Pass ssh remote command from bash to zsh

Question

I am running a remote putty session from windows to my Red Hat Enterprise Linux server.

Due to company restrictions, I am unable to change my Linux user's default shell from bash to zsh. I instead have read only access to ~./bash_profile, which executes ~/.bashrc (where I also have only read only access), which in turn executes ~/.bash_custom where I have write access. Inside ~/.bash_custom, I execute zsh to change my shell.

Including the line to execute zsh prevents the execution of any remote command provided by putty. I know this is the problem since if I comment out my execution of zsh it works fine.

I have several aliases and PATH modifications inside my zsh which I'd like to use as part of the remote command executed from putty. How can I pass the remote command from the bash shell to zsh for it to be executed there?

Answer 1

On some (most these days) systems, bash is configured at build time so that when run over ssh, bash reads ~/.bashrc (and corresponding system-wide file in /etc) even when run non-interactively like when running ssh some command where sshd runs bash -c 'some command'¹.

What you could do is add at the top of your ~/.bash_custom:

export SHELL="$(command -v zsh)" # tell everyone your preferred shell
case ${SSH_CONNECTION+s}:${-//[^ilc]}:$#:$SHLVL in
  (s:c:0:1) exec zsh -c -- "$BASH_EXECUTION_STRING";;
  (*:i:0:*) exec zsh;;
esac

That executes zsh with the same code when bash is run as bash -c 'some code' by sshd, and zsh alone when bash is run interactively.

You could limit that second case to cases where bash is invoked as a login shell only with:

if shopt -q login_shell; then
  exec zsh
fi

and/or only via ssh by replacing *:i:0:* with s:i:0:*

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^{¹ Strictly speaking, it should run bash -c -- 'some command' but since there are shells where that's not supported (or not needed like in csh, fish, rc...), it doesn't.}