Run a script on multiple shells?

The easiest way by far is to use the same shell on both systems. Just because one shell is preinstalled (there's no such thing as a “default shell” for scripts, the shell is whatever the shebang line says) doesn't mean that you can't install others. You can install bash on AIX (from the toolbox, for example), or ksh93 for Linux (with your distribution's package manager: install the ksh package).

If you pick ksh, beware that /usr/bin/ksh on AIX is ksh88. There is no version of ksh88 for Linux, only ksh93, which is available as /usr/bin/ksh93 on AIX.

It is easier if the same shell is installed at the same location on both systems (a symbolic link is fine), so that you can use the same path in the shebang line. If having the same location is too difficult, you can use something like #!/usr/bin/env bash, as long as you ensure that bash is in the PATH on both systems. This may be useful if you have to install bash or ksh in your home directory because you don't have root access.

If you really can't install the same shell on both machines — presumably because there's some silly compliance rule that only complicates people's life but is too entrenched to overturn — you have a few possibilities.

• Use #!/bin/sh as your shebang line and program in the intersection of what your systems offer. All modern unix systems provide a POSIX shell as /bin/sh. On AIX, /bin/sh is ksh88. On Red Hat, /bin/sh is bash (which behaves slightly differently when invoked as sh). Beware that on some other operating systems (for example, on many Linux distributions), /bin/sh is a smaller shell which may not have much more than POSIX features.

• Start your scripts with some boilerplate code that looks for a better shell and executes it.

  #!/bin/sh
  if [ -n "$BASH" ]; then
    … bash compatibility code …
  elif type whence >/dev/null 2>/dev/null; then
    … ksh compatibility code …
  elif type ksh93 >/dev/null 2>/dev/null; then
    exec ksh93 "$0" "$@"
  elif type ksh >/dev/null 2>/dev/null; then
    exec ksh "$0" "$@"
  elif type mksh >/dev/null 2>/dev/null; then
    exec mksh "$0" "$@"
  elif type bash >/dev/null 2>/dev/null; then
    exec bash "$0" "$@"
  else
    echo 1>&2 "Cannot find ksh or bash, aborting"
    exit 125
  fi
  

In either case, the intersection of ksh88 and bash provides some useful features beyond POSIX, but you may need a little compatibility code. In particular:

• Array assignment in ksh88 uses set -A. See assignation variable under different ksh environment
• Local variables are declared by typeset.
• Ksh88 doesn't have ${VAR/PATTERN/REPLACEMENT}, $'…' or FIGNORE. It does have [[ … ]].
• To enable @(…) and other ksh extended patterns in bash, run shopt -s extglob.

I would put some work into seeing if you can actually have one version with command that work on both systems.

I had this issue with my .bashrc file which I keep in my dotfiles repository. I want it to work on both OSX and Ubuntu and there were a few command I had to modify to do that. In this case it was mostly which switches worked for certain command or how a test command was written. I tackled each issue, finding a way to do whatever command I was trying in a form that would work on both OS's

I tried the 'two different files' and 'two different sections depending on OS' approaches and found them a pain to maintain.