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

True cross-platform solution, works properly on Windows:

# Cross-platform way of finding an executable in the $PATH.
#
#   which('ruby') #=> /usr/bin/ruby
def which(cmd)
  exts = ENV['PATHEXT'] ? ENV['PATHEXT'].split(';') : ['']
  ENV['PATH'].split(File::PATH_SEPARATOR).each do |path|
    exts.each do |ext|
      exe = File.join(path, "#{cmd}#{ext}")
      return exe if File.executable?(exe) && !File.directory?(exe)
    end
  end
  nil
end

This doesn't use host OS sniffing, and respects $PATHEXT which lists valid file extensions for executables on Windows.

Shelling out to which works on many systems but not all.

Use find_executable method from mkmf which is included to stdlib.

require 'mkmf'

find_executable 'ruby'
#=> "/Users/narkoz/.rvm/rubies/ruby-2.0.0-p0/bin/ruby"

find_executable 'which-ruby'
#=> nil

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).

Use MakeMakefile#find_executable0 with Logging Disabled

There are a number of good answers already, but here's what I use:

require 'mkmf'

def set_mkmf_log(logfile=File::NULL)
  MakeMakefile::Logging.instance_variable_set(:@logfile, logfile)
end

# Return path to cmd as a String, or nil if not found.
def which(cmd)
  old_mkmf_log = MakeMakefile::Logging.instance_variable_get(:@logfile)
  set_mkmf_log(nil)
  path_to_cmd = find_executable0(cmd)
  set_mkmf_log(old_mkmf_log)
  path_to_cmd
end

This uses the undocumented #find_executable0 method invoked by MakeMakefile#find_executable to return the path without cluttering standard output. The #which method also temporarily redirects the mkmf logfile to /dev/null to prevent cluttering the current working directory with "mkmf.log" or similar.