I learnt that ab exercises like sit ups are bad for the lower back and now I a looking for alternatives.
I found Stuart McGill's “Big Three” exercises, which are back-friendly and two of them train the abs. What are other back-friendly bodyweight exercises for the abs?
Edit: I found planks as one further alternative for abs.
Why sit ups are bad: "... McGill, a professor emeritus at the University of Waterloo, in Ontario, and the author of the book Back Mechanic, didn’t begin his academic career with a particular interest in the sit-up; his work focused on the spine. But throughout the 1990s and 2000s, he led research that changed the way fitness experts thought about exercise. His findings showed that sit-ups and crunches weren’t just mediocre strength-building moves; they were actually hurting lots of people. ... As McGill and other experts published their findings, he began to hear from people who had found injury patterns that matched his research—most notably, from trainers and physical therapists in the U.S. and Canadian military, who were questioning the sit-up’s primacy in their fitness instruction. In the past decade, every branch of the U.S. military has begun to phase out sit-ups and crunches from their required testing and training regimens, or else they have made them optional, alongside more orthopedically sound maneuvers such as the plank. Spokespeople for the Army and the Marines confirmed to me that these decisions in their branches were made in part to avoid the high rates of lower-back injury found among troops training for speed sit-up and crunch tests. ... If you hadn’t yet noticed crunches disappearing around you—or if you have a trainer who still puts you through your sit-up paces—McCall said he wouldn’t exactly be shocked. Like many other American industries, the fitness business is consolidating, but it still contains tons of independent instructors and small businesses. Sit-ups and crunches have been discouraged by educators within the industry for years, but there are no licensing or continuing-education requirements for teaching exercise, and if trainers don’t seek out new information and techniques, it can take a while for good information and new ideas to get through to them." (Source). Harvard Health Publishing writes: "One reason is that sit-ups (...) push your curved spine against the floor and work your hip flexors, the muscles that run from the thighs to the lumbar vertebrae in the lower back. When the hip flexors are too strong or too tight, they tug on the lower spine, which can create lower back discomfort." (source)