Using a different shell without running `chsh`

README.md

Problem

I wanted to create a jumpbox where the default shell was zsh, but I couldn't actually change the default shell of the users that would be connecting to the box. I also wanted to manage some symlinks in the user profile dynamically with Dotbot.

My initial idea was to add a script to /etc/profile.d that would just dump users into /bin/zsh, but this had several different problems, including the all-important question of "But what if I want to run Bash on purpose?"

Solution

Through a little bit of trial and error, I settled on this script located at /etc/profile.d/dotbot.sh

# Only run under bash
if [[ $0 == *"bash" ]]; then
  # Only for interactive sessions
  if [[ $- == *i* ]]; then
    # And only for the login session
    if shopt -q login_shell; then
      if command -v /usr/local/bin/dotbot > /dev/null; then
        /usr/local/bin/dotbot -c /srv/dotfiles/dotbot.conf.yaml -q
        # Enter zsh
        zsh; exit $?
      else
        >&2 echo "Error: Cannot configure user profile, unable to find Dotbot"
      fi
    fi
  fi
fi

Explanations

A bunch of nested ifs are always fun:

Only run under bash

# Only run under bash
if [[ $0 == *"bash" ]]; then

Early on, I thought I might actually switch the default shell at some point, so this was borne out of a desire for compatibility. The test is written as *"bash" because the login shell coming from our LDAP is /bin/bash, whereas my testing of $0 indicated it's just bash. This was so they'd both work.

Only for interactive sessions

# Only for interactive sessions
if [[ $- == *i* ]]; then

Without this step, things like rsync and scp would break.

And only for the login session

# And only for the login session
if shopt -q login_shell; then

What if you actually need bash though? This check will always be false except for the top-level shell, so users who are currently in a zsh session can run bash without the script dumping them right back into zsh.

Check for dotbot and run zsh

if command -v /usr/local/bin/dotbot > /dev/null; then
  /usr/local/bin/dotbot -c /srv/dotfiles/dotbot.conf.yaml -q
  # Enter zsh
  zsh; exit $?
else
  >&2 echo "Error: Cannot configure user profile, unable to find Dotbot"
fi

Check to make sure that dotbot is installed, then start zsh. The zsh; exit $? is there to pass exit codes up, but also to ensure that exit/logout in the zsh shell actually disconnect the user coming in via SSH. Otherwise they'd exit zsh and drop into the bash shell that's hosting it.