mjhjmtu

I have real trouble getting an if statement to work properly....
here is a link to a zip file with all code needed: https://www.dropbox.com/s/ojsj33oe9ac4g09/latex-rechnung.zip?dl=0

My code:

documentclass[a4paper]scrlttr2 
usepackage[top=2cm, bottom=2cm, left=2cm, right=2cm]geometry
usepackagegraphicx
usepackagelmodern
usepackage[utf8]inputenc
usepackage[T1]fontenc
usepackage[ngerman]babel
usepackagecolor
usepackage[hidelinks]hyperref
usepackagelipsum
usepackagexifthen

usepackage[german]invoice

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
makeatletter
defUnitDayPreis/Tag% <========================================

newcommandFeeDay@Title% <==========================================
 \
 noindenttextbfActivity&&UnitDay&Count&Amount (BC)\
 hline%
%

newcommandFeeDay[3]% Yields state 3, Print Fee Item <==============
 %
 % #1 Contents
 % #2 Fee per Day
 % #3 Unit Count
 %
 ifcaseFlag % 0: Invoice not started yet
 error@messageMissingOpening%
 %
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 or % 1: Start invoice
 %
 error@messageMissingProject%
 %
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 or % 2: Start project, print title
 %
 FeeDay@Title%
 Fee@Line#1#2#3%
 %
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 or % 3: Print remuneration item
 %
 Fee@Line#1#2#3%
 %
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 or % 4: Print subtotal remuneration
 %
 warning@messageFeeSTExists%
 %
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 or % 5: Expense item
 %
 error@messageFeeBeforeExpense%
 %
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 or % 6: Print subtotal expenses
 %
 error@messageFeeBeforeExpense%
 %
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 or % 7: Print subtotal project
 %
 error@messageProjectCompletedNoFee%
 %
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 or % 8: Print total, close invoice
 %
 error@messageInvoiceCompletedNoFee%
 %
 %
 else error@messageInternalError%
 fi%
%
makeatother


renewcommandfamilydefaultsfdefault

% include meta data
include_data % <============== delete in this file the following data!
% ################## invoice DATA ##################
newcommandinvoiceDatetoday % Datum der rechnungsstellung
newcommandinvoiceReference2019_02_04_1 % Rechnungsnummer (z.B. 20150122-4) YYYYMMDD-1
newcommandinvoiceSalutation % die Anrede
newcommandinvoiceTextFolgende Positionen werden in Rechnung gestellt: % Rechnungstext
newcommandinvoiceEnclosures % encl einfügen
newcommandinvoiceClosingMit freundlichen Grüßen
% ################## invoice DATA ##################


% ##################Job DATA ####################
def performaceStart 14.01.2019
def performaceEnd 01.02.2019
% ##################Job DATA ####################


% ################## Customer DATA ##################
newcommandcustomerCompanyFirma ABC %ggf. Firma
newcommandcustomerNameMustermann % Name
newcommandcustomerStreetStraße 123 % Straße
newcommandcustomerZIP12345 % Postleitzahl
newcommandcustomerCityMusterstadt % Ort
newcommandcustomerNumber1000 % Kundennummer <==================
% ################## Customer DATA ##################

setkomavarfromnamesenderName
setkomavarfromaddresssenderStreet \ senderZIP senderCity
setkomavarplacesenderCity
setkomavardateinvoiceDate
setkomavarfromphonesenderTelephone
setkomavarfrommobilephonesenderMobilephone
setkomavarfromemailhrefmailto:senderEmailnolinkurlsenderEmail
setkomavarsubjectRechnung

def invoicenrRechnungsnummer: % <========================================
def customernrKundennummer: %
def performanceDurationLeistungszeitraum: 

setkomavarlocationvspace*-1.99cm
 \[baselineskip] \[baselineskip]
 flushrightsmall invoicenr invoiceReference\[5pt]
 customernr\
 customerNumber\[5pt]
 performanceDuration\

 newcommanddownow
 if performanceEnd == 
 performaceStart -- performaceEnd
 else
 performanceStart
 
