Tutorial

How to Use Editors, Regex, and Hooks with Z-shell

Published on May 2, 2023

Writer / Developer / Musician

How to Use Editors, Regex, and Hooks with Z-shell

Introduction

zsh, otherwise known as the Z shell, was created by Paul Falstad in 1990 to provide an alternative to the Bourne Shell. zsh provides many customization options that are not available in other shells, making the Z shell a feature rich alternative to sh and bash, as well as lesser used shells like the Korn shell (ksh) and the C shell (csh). zsh has become popular for command line users who want to tailor their shell experience specifically to their needs.

This article will cover a sampling of the features offered by zsh, including using zsh to emulate other shells, editing on the command line using built in zsh features alongside the external tools vidir and vipe, using regular expressions in zsh commands, and setting up hooks to automate shell behavior.

Prerequisites

Before getting started with the code in this article, make sure that you have a terminal emulator installed on your system and your zsh –version is 5.8.1 or higher.

If you’re using Linux and your zsh version is lower than 5.8.1 you can update by using your system package manager. For example, using apt on Ubuntu will install zsh version 5.9

MacOS users will need to install an updated version of zsh which can be done via the homebrew tool:

  1. brew update && brew install zsh

To use vidir and vipe you must install the moreutils package on your machine. This package is available for Linux via joeyh.name/code/moreutils/, or installed with your package managers on Debian, Ubuntu, and Arch. Moreutils is also available on MacOS via homebrew or macports.

Finally, this article assumes that you have some familiarity with command-line tools, and editing files in the terminal. Familiarity with zsh dotfiles, including the .zshrc file is required for some of the examples below.

Now, let’s get started!

Compatibility with Other Shells

While zsh is one of several popular command line shells, the basic usage of zsh does not differ greatly from other shells. Additionally, zsh includes certain compatibility options with other shells for those times you prefer specific options offered by other shells, or you’re testing zsh out for the first time.

The tools provided by zsh to mimic the behavior of other shells fall into two categories:

  1. Using shell options (setopt)
  2. Using the emulate Builtin

Let’s take a look at what you can due using these commands, starting with setopt

1. Using Shell Options with setopt

The setopt tool offers many options for customizing your shell, but for the purposes of this article, we will cover only options that are most useful for making your zsh experience a bit more comfortable when coming from another environment. The options below all focus on bringing different behavior to the zsh shell whenever you need it.

But first, how do you even know what options are active in your shell? Running setopt without an argument will display a list of options you have enabled. While this is helpful, you may have to do a little digging to find a full list of options available in the zsh shell. However there is a way to see a full list of available options directly in your shell by using ksh_option_print.

ksh_option_print

The ksh_option_print option lets you change the list of options printed with setopt from showing only enabled options to showing each option and a corresponding on or off column for each.

# setopt kshoptionprint will also work
setopt ksh_option_print

Here’s an abridged list of options from setopt with an on/off indicator for each option.

  1. setopt
noaliases   off
allexport   on
noalwayslastprompt   off
alwaystoend   on
appendcreate   off
noappendhistory           off
autocd   on
autocontinue   off
noautolist   off
noautomenu   off
autonamedirs   off
noautoparamkeys   off
noautoparamslash   off
autopushd   on
noautoremoveslash   off
autoresume   off
nobgnice   off
braceccl   off
bsdecho   on

bsd_echo

By default zsh will print newline (\n) or tab (\t) characters as non-visible, but there may be times where you need to have the literal backslash escaped character present in a string. For these times, you can activate the bsd_echo option.

setopt bsd_echo
# prints tab and newline characters
echo "\t\n"

unsetopt bsd_echo

# will print the literal "\t\n"
echo "\t\n"

csh_junkie_loops

The csh_junkie_loops option offers a more terse syntax for looping over lists or arrays. Activating this option lets you use loops without requiring the do keyword before executing commands on each item in the loop. Additionally, the csh_junkie_loops option allows you to swap the done keyword for end, which saves a keystroke.

While this may not seem like a huge improvement, the shortened syntax can help you interact with your shell more quickly over time by eliminating the overhead of remembering the keywords necessary to create a syntactically correct loop in zsh.

The example below:

# the for command requires "do" before any actions
# can be performed on the list of items
# a traditional zsh loop
for f in 1 3 4 5; do print $f; done

With the csh_junkie_loops option enabled, you can use a shorter syntax to achieve the same result:

setopt csh_junkie_loops
for f in 1 3 4 5; echo $f; end
$ 1
3
4
5

csh_nullcmd

When clearing the contents of a file in your terminal, you may use a command similar to >file.txt which will redirect an empty string to replace the contents of your file. If you are working with sensitive data, or you want to avoid overwriting files with empty data, you can activate the csh_nullcmd option to prevent file redirections from occurring without a preceding command.

After activating this option, any redirection that does not follow a command will result in a error. For example

  1. setopt csh_nullcmd
  2. # attempting to overwrite file.txt will create an error
  3. >file.txt
zsh: redirection with no command

You can disable this option by entering unsetopt chs_nullcmd in your terminal.

ksh_arrays

In zsh, arrays are One-indexed, meaning that you must use $array[1] to retrieve the first item. This behavior is counterintuitive for developers who are used to using programming languages that include zero-indexed arrays.

Arrays in ksh allow you to use zero-indexing which mimics the behavior of many programming languages. This option also requires that you use curly braces to access array elements, instead of using square brackets, as with the default zsh behavior in which the curly braces are optional.

declare -a kitchen_items=(plates cutlery oven sink)
print ${kitchen_items[1]}
# plates

With the ksh_arrays option set, $kitchen_items[1] references “cutlery” instead of “plates”

setopt ksh_arrays
declare -a kitchen_items=(plates cutlery oven sink)

print ${kitchen_items[1]}
# cutlery

sh_word_split

By default zsh does not split multi-word variables into individual elements like sh and ksh shells. This means that a variable containing three words separated with spaces will be treated as a single item, rather than three individual words.

kitchen_items="plates cutlery oven sink"

# without sh_word_split
for word in $kitchen_items; do
    print "$word"
end

To force zsh to mimic the behavior of sh by splitting on spaces, use setopt sh_word_split

  1. setopt sh_word_split

Now using let’s print the contents of $kitchen_items using slightly different syntax:


kitchen_items="plates cutlery oven sink"
for word in $kitchen_items[@]; do
    print "$word"
end
# the loop above will print the following text:
# plates
# cutlery
# oven
# sink

The examples above can be used on-the-fly in your shell, or for more permanent usage of setopt you can add any of the above setopt commands to your .zshrc file to make these options available in any interactive shell session.

However, if you find yourself in need of a different way to mimic the behavior of other shells, let’s take a look at the emulate command.

2. Using the emulate Command

To emulate another shell in zsh, you can use the emulate builtin command. For example, to emulate the Bash shell, you can use the following command:

emulate bash

This will enable zsh to behave like the Bash shell in most respects. You can then run Bash scripts and commands in zsh, and they should work as expected.

To emulate other shells, such as the Bourne or Korn shells, you can use the corresponding name in place of bash in the emulate command. For example, to emulate the Korn shell, you can use the following command:

emulate ksh

If you have a command that you would like use in a sh emulation mode, you can use

emulate sh

Once set, you can print the current emulation mode by typing emulate in your terminal.

The emulate command also allows you to apply “sticky” emulation to functions by using the -c option. This means the emulated command will be in scope for any command that executes later in this function, after the emulate command is executed. Whenever the function is executed this emulation will be active.

function hello_world_sh () {
    emulate sh -c  "print hello world";
}

The emulate sh -c command can be useful for creating functions that require POSIX compatibility while still taking advantage of the zsh environment.

Editing on the Fly

During your daily work you may need to edit many kinds of files - configuration for the app you’re building or if you’re only checking an item off a to-do list. Being able to edit these files quickly is a benefit to working in the terminal in general, and is especially useful when using a shell as customizable as zsh.

There are several tools available for zsh that will allow you to edit various aspects of your environment within zsh seamlessly. Starting with the core of the interactive terminal, the zsh Line Editor.

zsh Line Editor

The zsh Line Editor, or ZLE is the editor that you use with zsh to enter and modify commands on the command line in your terminal. All shells have a similar line editor, and in most cases you won’t need to modify its behavior.

ZLE activates automatically when zsh is loaded, but it also exists as a zsh module called zsh/ZLE.

In zsh, the ZLE (zsh Line Editor) is the built-in line editing interface that allows you to edit the command line and navigate the command history. ZLE provides a number of keybindings and functions that you can use to interact with the command line, such as moving the cursor, deleting text, and accessing the command history.

If you are using zsh in an interactive shell, ZLE is enabled by default since it is the core of the interactive functionality of zsh. You can use the following command to verify the zle option is set:

# This command will print "ZLE: on" if the zsh Line Editor is enabled.
[[ -o zle ]] && echo "zle: on"

The main use for directly interacting with ZLE is through setting your default keybindings for your interactive shell environment. These options can allow you to use keybindings specific to your preferred editor, either emacs or vi. The zsh users guide provides a detailed explanation of the differences between these two editing modes.

If you prefer the emacs keybindings, enter the following command in your terminal.

bindkey -e

To enable vi keybindings, use the following command:

bindkey -v

Now that you learned a little bit about the zsh Line Editor, let’s see how you can use it with the zed editor.

Zed

Zed uses ZLE to let you edit scripts and functions on the command line without having to fire up your editor. This is great for quick edits as you navigate your file system, but can also be used as a basic editor for any general editing command. Zed exists as a short script so you won’t find the same bells and whistles as a full-featured editor, however simplicity is what makes zed and effective tool.

To get started, type zed in your terminal to check it is installed on your system.

zed

If the zed command is not available, you can grab the code from https://github.com/zsh-users/zsh/blob/master/Functions/Misc/zed and save it as an executable file, or wrap it in a function to be sourced.

Once zed is set up in your environment, typing zed in your terminal will create a new , where you can edit the command line as needed. When you save and exit the editor, the edited command line will be executed by zsh.

To edit a specific function in your zsh environment, you can use zed -f like so

zed -f precmd

Note that the changes made to your function with zed will only exist until you reload your configuration files via source .zshrc or similar command.

Vared

vared allows you to modify the value of a variable using your preferred text editor, rather than using the set or export commands to set the value manually on the command line. To use the vared command, you first need to specify the name of the variable that you want to edit. For example, to edit the value of your EDITOR environment variable, you can use the following command:

vared EDITOR

This will open the value of the EDITOR environment variable using ZLE, where you can edit the value as needed. When you are finished editing your variable you can use ctrl-c to save the changes and return to the command line.

Editing Directory Names with Vidir

Vidir allows you to edit the names of directories (or files) in your default editor. This is useful for quickly modifying directory names visually, rather than relying on pattern matching or loops. For example, to edit the names of all items in your $HOME directory, you can use the following command

vidir "$HOME"

Vidir will create a temporary file containing the names of each item in your $HOME directory. After you make your changes and save the file, vidir will update the name of any file or folder you modified.

You can also use Vidir to change the name of a single file name by passing your chosen filename to vidir as an argument instead of a directory. For example

vidir /path/to/file.txt

Will open file.txt in your default editor, where you can modify the filename as you please.

Using Editors in Pipelines with Vipe

It is sometimes useful to modify the contents of a pipe in your default editor, rather than rely on tools like awk or sed to perform actions on text within a pipeline. The vipe command can be inserted into any pipe and will allow you to edit the result of the previous command visually. This is especially useful for times when you aren’t sure what the result of the command will look like, and therefore you are unable to apply a specific command.

The following example uses vipe to modify the contents of the preceding curl command, and then saves the result to a file called example_modified.txt

curl "https://example.com" | vipe > example_modified.txt

Regular Expressions

zsh offers an extensive set of glob operators, a form of expansion, that can be overwhelming if you are coming from a more basic shell. Luckily zsh offers the ability to use regular expressions for pattern matching throughout your shell, which may be a better option if you find globbing to be confusing or unintuitive.

I tend to turn globbing off entirely in my environment by using the noglob shell option, but this is not required:

setopt noglob

Regex in Test Blocks

zsh provides basic Regular expression matching via the =~ comparison operator. This allows you to use regular expressions to match text when executing if blocks. For example, let’s check if $variable contains the text “txt” or “zsh”:

variable="file.txt"

if [[ "$variable" =~ (txt|zsh) ]];
    then
        print "variable contains txt or zsh";
    else
        print "variable does not contain txt or zsh"
fi

Using zsh/Regex

The zsh/regex module allows you to use a flag to enable regular expressions in test conditions. This flag uses POSIX extended regular expressions to match portions of text.

To load zsh modules into your environment, use the zmodload command:

zmodload zsh/regex

Once loaded, you can use the -regex-match flag in test blocks to apply regular expressions. For example, you can shorten the if block from the example above to a test one line command. When a match is found, the matching portion of the string is added to the environment variable $MATCH.

zmodload zsh/regex

test "txt" -regex-match (txt|zsh) && echo "contains txt or zsh" || print "does not contain txt or zsh"

And you can check for the specific match by printing the MATCH environment variable

  1. print $MATCH
  2. txt

Using zsh/PCRE

As with zsh/regex you can load the zsh/prce module via the zmodload command:

# note: pcre must be lowercase for the module to load
zmodload zsh/pcre

The zsh/pcre comes with three commands: pcre_compile, pcre_study, and pcre_match.

  • pcre_compile Compiles a perl-compatible regular expression.
    • The options for this command match the options for PCREs
  • pcre_match A standalone command to match strings against PCREs. Usage
pcre_compile -m "bark$"
string="dog says bark"
pcre_match -b -- $string
print $MATCH

Additionally, zsh/pcre offers a test condition called -pcre-match, which matches against strings, similar to the -regex-match flag available in the zsh/regex module mentioned above. The following code uses the -pcre-match flag to match a string against a perl-compatible regular expression.

test "txt" -regex-match (txt|zsh) && print "contains txt or zsh" || print "does not contain txt or zsh"

Respond to Events with Hooks

zsh Hooks are special functions that are automatically executed at specific points in the shell’s execution, such as when a command is entered at the prompt or when the shell starts up. Hooks allow you to perform certain actions or tasks at these points, such as modifying the command before it is executed or displaying a message to the user.

chpwd

In zsh, the chpwd function is a special function that is automatically executed whenever the current working directory is changed. This allows you to perform certain actions or tasks whenever the user changes directories.

To use the chpwd function, you first need to define it in your .zshrc file. To define the chpwd function, add the following lines to your configuration file:

chpwd () {
  # Your commands and actions go here
}

You could use the chpwd function to update the directory in the command prompt, or to display a message indicating the new working directory.

Once you have defined the chpwd function in your configuration file, it will be automatically executed whenever the current working directory is changed. You can test it by changing directories in your zsh session and observing the effects of the chpwd function.

Periodic

Writing a single function called periodic will run the containing commands every n seconds, where the seconds are stored in the environment variable $PERIOD. For example, say you want to run a function that downloads my bookmarks every six hours. To do this, you can use the following code.

The time is set using the environment variable $PERIOD to set the frequency in which the function runs.

periodic() {

}

precmd

This does not run when the terminal is redrawn. The redrawing happens when a background process sends a notification to your current prompt.

For a more concrete example, my precmd function contains a command that stores the current directory in a file. This allows me to open a new terminal in the most recently used directory. This is especially useful if you are testing your environment and have to restart your terminal to see the results, or if your terminal accidentally crashes while working on an important project.

precmd () {
  pwd >"$HOME/.zsh_reload_directory.txt" &
}

This appends the current directory to a file called .zsh_reload.txt in my $HOME directory. To automatically navigate to this directory when your terminal loads, add the following command elsewhere in your .zshrc file

# create the precmd function
precmd () {
  pwd >|"$HOME/.zsh_reload_directory.txt" &
}

# add the navigation command outside of the precmd function
cd "$HOME/.zsh_reload_directory.txt"

preexec

Executed after the command is read, but before the command prompt is displayed.

Typical use for the precmd function is to set the $PS1 command prompt variable with useful information, such as the time of your last executed command, however you are not limited to updating your prompt.

zshaddhistory

zshaddhistory is a special function that is automatically executed whenever a command is entered at the prompt. This allows you to perform certain actions or tasks whenever a command is entered, such as logging the command or modifying the command before it is executed.

To use zshaddhistory you can define it as a function in your .zshrc file:

zshaddhistory () {
  # Your commands and actions go here
}

You could use the zshaddhistory function to log the command to a file, or to modify the command before it is executed.

Once you have defined the zshaddhistory function in your configuration file, it will be automatically executed whenever a command is entered at the prompt. You can test it by entering a command at the prompt and observing the effects of the zshaddhistory function.

Adding New Hooks

To use the add-zsh-hook function, you first need to define a custom hook function that contains the commands and actions that you want to execute at the specified point. For example, you could define a hook function that logs the commands entered at the prompt, like this:

log_commands () {
  print "$(date): $1" >> ~/.zsh_command_log
}

Once you have defined your custom hook function, you can use the add-zsh-hook function to attach it to an existing zsh hook. For example, to attach the log_commands function to the zshaddhistory hook, which is executed whenever a command is entered at the prompt, you can use the following command:

add-zsh-hook zshaddhistory log_commands

This will cause the log_commands function to be executed whenever a command is entered at the prompt, and the command will be logged to the ~/.zsh_command_log file.

hooks-define-hook

In zsh, the hooks-define-hook function is used to define a custom hook. zsh hooks are special functions that are automatically executed at specific points in the shell’s execution, such as when a command is entered at the prompt or when the shell starts up. Defining a custom hook allows you to create your own points in the shell’s execution at which to execute your own commands or actions.

To use the hooks-define-hook function, you first need to decide when and where you want your custom hook to be executed. This will determine the name and arguments of the hook. For example, if you want your hook to be executed whenever a command is entered at the prompt, you could use the zshaddhistory hook, which is executed with the command as its only argument.

Once you have decided on the name and arguments of your hook, you can define the hook using the hooks-define-hook function. For example, to define a hook that is executed whenever a command is entered at the prompt and logs the command to a file, you could use the following command:

hooks-define-hook zshaddhistory log_command

This will define a hook named zshaddhistory that takes a single argument (the command entered at the prompt).

Once you have defined the hook, you can use the add-zsh-hook function to attach a function to the hook. This function will be executed whenever the hook is triggered.

Conclusion

In this article you learned about many ways you can customize your workflow with the Z shell by using editors on the fly, applying regex to match variables and other text, and setting up hooks in your environment to automatically respond to events in your shell. This article only touches on a portion of the customization options available through zsh - for more information about the shell visit the official documentation at https://zsh.sourceforge.io/.

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