Can I use a BASH script for apt-get post update?

Question

Q: Can a post-update / upgrade bash script be executed when performing apt-get? I'm experiencing an error where the post-upgrade/update script is running sh & not bash.

See below for details.

---

1. I'm building Debian packages with fpm, e.g.:

fpm -s dir -t deb \
    -n "<A NAME>" \
    -a all -v "<A VERSION>" \
    --after-install <BASH SCRIPT> \
    --after-upgrade <BASH SCRIPT> \
    -m <MODULE> \
    <DIR>=<INSTALL DIR>

2. Upon installation I receive the following error:

Syntax error: "(" unexpected (expecting "}")
dpkg: error processing package <MY PACKAGE> (--configure):
 installed <MY PACKAGE NAME> package post-installation script subprocess returned error exit status 2
Errors were encountered while processing:
 <MY PACKAGE NAME>
E: Sub-process /usr/bin/dpkg returned an error code (1)

3. Subsequently, I released that the --after-install & --after-upgrade scripts create a wrapper around my bash scripts using sh & not bash and failing because some bash syntax is not available in sh, i.e. cat /var/lib/dpkg/info/<MY PACKAGE>.postinst script:

#!/bin/sh


after_upgrade() {
    :
#!/bin/bash

# Executed after apt-get < install | upgrade >