Getting output of system() calls in Ruby

If I call a command using [Kernel#system](

[To see links please register here]

) in Ruby, how do I get its output?

system("ls")

You use backticks:

`ls`

Another way is:

f = open("|ls")
foo = f.read()

Note that's the "pipe" character before "ls" in open. This can also be used to feed data into the programs standard input as well as reading its standard output.

As a direct system(...) replacement you may use Open3.popen3(...)

Further discussion:

[To see links please register here]

I found that the following is useful if you need the return value:

result = %x[ls]
puts result

I specifically wanted to list the pids of all the Java processes on my machine, and used this:

ids = %x[ps ax | grep java | awk '{ print $1 }' | xargs]

You can use system() or %x[] depending what kind of result you need.

system() returning true if the command was found and ran successfully, false otherwise.

>> s = system 'uptime'
10:56 up 3 days, 23:10, 2 users, load averages: 0.17 0.17 0.14
=> true
>> s.class
=> TrueClass
>> $?.class
=> Process::Status

%x[..] on the other hand saves the results of the command as a string:

>> result = %x[uptime]
=> "13:16 up 4 days, 1:30, 2 users, load averages: 0.39 0.29 0.23\n"
>> p result
"13:16 up 4 days, 1:30, 2 users, load averages: 0.39 0.29 0.23\n"
>> result.class
=> String

Th [blog post by Jay Fields][1] explains in detail the differences between using system, exec and %x[..] .


[1]:

[To see links please register here]

Just for the record, if you want both (output and operation result) you can do:



output=`ls no_existing_file` ; result=$?.success?

If you need to escape the arguments, in Ruby 1.9 [IO.popen](

[To see links please register here]

) also accepts an array:

p IO.popen(["echo", "it's escaped"]).read

In earlier versions you can use [Open3.popen3](

[To see links please register here]

):

require "open3"

Open3.popen3("echo", "it's escaped") { |i, o| p o.read }

If you also need to pass stdin, this should work in both 1.9 and 1.8:

out = IO.popen("xxd -p", "r+") { |io|
io.print "xyz"
io.close_write
io.read.chomp
}
p out # "78797a"

If you want the output redirected to a file using `Kernel#system`, you can do modify descriptors like this:

redirect stdout and stderr to a file(/tmp/log) in append mode:

```
system('ls -al', :out => ['/tmp/log', 'a'], :err => ['/tmp/log', 'a'])
```

For a long running command, this will store the output in real time. You can also, store the output using a IO.pipe and redirect it from Kernel#system.

As [Simon Hürlimann already explained](

[To see links please register here]

), [Open3][1] is safer than backticks etc.



require 'open3'
output = Open3.popen3("ls") { |stdin, stdout, stderr, wait_thr| stdout.read }


Note that the block form will auto-close stdin, stdout and stderr- otherwise they'd have to be [closed explicitly][2].

[1]:

[To see links please register here]

[2]:

[To see links please register here]