"which in ruby": Checking if program exists in $PATH from ruby

Question

Hiya, my scripts rely heavily on external programs and scripts. I need to be sure that a program I need to call exists. Manually, I'd check this using 'which' in the commandline.

Is there an equivalent to File.exists? for things in $PATH?

(yes I guess I could parse %x[which scriptINeedToRun] but that's not super elegant.

Thanks! yannick

---

UPDATE: Here's the solution I retained:

def commandExists?(command)
        system("which #{command} > /dev/null 2>/dev/null")
        return false if $?.exitstatus == 127
        return true
end

Answer 1

You can access system environment variables with the ENV hash:

puts ENV['PATH']

It will return the PATH on your system. So if you want to know if program nmap exists, you can do this:

ENV['PATH'].split(':').each {|folder| puts File.exists?(folder+'/nmap')}

This will print true if file was found or false otherwise.

Answer 2

There was a GEM called which_rubythat was a pure-Ruby which implementation. It's no longer available.

However, I found this pure-Ruby alternative implementation.

Answer 3

This is a tweak of rogeriopvl's answer, making it cross platform:

require 'rbconfig'

def is_windows?
  Config::CONFIG["host_os"] =~ /mswin|mingw/
end

def exists_in_path?(file)
  entries = ENV['PATH'].split(is_windows? ? ";" : ":")
  entries.any? {|f| File.exists?("#{f}/#{file}")}
end

Answer 4

def command?(name)
  `which #{name}`
  $?.success?
end

Initially taken from hub, which used type -t instead of which though (and which failed for both zsh and bash for me).

Answer 5

Here's what I'm using. This is platform neutral (File::PATH_SEPARATOR is ":" on Unix and ";" on Windows), only looks for program files that actually are executable by the effective user of the current process, and terminates as soon as the program is found:

##
# Returns +true+ if the +program+ executable is found in the user's path.
def has_program?(program)
  ENV['PATH'].split(File::PATH_SEPARATOR).any? do |directory|
    File.executable?(File.join(directory, program.to_s))
  end
end

Answer 6

What is not elegant in unix philosophy? Why go through $PATH if there is a tool which does one thing well?

Also I can't find any man page telling that which could return 127.

Ruby system method returns true on 0 exit status, so you need only first line of your method. Also this method doesn't care about escaping command name: use builtin shellwords if you use 1.8.7 or higher version, or something alike if not.

Answer 7

I'd like to add that which takes the flag -s for silent mode, which only sets the success flag, removing the need for redirecting the output.