I'm working on implementing Project Euler solutions as semantic Ruby one-liners. It would be extremely useful if I could coerce Ruby to automatically puts the value of the last expression. Is there a way to do this? For example:
#!/usr/bin/env ruby -Ilib -rrubygems -reuler
1.upto(100).into {|n| (n.sum.squared - n.map(&:squared).sum)
I realize I can simply puts the line, but for other reasons (I plan to eval the file in tests, to compare against the expected output) I would like to avoid an explicit puts. Also, it allots me an extra four characters for the solution. :)
Is there anything I can do?
You might try running it under irb instead of directly under a Ruby interpreter.
It seems like the options -f --noprompt --noverbose might be suitable (.
#!/usr/bin/env irb -f --noprompt --noverbose -Ilib -rrubygems -reuler
'put your one-liner here'
The options have these meanings:
-f: do not use .irbrc (or IRBRC)
--noverbose: do not display the source lines
--noprompt: do not prefix the output (e.g. with =>)
result = calculate_result
puts result if File.exist?(__FILE__)
result of eval is last executed operation just like any other code block in ruby
is doing
puts eval(file_contents)
an option for you?
EDIT
you can make use of eval's second parameter which is variables binding
try the following:
do_not_puts = true
eval(file_contents, binding)
and in the file:
....
result = final_result
if defined?(do_not_puts)
result
else
puts(result)
end
Is it an option to change the way you run scripts?
script.rb:
$_= 1.upto(100).into {|n| (n.sum.squared - n.map(&:squared).sum)
invoke with
echo nil.txt | /usr/bin/env/ruby -Ilib -rrubygems -reuler -p script.rb, where nil.txt is a file with a single newline.
Related
My question is a follow up to this question: No return on command line when running Ruby script because the answer doesn't offer an explanation:
Take the following file, script.rb:
def hello(names)
names.map {|name| "#{name} is awesome!"}
end
hello(["mark", "tony", "scott"])
When executing the file on the command line with ruby script.rb the return value of the following function does not appear. However, testing the method in IRB or by dropping into the code with PRY outputs an explicit return value. Why isn't the return value visible via script execution?
P.S. I am aware that using puts will output code into the terminal but I'm interested in why the return value doesn't output.
Because both IRB or Pry are REPL's
REPL stands for: read, evaluate, print and loop. And that's exactly what both Pry and IRB are doing.
They will first read your input, evaluate your code, print the result of the code execution and then start over.
A Ruby script can't return a value directly like you want it to, the Bash shell works in the same way, a Bash function can't return a string directly. You can return a string (with stdout) and assign it to the variable.
~:$~ cat scr.rb
~:$~ puts "Something here"
~:$~ echo $(ruby ./scr.rb)
Something here
~:$~ myvar=$(echo $(ruby ./scr.rb))
~:$~ echo $myvar
Something here
It's really simple: Bash (or whatever shell you are using) and Ruby are different programming languages. Bash doesn't know anything about Ruby. It doesn't know what a " Ruby return" is, it doesn't know what a "Ruby array" is, it doesn't know what a "Ruby string" is. Therefore, you simply cannot possibly return a Ruby object to Bash.
In fact, the shell usually just uses some operating system function to execute the Ruby script (e.g. the classical fork and exec or something like vfork or clone). If you wanted to return values this way, the operating system kernel would have to know about the semantics of every programming language ever invented plus every programming language that is going to be invented in the future. That is just not feasible.
Note that a command can return a value to the shell, namely an integer between 0 and 255 intended as a status code (with 0 meaning "success" and nonzero meaning "error"), and you can set that return value by calling Kernel#exit.
I used to have the same question myself when I started coding. If you have a closer look at your code you can see why it doesn't print anything. You are actually no asking it in your code. Imagine having a huge script of thousands of lines and you want to execute it. You would have millions of pointless outputs if ruby myscript.rb worked the same way as the REPLs.
In addition, if you do want it to work that way, you can just do require the script inside the REPL session ( require_relative 'your_script' ) and then if you call your function hello it will work the way you describe.
I can use the ruval gem. It evaluates each statement and returns its value.
