/usr/bin/between-two-genes is in ncbi-entrez-direct 7.40.20170928+ds-1.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
1 2 | #!/bin/bash -norc
awk -F '\t' -v 'OFS=\t' "/^$1\t/{a++}/^$2\t/{a++}a>0{print}a>1{exit}"
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