why do I need .zshrc?

Nope. Start with a clean .zshrc file and do not base it on .bashrc. The export PATH syntax is identical, but as I stated earlier, the export PS1 (shell prompt) syntax is different, and some of the aliases may behave differently owing to the different shell syntax and built-in commands.

In Terminal settings > Profiles > Shell tab, leave the Run command unchecked and blank.

As you would be just starting with Zsh, I would stay away from Oh-my-Zsh as it has caused novice usability issues for several users in these communities. I do not use it nor have any intention of doing so.

Some Zsh help:

Zsh documentation

How to customize the zsh prompt in macOS Terminal

Master your Z shell with these outrageously useful tips.

Moving to zsh - MacSysAdmin 2019 - Scripting OS X

Zsh: Helpful wiki

To set up Zsh history (also different than Bash) in the ~/.zshrc file, use these settings:

export HISTFILE=~/.zsh_history
export HISTFILESIZE=10000
export HISTSIZE=10000
export SHELL_SESSIONS_DISABLE=1
# any command preceded by a space is ignored by history
setopt HIST_IGNORE_SPACE
setopt INC_APPEND_HISTORY
alias hist_reset='history -p'
alias hist='history 1 | more'
# a handy history grep tool
hgrep () { fc -Dlim "*$@*" 1 }
# see your current Mac IP address
alias localIP='ifconfig -l | xargs -n1 ipconfig getifaddr'
# open a provided manpage name and transform it as a PDF in Preview (e.g. manpdf ls)
manpdf () { mandoc -T pdf -mdoc $(man -w "$@") | open -f -a Preview }

Let's say that you want to see the history items that you accessed file foo.pdf or issued an awk command:

hgrep foo.pdf

hgrep awk

Dannymac22 wrote:

iMac was purchased 7/21 (iMac M1 mid-2021). I believe it was delivered with Monterey. That should have been well after acceptance of zsh, so yes, it's unlikely that the default was in delivered firmware. So you're saying that this default is NOT part of the operating system, (which is what I was suggesting), but was just passed on by an earlier installation, most likely as part of individual accounts. This Mac replaced an earlier Mac, and I have to suppose that Migration Assistant was responsible for preserving bash as a default. That seems very odd, that if Apple wants to migrate from bash as a default to zsh as a default it doesn't happen in the operating system itself. That is, Mac is depending on YOU to make the change, rather than them.

Apple used tcsh as the default shell from OS X 10.0 to 10.2 inclusive, switched the default login shell for newly-created logins from tcsh to bash starting with OS X 10.3, and switched new logins from bash to zsh starting with macOS 10.15.

Newly-created logins get whatever was selected at creation, or get the default shell.

Older logins maintain the selected shell across macOS upgrades.

Logins that were migrated from earlier Mac installations will preserve (copy) the shell that the original login was using.

Really old logins will have tcsh, for instance. Unless the user switched the shell somewhere along the way.

So you might well have two Macs running the same macOS version, but different logins can have any of the Apple-provided shells, or any add-on shell.

This approach avoids breaking stuff. Particularly scripts that might not include a shebang, or that might make shell-related assumptions Folks would be unhappy if Apple shifted their login shell as part of a macOS upgrade.

Dannymac22 wrote:

OK, so then if the default is account-specific, that's why Migration Assistant preserved it.

Past the bootstrap environment, I’m unclear how a shell selection can be anything other than login-specific.

To the extent that bash and zsh are not wholly compatible, I guess that makes sense that Apple would leave it up to the user to make the change.

There are similarities among various shells, and there are differences.

zsh differs from bash in various ways: https://scriptingosx.com/2019/06/moving-to-zsh/

In my case, they were wholly compatible.

If within the subset of compatibility, sure. Or if the script includes a shebang, then the user’s shell choice doesn’t matter.

This depends on what shell is used.

The older bash uses files like .bashrc (and other files)

whereas zsh (which is now the default shell on Mac Terminal) uses .zshrc (and other files)

You can easily check which shell is being used by issuing the ps command or merely by the title of the Terminal window:

The Bash shell used by the Terminal in macOS Sequoia dates back to 2007 and Apple no longer supports it. It will be the default shell unless you choose to change it to the Zsh shell which Apple now encourages users to switch. Thus, the default message you see when you launch the Terminal on a Mac where the Bash shell is still active will suggest using chsh -s /bin/zsh to do this. If you are comfortable with the Bash shell you do not have to switch to the Zsh shell — though some (unconfirmed) future releases of macOS may ditch Bash altogether.

The Bash and Zsh shells have some things in common and many that are not, making the Zsh shell far more powerful than Bash. The Bash shell understands the Terminal dotfiles containing the word bash, and the Zsh shell does not read these, having its separate dotfiles (.e.g. ~/.zshrc, ~/.zshenv, ~/.zprofile, etc.). I can create the same visual prompt in either shell, but the syntax to achieve that is different between the two shells.

One machine (iMac) was older and using Bash originally - it would have to be manually switched to Zsh. The other (Air) is newer and had Zsh as the default shell out of the box.

But no, do not simply use mv to change the .bashrc file to .zshrc. The two don't use the same syntax for everything and if you do that you may have lines in .bashrc that will cause Zsh to experience a fatal error and crash. And if you use third party apps like iTerm instead of Terminal - and installed the Shell Integrations - then there will be non fatal but incompatible lines and it'll be errors everywhere.

I recommend that you review your .bashrc file and copy the right lines into a new .zshrc file.

Or...wild thought...there's also an opportunity here for you to really dig into shell customization with Oh My Zsh and its many themes: https://ohmyz.sh/

Did the oler machine come with Catalina, or did you get it before 2019 when Bash was still the default? If so, Bash was the default shell and Apple didn't change the default on existing installations when the OS upgaded, just new installations. Which I said the first time I posted in this thread when I said that if Bash was the original shell it would have to have been manually changed. Read harder yourself.

FYI, by default macOS has no default shell profiles within your home user folder (at least with later versions of macOS). Any such files are created either by you or by some software you have installed. IIRC, there is a basic default shell profile within the "/etc" folder that really should not be touched under normal use and is used if a shell profile is not within your home user folder.