$ ruval names.rb
def hello(names)
names.map {|name| "#{name} is awesome!"}
end
=> hello
hello(["mark", "tony", "scott"])
=> ["mark is awesome!", "tony is awesome!", "scott is awesome!"]
I have a task of writing a simple Ruby script which would do the following.
Upon execution from the UNIX command line, it would present the user with a prompt at which he should be able to run certain commands, like "dir", "help" or "exit". Upon "exit" the user should return to the Unix shell.
I'm not asking for the solution; I would just like to know how this "shell" functionality can be implemented in Ruby. How do you present the user with a prompt and interpret commands.
I do not need a CLI script that takes arguments. I need something that creates a shell interface.
The type of program you require can easily be made with just a few simple constructs.
I know you're not asking for a solution, but I'll just give you a skeleton to start off and play around with:
#!/usr/bin/env ruby
def prnthelp
puts "Hello sir, what would you like to do?"
puts "1: dir"
puts "2: exit"
end
def loop
prnthelp
case gets.chomp.to_i
when 1 then puts "you chose dir!"
when 2 then puts "you chose exit!"
exit
end
loop
end
loop
Anyways, this is a simplistic example on how you could do it, but probably the book recommended in the comments is better. But this is just to get you off.
Some commands to get you started are:
somevar = gets
This gets user input. Maybe learn about some string methods to manipulate this input can do you some good. http://ruby-doc.org/core-2.0/String.html
chomp will chop off any whitespace, and to_i converts it to an integer.
Some commands to do Unix stuff:
system('ls -la') #=> outputs the output of that command
exit #=> exits the program
Anyways, if you want this kind of stuff, I think it's not a bad idea to look into http://www.codecademy.com/ basically they teach you Ruby by writing small scripts such as these. However, they maybe not be completely adapted to Unix commands, but user input and the likes are certainly handled.
Edit:
As pointed out do use this at the top of your script:
#!/usr/bin/env ruby
Edit:
Example of chomp vs. chop:
full_name = "My Name is Ravikanth\r\n"
full_name.chop! # => "My Name is Ravikanth"
Now if you run chop and there are no newline characters:
puts full_name #=> "My Name is Ravikanth"
full_name.chop! #=> "My Name is Ravikant"
versus:
puts full_name #=> "My Name is Ravikanth\r\n"
full_name.chomp! #=> "My Name is Ravikanth"
full_name.chomp! #=> "My Name is Ravikanth"
See: "Ruby Chop vs Chomp"
Here's a really basic loop:
#!/user/bin/ruby
#
while true do
print "$ "
$stdout.flush
inputs = gets.strip
puts "got your input: #{inputs}"
# Check for termination, like if they type in 'exit' or whatever...
# Run "system" on inputs like 'dir' or whatever...
end
As Stefan mentioned in a comment, this is a huge topic and there are scenarios that will make this complicated. This is, as I say, a very basic example.
Adding to the two other (valid) answers posted so far be wary of using #!/usr/bin/ruby, because ruby isn't always installed there. You can use this instead:
#!/usr/bin/env ruby
Or if you want warnings:
#!/usr/bin/env ruby -w
That way, your script will work irrespective of differences where ruby might be installed on your server and your laptop.
Edit: also, be sure to look into Thor and Rake.
http://whatisthor.com
http://rake.rubyforge.org
Use irb.
I was looking into an alternative to bash and was thinking along the same lines... but ended up choosing fish: http://fishshell.com/
Nonetheless, I was thinking of using irb and going along the lines of irbtools: https://github.com/janlelis/irbtools
Example:
> irb
Welcome to IRB. You are using ruby 1.9.3p0 (2011-10-30 revision 33570) [x86_64-linux]. Have fun ;)
>> ls #=> ["bin", "share", "opt", "lib", "var", "etc", "src"]
>>
In any case, irb is the ruby shell.
Take a look at cliqr which comes with inbuilt support for build a custom shell https://github.com/anshulverma/cliqr/
I'm using pry and I want to capture, and work with output of a shell command.
For example, If I run
pry(main)> .ls
I want to get the list of files into an array that I can work with in Ruby.
How can I do this?
