Bash script needed to read variables from file and write back

I need a small shell script which does some simple operations of reading a var from one file and write it into another. Read Var 1, Var1_book, write Var 1 into file Execute a program with filename 1 --> creates file2 Read Var 2 from file 2 --> execute curl and save xml file to disk (xml file 1) Read Var 3 from XML file 1 and - write to xml file 2 - write Var1_book to xml file 2 execute program with filename 2 execute program with filename 3 --> creates file 3 Read some Vars from file 3 and write into filename 3 execute program with filename 4 --> creates file 4 Read a Var from file 4 and write into filename 3