BASH: Split a large file, retaining extension

# Split into 10 files
split -n l/10 --additional-suffix=.txt largefile.txt output_prefix_

# Split into files containing 10000 lines each
split -l 10000 --additional-suffix=.txt largefile.txt output_prefix_

# Note on MacOS (BSD, not GNU), first... 
brew install coreutils

# then use 
gsplit ....

# Merge the split files using:
cat output_prefix_* > merged_file.txt

# or, to merge in a predictable order:
cat output_prefix_aa output_prefix_ab output_prefix_ac > merged_file.txt

# Or, if the files are numerically named, e.g output_prefix_01.txt:
cat output_prefix_*.txt > merged_file.txt