POSTIN

/bin/sh

grep -q "^/bin/mksh$" /etc/shells 2>/dev/null || \
  echo "/bin/mksh" >> /etc/shells
grep -q "^/usr/bin/mksh$" /etc/shells 2>/dev/null || \
  echo "/usr/bin/mksh" >> /etc/shells

POSTUN

/bin/sh

if [ ! -x /usr/bin/mksh ]; then
  sed -e 's@^/bin/mksh$@POSTUNREMOVE@' -e '/^POSTUNREMOVE$/d' -i /etc/shells
  sed -e 's@^/usr/bin/mksh$@POSTUNREMOVE@' -e '/^POSTUNREMOVE$/d' -i /etc/shells
fi