why is bash replaced with zsh?

I like zsh better. Unless you have a large library of shell scripts that are dependent on bash features, you shouldn't see any difference.

The current (i.e. updated) version of bash is under the GPLv3 license which is incompatible with Apple's (or any hardware manufacturer's) business model.

zsh is still under the GPLv2 license.

You can still use the old version of bash if you want, or you can install your own current version of bash. You may have to use some type of package manager like HomeBrew.

Apple has shipped the 2007 vintage of Bash v3 and cannot upgrade to Bash 4 due to GPL3 restrictions. For the same reason, Apple is not including any of the GNU software. As Barney pointed out, the homebrew package manager can install Bash4 and that will go into /usr/local/bin. You could also install the GNU Coreutils in the same manner, which I did.

By shifting to Zsh, Apple can keep it version current. Zsh has far more power than Bash 4. By default, Zsh ignores white-space in filenames, so you do not have to quote Zsh variables to avoid Bash white-space issues. Want an absolute path in Zsh, it is simply ${var:a}. Zsh supports globing so you can do directory recursion. It also supports associative arrays, and ton of other stuff.

The Zsh site has a Zsh Manual for Zsh v5.3 which is current. The Zsh user guide however is dated 2003.

Much of the Bash script logic just works with Zsh, but now, when you open the Terminal, your Bash associated dot files do not get run, so your PATH and other shell customizations need to be set up for Zsh in the ~/.zshrc file. Here is what I have done with mine:

# .zshrc
export PATH=".:$HOME/bin:/usr/local/bin:$PATH"
export MANPATH=.:/usr/share/man:$MANPATH
TPUT="/usr/bin/tput"
Bluebld="$(${TPUT} bold; ${TPUT} setaf 63)"
White="$(${TPUT} bold; ${TPUT} setaf 15)"
Green="$(${TPUT} bold; ${TPUT} setaf 42)"
Normal="$(${TPUT} sgr0)"
MyHost="$(networksetup -getcomputername)"
export CLICOLOR=1
export GREP_COLOR='00;38;5;226'
export GREP_OPTIONS='--color=auto --extended-regexp'
export LSCOLORS='gxfxcxdxbxegedabagacad'
PS1="${Green}${MyHost}: %~ %% ${White}"
export EDITOR="/usr/bin/vim"
bindkey -v

That MyHost is because I do not want a hostname assigned by a remote VPN DNS server. The e/grep color is vivid yellow, and the Zsh uses an entirely different PS1 syntax from Bash. Finally, when I press the escape key in a command line, the bindkey -v allows me to use Vim editor syntax when editing the command-line.

solaris.neon wrote:

"The default interactive shell is now zsh.

To update your account to use zsh, please run `chsh -s /bin/zsh`.

For more details, please visit https://support.apple.com/kb/HT208050."

-- HOW TO DISABLE THIS MESSAGE?????

Different question.

export BASH_SILENCE_DEPRECATION_WARNING=1

That and additional details are available from various discussions, including:

https://scriptingosx.com/2019/06/moving-to-zsh/

MrHoffman wrote:

export BASH_SILENCE_DEPRECATION_WARNING=1

https://scriptingosx.com/2019/06/moving-to-zsh/

Thank You. Thank You. Thank You!

I do not have anything against zsh, but I work in a cross platform environment with Linux, Solaris, and AIX, and I have a ton of bash based shell scripts.

Yes I know I do not need to run bash to use a bash shell script. After all, I used csh for years, but wrote /bin/sh (and as in pre-POSIX sh) and ksh scripts.

As I switched to bash in 2005 and it is more comfortable using the same shell when ssh'ing Linux, Solaris and AIX platforms.

So I thank you very much for finding the "export BASH_SILENCE_DEPRECATION_WARNING=1" settings.

"The default interactive shell is now zsh.

To update your account to use zsh, please run `chsh -s /bin/zsh`.

For more details, please visit https://support.apple.com/kb/HT208050."

-- HOW TO DISABLE THIS MESSAGE?????

I need BASH its already default I don't need to Read this message any time I start terminal on catalina

thank you

That's the problem -- I'm using package manager (macports) and it (zsh) fails to launch any application. Bash -- no problem.

You need to update your path in zsh. MacPorts likely does not do that for zsh.

MrHoffman wrote:

Might eventually be headed for the following, if we’re going to be adding our own bash locally:

#!/usr/bin/env bash

Not that I’ve sorted all the links already used in Catalina, either.

Yea, I've been using #!/usr/bin/env bash for a few years now.

This trick has been helpful when platforms such as Solaris and AIX do not ship bash by default, so it is often in an alternate directory. But many of my script have to work cross-platform.

De nada.

Might eventually be headed for the following, if we’re going to be adding our own bash locally:

#!/usr/bin/env bash

Not that I’ve sorted all the links already used in Catalina, either.

Remember that the shell is just a program, you can still get any program you want, just not from Apple. I prefer tcsh.