 % performaceStart -- performaceEnd
 


setkomavarfirstheadhfill
 parbox[t][headheight][t]7.3cm
 % 
 footnotesize
 raggedright
 flushright
 color[gray].3% 
 %



begindocument
 beginlettercustomerCompany \ customerName \ 
 customerStreet \ customerZIP customerCityvspace*-3.5cm


 openinginvoiceSalutation
 invoiceText
 begininvoiceEuro0

ProjectTitleTagesleistungen % 
FeeDayFreelancing 300 1

%ProjectTitleProjekttitel % 
%FeeÜberstunden 37.5 4



% Auslagen
%EBCSoftwarenutzung 150.00 
%EBCiHotel, 12 Nächte 2400.00
%STExpenses

% Rabatt
DiscountRabatt 0 % 
 endinvoice
 ps Bitte zahlen Sie den Gesamtbetrag unter Angabe der Rechnungsnummer auf das aufgeführte Konto.
 ps Kleinunternehmerregelung:\
 Im ausgewiesenen Betrag ist gemäß § 19 UStG keine Umsatzsteuer 
 enthalten.

 closinginvoiceClosing
 invoiceEnclosures
 vspace*3.9cm
 footnotesize ps textbfBankverbindung:hspace6mmaccountBankName, accountIBAN, accountBIC
 footnotesize ps textbfSteuernummer:hspace8mmtaxID



 endletter
enddocument 

I would like to have an if statement that checks if performaceEnd is undefined or empty. If it is empty, under performanceDuration it shall only say performanceStart, if performanceEnd is defined it shall say performanceStart -- performanceEnd

Ho do I do that?

The correct way to write your test is

ifxperformanceEndundefined
 performanceStart
elseifxperformanceEndempty
 performanceStart
else
 performanceStart -- performanceEnd
fifi

You can also declare a boolean ifperformanceEnd by

newififperformanceEnd

anywhere in the preamble and check performanceEnd with

performanceEndfalse
ifxperformanceEndundefinedelse
 ifxperformanceEndemptyelse
 performanceEndtrue
fifi

then you can write

ifperformanceEnd
 performanceStart -- performanceEnd
else
 performanceStart
fi

or even

performanceStartifperformanceEnd -- performanceEndfi

Okay, here we go: as already pointed out by @eric-domenjoud you can use ifx to build the variations of text you need. BTW think about adding an "seit " to make clear that the performance time is not over.

I corrected in the following code the wrong written variables performaceStart and performaceEnd to the correct one performanceStart and performanceEnd in the complete following code.

As we did in your last question we once again have to change the code for command location like:

setkomavarlocation%
 \[baselineskip] \[baselineskip]
 flushrightsmall invoicenr invoiceReference\[5pt]
 customernr\
 customerNumber\[5pt]
 performanceDuration\ 
ifxperformanceEndundefined % <=======================================
 performanceStart
elseifxperformanceEndempty
 performanceStart
else
 performanceStart -- performanceEnd
fifi % <==============================================================
 

Then we get with the following code (you need to use all files in the question linked zip file to be able to compile it!)

 documentclass[a4paper]scrlttr2

usepackage[top=2cm, bottom=1cm, left=2cm, right=2cm]geometry
usepackagegraphicx
usepackagelmodern
usepackage[utf8]inputenc
usepackage[T1]fontenc
usepackage[ngerman]babel
usepackagecolor
usepackage[hidelinks]hyperref
%usepackage ifthen

usepackage[german]invoice

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
makeatletter
defUnitDayPreis/Tag% <========================================

newcommandFeeDay@Title% <==========================================
 \
 noindenttextbfActivity&&UnitDay&Count&Amount (BC)\
 hline%
%

newcommandFeeDay[3]% Yields state 3, Print Fee Item <==============
 %
 % #1 Contents
 % #2 Fee per Day
 % #3 Unit Count
 %
 ifcaseFlag % 0: Invoice not started yet
 error@messageMissingOpening%
 %
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 or % 1: Start invoice
 %
 error@messageMissingProject%
 %
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 or % 2: Start project, print title
 %
 FeeDay@Title%
 Fee@Line#1#2#3%
 %
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 or % 3: Print remuneration item
 %
 Fee@Line#1#2#3%
 %
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 or % 4: Print subtotal remuneration
 %
 warning@messageFeeSTExists%
 %
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 or % 5: Expense item
 %
 error@messageFeeBeforeExpense%
 %
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 or % 6: Print subtotal expenses
 %
 error@messageFeeBeforeExpense%
 %
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 or % 7: Print subtotal project
 %
 error@messageProjectCompletedNoFee%
 %
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 or % 8: Print total, close invoice
 %
 error@messageInvoiceCompletedNoFee%
 %
 %
 else error@messageInternalError%
 fi%
%
makeatother