This is a pretty old question but I'll answer it anyways. There are two primary methods of getting data out of pry commands. The first is if the command sets the keep_retval option to true, which the shell command does not. The second, is to use the virtual pipe. In your example this can be done as:
fizz = []
.ls | {|listing| fizz = listing.split("\n")} # can also be written as
.ls do |listing|
fizz = listing.split("\n")
end
I assume it's some kind of pry's magic ;-)
After quick look at what's happening (I didn't look at pry's source), you might want to use this:
`ls`.split("\n")
or
Dir['./*']
What's good about this solution is that it will work outside of pry
Ok this is driving me crazy:
`ls #{"/media/music/Miles Davis"}`
fails because of the space between "Miles" and "Davis"
Say I write a ruby script and a user passes file path as an argument. How do I escape it and feed to a shell-out command. Yes, yes, I know, shelling out should be avoided. But this is a contrived example, I still need this.
I would do system("ls", ARGV[0]), but it doesn't return the stdout output of ls as a string, which is what backticks do well.
How do escape whatever you insert in a shellout?
Use require 'shellwords' and Shellwords.escape, which will fix this sort of stuff for you:
http://apidock.com/ruby/Shellwords/shellescape
Stay away from building shell strings
This is a fine vector for arbitrary code execution.
In this case, you could use popen, which does the escaping for you, e.g.:
#!/usr/bin/env ruby
IO.popen(['printf', 'a b']) do |f|
puts f.read
end
This outputs:
a b
just as if we had run on the terminal:
/usr/bin/printf 'a b'
If a b hadn't been escaped, we wouldn't get a b as expected, because running an unquoted:
/usr/bin/printf a b
in the terminal gives:
a/usr/bin/printf: warning: ignoring excess arguments, starting with ‘b’
Tested in Ubuntu 20.02, Ruby 2.6.
Double quotes also works:
`ls "#{'/media/music/Miles Davis'}"`
or
`ls "#{ARGV[0]}"`
I'm new to ruby ... wondering if the following is possible:
I currently run a test app within irb (irb -r test.rb) and manually execute
various command implemented in test.rb. One of these functions is currently implemented as follows:
def cli(cmd)
ret=$client.Cli(cmd)
print ret, "\n"
end
Where $client.Cli() takes a string. I currently type the following in the IRB prompt
> cli "some command with parameters"
This is sent over socket and results are returned
I would like to be able to do this WITHOUT the quotes. This would be just for this command
Is there a way to do this generally in ruby? if not how would you extend irb to do this?
For those who know 'C' this would be like the following:
#define CLI(CMD) cli(#CMD)
CLI(Quadafi and Sheen walk into a bar...)
where the pre-processed output is:
cli("Quadafi and Sheen walk into a bar...")
Thanks
You could actually monkey patch the gets method of the IRB::StdioInputMethod and IRB::ReadlineInputMethod classes, and perform a rewrite if the cli method is called, by adding the following to your test.rb file:
module IRB
def self.add_quotes(str)
str.gsub(/^cli (..+?)(\\+)?$/, 'cli "\1\2\2"') unless str.nil?
end
class StdioInputMethod
alias :old_gets :gets
def gets
IRB::add_quotes(old_gets)
end
end
class ReadlineInputMethod
alias :old_gets :gets
def gets
IRB::add_quotes(old_gets)
end
end
end
This way, any input line matching cli ... will be replaced with cli "..." before it's evaluated.
I don't think it's possible, because the command that you type to irb has to parse as ruby, and all those bare words will report errors like this:
NameError: undefined local variable or method `hello' for main:Object
(My first attempt, I just called it via cli hello.)
But if you didn't mind a more radical change, you could do something like this:
$ cat /tmp/test_cases
hello world
one
two
three
riding the corpse sled
$ ruby -e 'def f(arg) puts arg end' -ne 'f($_)' < /tmp/test_cases
hello world
one
two
three
riding the corpse sled
I just defined a simple function f() here that would show how it works; you could replace f($_) with $Client.cli($_), and set up the $Client global variable in the first -e argument. And you can leave off the < /tmp/test_cases if you want to type them in interactively:
$ ruby -e 'def f(arg) puts arg end' -ne 'f($_)'
hello
hello
world
world
Of course, if you want it to be any more advanced than this, I'd just write a script to do it all, rather than build something hideous from -pe or -ne commands.