Conversion between Puppet variables and Ruby variables in ERB is not always obvious and, in one notable way, inconsistent. Notably, an empty string is false in the Puppet language, but true in Ruby (and ERB).
The following is a Puppet manifest that can be “run” with puppet apply xxx/puppet-truthiness.pp:
$fednu = undef
$eslaf = false
$lambda = ''
$empty_array = []
$hiera_undef = hiera('nevergonnagiveyouup', undef)
#$no_such_var
notice("* Versions: Puppet ${puppetversion}, Hiera: ${hieraversion}, Facter: ${facterversion}")
notice("")
notice("* Checking for truthiness in ERB *")
notice("")
notice(inline_template("<%= if @fednu; '@fednu is truthy'; else '@fednu is not truthy'; end %>"))
notice(inline_template("<%= if @eslaf; '@eslaf is truthy'; else '@eslaf is not truthy'; end %>"))
notice(inline_template("<%= if @lambda; '@lambda is truthy'; else '@lambda is not truthy'; end %>"))
notice(inline_template("<%= if @empty_array; '@empty_array is truthy'; else '@empty_array is not truthy'; end %>"))
notice(inline_template("<%= if @hiera_undef; '@hiera_undef is truthy'; else '@hiera_undef is not truthy'; end %>"))
notice(inline_template("<%= if @no_such_var; '@no_such_var is truthy'; else '@no_such_var is not truthy'; end %>"))
notice("")
notice("* Checking for truthiness in Puppet lang *")
notice("")
if $fednu { notice('$fednu is truthy') } else { notice('$fednu is not truthy') }
if $eslaf { notice('$eslaf is truthy') } else { notice('$eslaf is not truthy') }
if $lambda { notice('$lambda is truthy') } else { notice('$lambda is not truthy') }
if $empty_array { notice('$empty_array is truthy') } else { notice('$empty_array is not truthy') }
if $hiera_undef { notice('$hiera_undef is truthy') } else { notice('$hiera_undef is not truthy') }
if $no_such_var { notice('$no_such_var is truthy') } else { notice('$no_such_var is not truthy') }
notice("")
notice("* Checking for nil *")
notice("")
notice(inline_template("<%= if @fednu.nil?; '@fednu is nil'; else '@fednu is not nil'; end %>"))
notice(inline_template("<%= if @eslaf.nil?; '@eslaf is nil'; else '@eslaf is not nil'; end %>"))
notice(inline_template("<%= if @lambda.nil?; '@lambda is nil'; else '@lambda is not nil'; end %>"))
notice(inline_template("<%= if @empty_array.nil?; '@empty_array is nil'; else '@empty_array is not nil'; end %>"))
notice(inline_template("<%= if @hiera_undef.nil?; '@hiera_undef is nil'; else '@hiera_undef is not nil'; end %>"))
notice(inline_template("<%= if @no_such_var.nil?; '@no_such_var is nil'; else '@no_such_var is not nil'; end %>"))
notice("")
notice("* Checking rendering in ERB *")
notice("")
notice(inline_template("@fednu is '<%= @fednu.to_s %>'"))
notice(inline_template("@eslaf is '<%= @eslaf.to_s %>'"))
notice(inline_template("@lambda is '<%= @lambda.to_s %>'"))
notice(inline_template("@empty_array is '<%= @empty_array.to_s %>'"))
notice(inline_template("@hiera_undef is '<%= @hiera_undef.to_s %>'"))
notice(inline_template("@no_such_var is '<%= @no_such_var.to_s %>'"))
notice("")
notice("* Checking rendering in Puppet lang *")
notice("")
notice("\$fednu is '${fednu}'")
notice("\$eslaf is '${eslaf}'")
notice("\$lambda is '${lambda}'")
notice("\$empty_array is '${empty_array}'")
notice("\$hiera_undef is '${hiera_undef}'")
notice("\$no_such_var is '${no_such_var}'")
notice("")
notice("DONE")
And the following is the resulting output:
Notice: Scope(Class[main]): * Checking for truthiness in ERB *
Notice: Scope(Class[main]):
Notice: Scope(Class[main]): @fednu is not truthy
Notice: Scope(Class[main]): @eslaf is not truthy
Notice: Scope(Class[main]): @lambda is truthy
Notice: Scope(Class[main]): @empty_array is truthy
Notice: Scope(Class[main]): @hiera_undef is truthy
Notice: Scope(Class[main]): @no_such_var is not truthy
Notice: Scope(Class[main]):
Notice: Scope(Class[main]): * Checking for truthiness in Puppet lang *
Notice: Scope(Class[main]):
Notice: Scope(Class[main]): $fednu is not truthy
Notice: Scope(Class[main]): $eslaf is not truthy
Notice: Scope(Class[main]): $lambda is not truthy
Notice: Scope(Class[main]): $empty_array is truthy
Notice: Scope(Class[main]): $hiera_undef is not truthy
Notice: Scope(Class[main]): $no_such_var is not truthy
Notice: Scope(Class[main]):
Notice: Scope(Class[main]): * Checking for nil *
Notice: Scope(Class[main]):
Notice: Scope(Class[main]): @fednu is nil
Notice: Scope(Class[main]): @eslaf is not nil
Notice: Scope(Class[main]): @lambda is not nil
Notice: Scope(Class[main]): @empty_array is not nil
Notice: Scope(Class[main]): @hiera_undef is not nil
Notice: Scope(Class[main]): @no_such_var is nil
Notice: Scope(Class[main]):
Notice: Scope(Class[main]): * Checking rendering in ERB *
Notice: Scope(Class[main]):
Notice: Scope(Class[main]): @fednu is ''
Notice: Scope(Class[main]): @eslaf is 'false'
Notice: Scope(Class[main]): @lambda is ''
Notice: Scope(Class[main]): @empty_array is '[]'
Notice: Scope(Class[main]): @hiera_undef is ''
Notice: Scope(Class[main]): @no_such_var is ''
Notice: Scope(Class[main]):
Notice: Scope(Class[main]): * Checking rendering in Puppet lang *
Notice: Scope(Class[main]):
Notice: Scope(Class[main]): $fednu is ''
Notice: Scope(Class[main]): $eslaf is 'false'
Notice: Scope(Class[main]): $lambda is ''
Notice: Scope(Class[main]): $empty_array is ''
Notice: Scope(Class[main]): $hiera_undef is ''
Notice: Scope(Class[main]): $no_such_var is ''
Notice: Scope(Class[main]):
Notice: Scope(Class[main]): DONE
Notice: Compiled catalog for coolair.oit.pdx.edu in environment production in 0.07 seconds
Notice: Finished catalog run in 0.05 seconds