renewcommandfamilydefaultsfdefault

% include meta data
include_data % <============== delete in this file the following data!
% ################## invoice DATA ##################
newcommandinvoiceDatetoday % Datum der rechnungsstellung
newcommandinvoiceReference2019_01_01_1 % Rechnungsnummer (z.B. 20150122-4) YYYYMMDD-1
newcommandinvoiceSalutationSehr geehrte Damen und Herren, % die Anrede
newcommandinvoiceTextFolgende Positionen werden in Rechnung gestellt: % Rechnungstext
newcommandinvoiceEnclosures % encl einfügen
newcommandinvoiceClosingMit freundlichen Grüßen
% ################## invoice DATA ##################


% ##################Job DATA ####################
def performanceStart 01.01.2019
%def performanceEnd 02.01.2019
% ##################Job DATA ####################


% ################## Customer DATA ##################
newcommandcustomerCompanyFirma ABC %ggf. Firma
newcommandcustomerNameMax Mustermann % Name
newcommandcustomerStreetMusterstraße 10 % Straße
newcommandcustomerZIP12345 % Postleitzahl
newcommandcustomerCityMusterort % Ort
newcommandcustomerNumber1000 % Kundennummer <==================
% ################## Customer DATA ##################

setkomavarfromnamesenderName
setkomavarfromaddresssenderStreet \ senderZIP senderCity
setkomavarplacesenderCity
setkomavardateinvoiceDate
setkomavarfromphonesenderTelephone
setkomavarfrommobilephonesenderMobilephone
setkomavarfromemailhrefmailto:senderEmailnolinkurlsenderEmail
setkomavarsubjectRechnung

def invoicenrRechnungsnummer: % <========================================
def customernrKundennummer: %
def performanceDurationLeistungszeitraum: 

setkomavarlocation%
 \[baselineskip] \[baselineskip]
 flushrightsmall invoicenr invoiceReference\[5pt]
 customernr\
 customerNumber\[5pt]
 performanceDuration\ 
ifxperformanceEndundefined % <=======================================
 performanceStart
elseifxperformanceEndempty
 performanceStart
else
 performanceStart -- performanceEnd
fifi % <==============================================================
 


setkomavarfirstheadhfill
 parbox[t][headheight][t]7.3cm% 
 footnotesize
 raggedright
 flushright
 color[gray].3%
 begintabularrl
 Anschrift & usekomavarfromname\
 & senderStreet\
 & senderZIP senderCity \
 \
 %Telefon Telefon: ` senderTelephone \
 Mobil & usekomavarfrommobilephone \
 E-Mail & usekomavarfromemail \
 %Webseite & normalfontttfamily senderWeb \
 \
 Steuernummer & taxID \
 \
 Institut & accountBankName \
 IBAN & accountIBAN \
 BIC & accountBIC
 endtabular 
 %



begindocument
 beginlettercustomerCompany \ customerName \ 
 customerStreet \ customerZIP customerCity


 openinginvoiceSalutation
 invoiceText
 begininvoiceEuro0

ProjectTitleTagesleistungen % 
FeeDayFreelance Arbeit 10 1

ProjectTitleProjekttitel % 
FeeÜberstunden 20 1



% Auslagen
%EBCHotel, 12 Nächte 2400.00 % EBCiHotel, 12 Nächte 2400.00
%STExpenses

% Rabatt
DiscountRabatt 0 % 
 endinvoice
 ps Bitte zahlen Sie den Gesamtbetrag unter Angabe der Rechnungsnummer auf das aufgeführte Konto.
 ps Kleinunternehmerregelung:\
 Im ausgewiesenen Betrag ist gemäß § 19 UStG keine Umsatzsteuer 
 enthalten.

 closinginvoiceClosing
 invoiceEnclosures



 endletter
enddocument

the following result (see that I commented definition of performanceEnd):

If you want to add "seit " as remarked at the beginning of this answer you can use

setkomavarlocation%
 \[baselineskip] \[baselineskip]
 flushrightsmall invoicenr invoiceReference\[5pt]
 customernr\
 customerNumber\[5pt]
 performanceDuration\ 
ifxperformanceEndundefined % <=======================================
 seit performanceStart
elseifxperformanceEndempty
 seit performanceStart
else
 performanceStart -- performanceEnd
fifi % <==============================================================
 

with the result: