Moving to zsh, part 6 – Customizing the zsh Prompt

Apple has announced that in macOS 10.15 Catalina the default shell will be zsh.

I will be giving a half-day ‘Moving to zsh’ class at our offices in Amsterdam on September 6! Visit the website for details!

In this series, I will document my experiences moving bash settings, configurations, and scripts over to zsh.

As I have mentioned in the earlier posts, I am aware that there are many solutions out there that give you a pre-configured ‘shortcut’ into lots of zsh goodness. But I am interested in learning this the ‘hard way’ without shortcuts. Call me old-fashioned. (“Uphill! In the snow! Both ways!”)

The default bash prompt on macOS is quite elaborate. It shows the username, the hostname, and the current directory.

Calypso:~ armin$

On the other hand, the default bash prompt doesn’t show the previous command’s exit code, a piece of information I find very useful. I have written before how I re-configured my bash prompt to have the information I want:

Of course, I wanted to recreate the same experience in zsh.

Minimal Zsh Prompt

The only (visual) difference to my bash prompt is the % instead of the $.

Note: creating a file ~/.hushlogin will suppress the status message at the start of each Terminal session in zsh as well as in bash (or any other shell).

Basic Prompt Configuration

The basic zsh prompt configuration works similar to bash, even though it uses a different syntax. The different placeholders are described in detail in the zsh manual.

zsh uses the same shell variable PS1 to store the default prompt. However, the variable names PROMPT and prompt are synonyms for PS1 and you will see either of those three being used in various examples. I am going to use PROMPT.

The default prompt in zsh is %m%#. The %m shows the first element of the hostname, the %# shows a # when the current prompt has super-user privileges (e.g. after a sudo -s) and otherwise the % symbol (the default zsh prompt symbol).

The zsh default prompt is far shorter than the bash default, but even less useful. Since I work on the local system most of the time, the hostname bears no useful information, and repeating it every line is superfluous.

Note: you can argue that the hostname in the prompt is useful when you frequently have multiple terminal windows open to different hosts. This is true, but then the prompt is defined by the remote shell and its configuration files on the remote host. In your configuration file, you can test if the SSH_CLIENT variable is set and show a different prompt for remote sessions. There are more ways of showing the host in remote shell sessions, for example in the Terminal window title bar or with different window background colors.

In our first iteration, I want to show the current working directory instead of the hostname. When you look through the list of prompt placeholders in the zsh documentation, you find %d, %/, and %~. The first two do exactly the same. The last substitution will display a path that starts with the user’s home directory with the ~, so it will shorten /Users/armin/Projects/ to ~/Projects.

Note: in the end you want to set your PROMPT variable in the .zshrc file, so it will take effect in all your zsh sessions. For testing, however, you can just change the PROMPT variable in the interactive shell. This will give you immediate feedback, how your current setup works.

% PROMPT='%/ %# '
/Users/armin/Projects/dotfiles/zshfunctions % 

% PROMPT='%~ %# '
~/Projects/dotfiles/zshfunctions % 

Note the trailing space in the prompt string, to separate the final % or # from the command entry.

I prefer the shorter output of the %~ option, but it can still be quite long, depending on your working directory. zsh has a trick for this: when you insert a number n between the % and the ~, then only the last n elements of the path will be shown:

% PROMPT='%2~ %# '
dotfiles/zshfunctions %                       

When you do %1~ it will show only the name of the working directory or ~ if it is the home directory. (This also works with %/, e.g. %2/.)

Adding Color

Adding a bit of color or shades of gray to the prompt can make it more readable. In bash you need cryptic escape codes to switch the colors. zsh provides an easier way. To turn the directory in the path blue, you can use:

PROMPT='%F{blue}%1~%f %# '

The F stands for ‘Foreground color.’ zsh understands the colors black, red, green, yellow, blue, magenta, cyan and white. %F or %f resets to the default text color. Furthermore, Terminal.app represents itself as a 256-color terminal to the shell. You can verify this with

% echo $TERM
xterm-256color

You can access the 256 color pallet with %F{0} through %F{255}. There are tables showing which number maps to which color:

So, since I want a dark gray for my current working dir in my prompt, I chose 240, I also set it to bold with the %B code:

PROMPT='%B%F{240}%1~%f%b %# '

You can find a detailed list of the codes for visual effects in the documentation.

Dynamic Prompt

I wrote an entire post on how to get bash to show the color-coded exit code of the last command. As it turns out, this is much easier in zsh.

One of the prompt codes provides a ‘ternary conditional,’ which means it will show one of two expressions, depending on a condition. There are several conditions you can use. Once again the details can be found in the documentation.

There is one condition for the previous commands exit code:

%(?.<success expression>.<failure expression>)

This expression will use the <success expression> when the previous command exited successfully (exit code zero) and <failure expression> when the previous command failed (non-zero exit code). So it is quite easy to build an conditional prompt:

% PROMPT='%(?.√.?%?) %1~ %# ' 
√ ~ % false
?1 ~ % 

You can get the character with option-V on the US or international macOS keyboard layout. The last part of the ternary ?%? looks confusing. The first ? will print a literal question mark, and the second part %? will be replaced with previous command’s exit code.

You can add colors in the ternary expression as well:

PROMPT='%(?.%F{green}√.%F{red}?%?)%f %B%F{240}%1~%f%b %# ' 

Another interesting conditional code is ! which returns whether the shell is privileged (i.e. running as root) or not. This allows us to change the default prompt symbol from % to something else, while maintaining the warning functionality when running as root:

% PROMPT='%1~ %(!.#.>) ' 
~ > sudo -s
~ # exit
~ > 

Complete Prompt

Here is the complete prompt we assembled, with all the parts explained:

PROMPT='%(?.%F{green}√.%F{red}?%?)%f %B%F{240}%1~%f%b %# '
%(?.√.?%?) if return code ? is 0, show , else show ?%?
%? exit code of previous command
%1~ current working dir, shortening home to ~, show only last 1 element
%# # with root privileges, % otherwise
%B %b start/stop bold
%F{...} text (foreground) color, see table
%f reset to default textcolor

Right Sided Prompt

zsh also offers a right sided prompt. It uses the same placeholders as the ‘normal’ prompt. Use the RPROMPT variable to set the right side prompt:

% RPROMPT='%*'
√ zshfunctions %                    11:02:55

zsh will automatically hide the right prompt when the cursor reaches it when typing a long command. You can use all the other substitutions from the left side prompt, including colors and other visual markers in the right side prompt.

Git Integration

zsh includes some basic integration for version control systems. Once again there is a voluminous, but hard to understand description of it in the documentation.

I found a better, more specific example in the ‘Pro git’ documentation. This example will show the current branch on the right side prompt.

I have changed the example to include the repo name and the branch, and to change the color.

autoload -Uz vcs_info
precmd_vcs_info() { vcs_info }
precmd_functions+=( precmd_vcs_info )
setopt prompt_subst
RPROMPT=\$vcs_info_msg_0_
zstyle ':vcs_info:git:*' formats '%F{240}(%b)%r%f'
zstyle ':vcs_info:*' enable git

In this case %b and %r are placeholders for the VCS (version control system) system for the branch and the repository name.

There are git prompt solutions other than the built-in module, which deliver more information. There is a script in the git repository, and many of the larger zsh theme projects, such as ‘oh-my-zsh’ and ‘prezto’ have all kinds of git status widgets or modules or themes or what ever they call them.

Summary

You can spend (or waste) a lot of time on fine-tuning your prompt. Whether these modifications really improve your productivity is a matter of opinion.

In the next post, we will cover some miscellaneous odds and ends that haven’t yet really fit into any of preceding posts.

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