Apple has announced that in macOS 10.15 Catalina the default shell will be zsh
.
In this series, I will document my experiences moving bash
settings, configurations, and scripts over to zsh
.
- Part 1: Moving to zsh (this article)
- Part 2: Configuration Files
- Part 3: Shell Options
- Part 4: Aliases and Functions
- Part 5: Completions
- Part 6: Customizing the
zsh
Prompt - Part 7: Miscellanea
- Part 8: Scripting
zsh
zsh
(I believe it is pronounced zee-shell, though zish is fun to say) will succeed bash
as the default shell. bash
has been the default shell since Mac OS X 10.3 Panther.
This series has grown into a book: reworked and expanded with more detail and topics. Like my other books, I plan to update and add to it after release as well, keeping it relevant and useful. You can order it on the Apple Books Store now.
Why?
The bash
binary bundled with macOS has been stuck on version 3.2 for a long time now. bash
v4 was released in 2009 and bash
v5 in January 2019. The reason Apple has not switched to these newer versions is that they are licensed with GPL v3. bash
v3 is still GPL v2.
zsh
, on the other hand, has an ‘MIT-like’ license, which makes it much more palatable for Apple to include in the system by default. zsh
has been available as on macOS for a long time. The zsh
version on macOS 10.14 Mojave is fairly new (5.3). macOS 10.15 Catalina has the current zsh
5.7.1.
Is bash gone!?
No.
macOS Catalina still has the same /bin/bash
(version 3.2.57) as Mojave and earlier macOS versions. This change is only for new accounts created on macOS Catalina. When you upgrade to Catalina, a user’s default shell will remain what it was before.
Many scripts in macOS, management systems, and Apple and third party installers rely on /bin/bash
. If Apple just yanked this binary in macOS 10.15 Catalina or even 10.16. Many installers and other solutions would break and simply cease to function.
Users that have /bin/bash
as their default shell on Catalina will see a prompt at the start of each Terminal session stating that zsh
is now the recommended default shell. If you want to continue using /bin/bash
, you can supress this message by setting an environment variable in your .bash_profile
or .bashrc
.
export BASH_SILENCE_DEPRECATION_WARNING=1
You can also download and install a newer version of bash yourself. Keep in mind that custom bash installations reside in a different directory, usually /usr/local/bin/bash
.
Will bash remain indefinitely?
Apple is strongly messaging that you should switch shells. This is different from the last switch in Mac OS X 10.3 Panther, when Apple switched the default to bash
, but didn’t really care if you remained on tcsh
. In fact, tcsh
is still present on macOS.
Apple’s messaging should tell us, that the days of /bin/bash
are numbered. Probably not very soon, but eventually keeping a more than ten year old version of bash
on the system will turn into a liability. The built-in bash had to be patched in 2014 to mitigate the ‘Shellshock’ vulnerability. At some point Apple will consider the cost of continued maintenance too high.
Another clue is that a new shell appeared on macOS Catalina (and is mentioned in the support article). The ‘Debian Almquist Shell’ dash
has been added to the lineup of shells. dash
is designed to be a minimal implementation of the Posix standard shell sh
. So far, in macOS (including Catalina),sh
invokes bash
in sh
-compatibility mode.
As Apple’s support article mentions, Catalina also adds a new mechanism for users and admins to change which shell handles sh
invocations. MacAdmins or users can change the symbolic link stored in /var/select/sh
to point to a shell other than /bin/bash
. This changes which shell interprets scripts the #!/bin/sh
shebang or scripts invoked with sh -c
. Changing the interpreter for sh
should not, but may change the behavior of several crucial scripts in the system, management tools, and in installers, but may be very useful for testing purposes.
All of these changes are indicators that Apple is preparing to remove /bin/bash
at some, yet indeterminate, time in the future.
Do I need to wait for Catalina to switch to zsh?
No, zsh
is available Mojave and on older macOS versions. You can start testing zsh
or even switch your default shell already.
If you want to just see how zsh
works, you can just open Terminal and type zsh
:
$ zsh
MacBook%
The main change you will see is that the prompt looks different. zsh
uses the %
character as the default prompt. (You can change that, of course.) Most navigation keystrokes and other behaviors will remain the same as in bash
.
If you want to already switch your default shell to zsh
you can use the chsh
command:
$ chsh -s /bin/zsh
This will prompt for your password. This command will not change the current shell, but all new ones, so close the current Terminal windows and tabs and open a new one.
How is zsh different?
Like bash
(‘Bourne again shell’ ), zsh
derives from the ‘Bourne’ family of shells. Because of this common ancestry, it behaves very similar in day-to-day use. The most obvious change will be the different prompt.
The main difference between bash
and zsh
is configuration. Since zsh
ignores the bash
configuration files (.bash_profile
or .bashrc
) you cannot simply copy customized bash settings over to zsh
. zsh
has much more options and points to change zsh
configuration and behavior. There is an entire eco-system of configuration tools and themes called oh-my-zsh
which is very popular.
zsh
also offers better configuration for auto-completion which is far easier than in bash
.
I am planning a separate post, describing how to transfer (and translate) your configurations from bash
to zsh
.
What about scripting?
Since zsh
has been present on macOS for a long time, you could start moving your scripts from bash
to zsh
right away and not lose backwards compatibility. Just remember to set the shebang in your scripts to #!/bin/zsh
.
You will gain some features where zsh
is superior to bash
v3, such as arrays and associative arrays (dictionaries).
There is one exception where I would now recommend to use /bin/sh
for your scripts: the Recovery system does not contain the /bin/zsh
shell, even on the Catalina beta. This could still change during the beta phase, or even later, but then you still have to consider older macOS installations where zsh
is definitely not present in Recovery.
When you plan to use your scripts or pkgs with installation scripts in a Recovery (or NetInstall, or bootable USB drive) context, such as Twocanoes MDS, installr or bootstrappr, then you cannot rely on /bin/zsh
.
Since we now know that bash
is eventually going away, the only common choice left is /bin/sh
.
When you build an installer package, it can be difficult to anticipate all the contexts in which it might be deployed. So, for installation pre- and postinstall scripts, I would recommend using /bin/sh
as the shebang from now on.
I used to recommend using /bin/bash
for everything MacAdmin related. /bin/sh
is definitely a step down in functionality, but it seems like the safest choice for continued support.
Summary
Overall, while the messaging from Apple is very interesting, the change itself is less dramatic than the headlines. Apple is not ‘replacing’ bash
with zsh
, at least not yet. Overall, we will have to re-think and re-learn a few things, but there is also much to be gained by finally switching from a ten-year-old shell to a new modern one!
Next
In the next part we will look at the configuration files for zsh
.