Kindlegen output pipe while not generating images

kindlegen.log

*************************************************************
 Amazon kindlegen(Linux) V2.9 build 1028-0897292 
 A command line e-book compiler 
 Copyright Amazon.com and its Affiliates 2014 
*************************************************************

Info(prcgen):I1047: Added metadata dc:Title        "test 4"
Info(prcgen):I1047: Added metadata dc:Creator      "webook"
Info(prcgen):I1047: Added metadata dc:Publisher    "webook"
Info(prcgen):I1002: Parsing files  0000002
Warning(inputpreprocessor):W29004: Forcefully closed opened Tag: <p class="l-box-7-5">
      in file: /home/profiart/domains/webookdisk.profiart.pl/public_html/releases/20170722102814/books_drafts/5_125157_22072017/text.html     line: 0000014
Warning(prcgen):W14010: media file not found  /home/profiart/domains/webookdisk.profiart.pl/public_html/releases/20170722102814/books_drafts/5_125157_22072017/Blog-w-sklepie-300x225.jpg
Warning(prcgen):W14010: media file not found  /home/profiart/domains/webookdisk.profiart.pl/public_html/releases/20170722102814/books_drafts/5_125157_22072017/Blog-w-sklepie-internetowym-300x201.jpg
Warning(prcgen):W14010: media file not found  /tmp/mbt-qLoqKA7E1_7_16_D_D_21_2EE_543E_B79F860_A
Info(prcgen):I1015: Building PRC file
Info(prcgen):I1006: Resolving hyperlinks
Info(prcgen):I1008: Resolving start reading location
Warning(prcgen):W14016: Cover not specified
Info(pagemap):I8000: No Page map found in the book
Info(prcgen):I1045: Computing UNICODE ranges used in the book
Info(prcgen):I1046: Found UNICODE range: Basic Latin [20..7E]
Info(prcgen):I1046: Found UNICODE range: Latin Extended-A [100..17F]
Info(prcgen):I1046: Found UNICODE range: Latin-1 Supplement [A0..FF]
Info(prcgen):I1046: Found UNICODE range: General Punctuation - Windows 1252 [2013..2014]
Info(prcgen):I1017: Building PRC file, record count:   0000003
Info(prcgen):I1039: Final stats - text compressed to (in % of original size):  59.97%
Info(prcgen):I1040: The document identifier is: "test_4"
Info(prcgen):I1041: The file format version is V5
Info(prcgen):I1031: Saving PRC file
Info(prcgen):I1033: PRC built with WARNINGS!
Info(prcgen):I1016: Building enhanced PRC file
Info(prcgen):I1007: Resolving mediaidlinks
Info(prcgen):I1011: Writing mediaidlinks
Info(prcgen):I1009: Resolving guide items
Info(prcgen):I1039: Final stats - text compressed to (in % of original size):  59.36%
Info(prcgen):I1041: The file format version is V8
Info(prcgen):I1032: PRC built successfully
Info(prcgen):I15000:  Approximate Standard Mobi Deliverable file size :   4194301KB
Info(prcgen):I15001:  Approximate KF8 Deliverable file size :   0000001KB
Info(prcgen):I1037: Mobi file built with WARNINGS!