Instead of using sh trying using bash. This works for me:
$ bash intro
Hello World
/usr/bin/neqn
#! /bin/sh
# Provision of this shell script should not be taken to imply that use of
# GNU eqn with groff -Tascii|-Tlatin1|-Tutf8|-Tcp1047 is supported.
GROFF_RUNTIME="${GROFF_BIN_PATH=/usr/bin}:"
PATH="$GROFF_RUNTIME$PATH"
export PATH
exec eqn -Tascii ${1+"$@"}
# eof
When you invoke your script using sh you're actually using bash but it's running in a mode which makes it behave as if it's a more ancient version of the shell that is the predecessor to Bash.
excerpt from man bash:
If bash is invoked with the name sh, it tries to mimic the startup
behavior of historical versions of sh as closely as possible, while
conforming to the POSIX standard as well. When invoked as an interactive
login shell, or a non-interactive shell with the --login option, it first
attempts to read and execute commands from /etc/profile and ~/.profile,
in that order. The --noprofile option may be used to inhibit this
behavior. When invoked as an interactive shell with the name sh, bash
looks for the variable ENV, expands its value if it is defined,
and uses the expanded value as the name of a file to read and execute.
Since a shell invoked as sh does not attempt to read and execute
commands from any other startup files, the --rcfile option has no
effect. A non-interactive shell invoked with the name sh does not
attempt to read any other startup files. When invoked as sh, bash enters
posix mode after the startup files are read.
