1.9.3 $
I see the result but since the irb process was replaced by the exec process I am no longer in irb .
Behind the scene both system and backtick operations use fork to fork the current process and then they execute the given operation using exec .
Since exec replaces the current process it does not return anything if the operation is a success. If the operation fails then `SystemCallError is raised.
sh : 
sh actually calls system under the hood. However it is worth a mention here. This method is added by FileUtils in rake. It allows an easy way to check the exit status of the command.
require 'rake' sh %w(xxxxx) do |ok, res| if !ok abort 'the operation failed' end end
popen3 : 
If you are going to capture stdout and stderr then you should use popen3 since this method allows you to interact with stdinstdout and stderr .
I want to execute git push heroku master programmatically and I want to capture the output. Here is my code.
require 'open3' cmd = 'git push heroku master' Open3.popen3(cmd) do |stdin, stdout, stderr, wait_thr| puts "stdout is:" + puts "stderr is:" + end
And here is the output. It has been truncated since rest of output is not relevant to this discussion.
stdout is: stderr is: Heroku receiving push Ruby/Rails app detected Installing dependencies using Bundler version 1.2.1
Lets see an example of capturing streaming output.
require 'open3' cmd = 'ping' Open3.popen3(cmd) do |stdin, stdout, stderr, wait_thr| while line = stdout.gets puts line end end
In the above case you will get the output of ping on your terminal as if you had typed ping on your terminal .
Now let’s see how to check if command succeeded or not.
require 'open3' cmd = 'ping' Open3.popen3(cmd) do |stdin, stdout, stderr, wait_thr| exit_status = wait_thr.value unless exit_status.success? abort "FAILED !!! #{cmd}" end end

Hope this post helps in understanding the basics of executing commands from our program.
Happy Coding... :)

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