[info exists env($envVarName)]} then {
      return $env($envVarName)
    } elseif {[info exists env(TEMP)]} then {
      return $env(TEMP)
    } elseif {[info exists env(TMP)]} then {
      return $env(TMP)
    } else {
      if {[string length $envVarName] > 0} then {
        set defEnvVarName $envVarName
      } elseif {[isWindows]} then {
        set defEnvVarName TEMP
      } else {
        set defEnvVarName TMP
      }

      error [appendArgs \
          "please set " $envVarName \
          " (via environment) to temporary directory"]
          "please set the \"" $defEnvVarName \
          "\" environment variable to the path of a temporary directory"]
    }
  }

  #
  # NOTE: This procedure returns a unique temporary file name.  A script
  #       error is raised if this task cannot be accomplished.  There are
  #       no arguments.