Executing commands in ruby
Ruby allows many different ways to execute a command or a sub-process. In this article we are going to see some of them.
Returns standard output
backtick returns the standard output of the operation.
Result of above code is
Exception is passed on to the master program
Backtick operation forks the master process and the operation is executed in a new process. 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.
Result of above code is given below. Notice that
puts was never executed because the backtick operation raised exception.
Backtick is a blocking operation. The main application waits until the result of backtick operation completes.
Checking the status of the result
To check the status of the backtick operation you can execute
Notice that the last line of the result contains
true because the backtick operation was a success.
STDOUT. backtick does not capture
STDERR . If you want to learn about
STDERR then checkout this excellent article .
You can redirect
STDOUT if you want to capture
STDERR using backtick.
Different delimiter and string interpolation
%x does the same thing as backtick. It allows you to have different delimeter.
backtick runs the command via shell. So shell features like string interpolation and wild card can be used. Here is an example.
system behaves a bit like backtick operation. However there are some differences.
First let’s look at similarities.
system is a blocking operation. You can get the result of the operation using ` $?.success?
. system` operations are also executed in a subshell.
Now the differneces between
system eats up all the exceptions. So the main operation never needs to worry about capturing an exception raised from the child process.
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.
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 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
I see the result but since the irb process was replaced by the
exec process I am no longer in
Behind the scene both
backtick operations use
fork to fork the current process and then they execute the given operation using
exec replaces the current process it does not return anything if the operation is a success. If the operation fails then `SystemCallError</e> is raised.
sh actually calls
system under the hood. However it is worth a mention here. This method is added by
rake. It allows an easy way to check the exit status of the command.
If you are going to capture
stderr then you should use popen3 since this method allows you to interact with
I want to execute
git push heroku master programmatically and I want to capture the output. Here is my code.
And here is the output. It has been truncated since rest of output is not relevant to this discussion.
The important thing to note here is that when I execute the program
ruby lab.rb I do not see any output on my terminal for first 10 seconds. Then I see the whole output as one single dump.
The other thing to note is that heroku is writing all this output to
stderr and not to
Above solution works but it has one major drawback. The push to heroku might take 10 to 20 seconds and for this period we do not get any feedback on the terminal. In reality when we execute
git push heroku master we start seeing result on our terminal one by one as heroku is processing things.
So we should capture the output from heroku as it is being streamed rather than dumping the whole output as one single chunk of string at the end of processing.
Here is the modified code.
Now when I execute above command using
ruby lab.rb I get the output on my terminal incrementally as if I had typed
git push herokou master .
Here is another example of capturing streaming output.
In the above case you will get the output of ping on your terminal as if you had typed
ping www.google.com on your terminal .
Now let’s see how to check if command succeeded or not.
popen2e is similar to popen3 but merges the standard output and standard error .
In all other areas this method works similar to
Kernel.spawn executes the given command in a subshell. It returns immediately with the process id.