This Zsh function, dedup_env_var, is designed to remove duplicate entries from a colon-separated environment variable, such as $PATH.

dedup_env_var.zsh

#!/bin/zsh

#
# Function: Removes duplicate, colon-separated entries from a specified
#           environment variable.
# Usage:    dedup_env_var <VARIABLE_NAME>
# Example:  dedup_env_var PATH
#
dedup_env_var () {
    # Store the first argument (the environment variable's name) in a local variable.
    local var_name="$1"

    # Check if the target environment variable is unset or empty.
    # (P)var_name is Zsh's parameter indirection, which gets the value of the
    # variable whose name is stored in the var_name variable.
    if [ -z "${(P)var_name}" ]; then
        # Print the error message to standard error (stderr).
        echo "Error: Environment variable '$var_name' is not set." >&2
        return 1
    fi

    # Declare a local (-a for array) array with unique (-U) elements.
    # This is a more explicit way of writing 'typeset -U array'.
    local -aU array

    # This is the updated, robust line:
    # 1. `${(P)var_name}`: Gets the content of the environment variable (e.g., the value of $PATH).
    # 2. `(@s/:/)`: Splits the string by colons. The `@` flag ensures proper array
    #    treatment and preserves empty fields.
    # 3. `("...")`: The outer double quotes prevent word splitting, ensuring that
    #    paths with spaces are treated as a single, complete element.
    # 4. Because 'array' was declared as unique (-U), duplicates are automatically
    #    removed during this assignment.
    array=("${(@s/:/)${(P)var_name}}")

    # `(j/:/)` joins the array elements back into a single string, separated by colons.
    # Finally, export the cleaned-up string, overwriting the original environment variable.
    export "$var_name"="${(j/:/)array}"
}

Of course. Here is an explanation of the Zsh function in English.

This Zsh function, dedup_env_var, is designed to remove duplicate entries from a colon-separated environment variable, such as $PATH.

It's a useful utility for cleaning up variables that may have accumulated redundant paths after sourcing multiple configuration files.

Line-by-Line Code Analysis
Here is a breakdown of what each line in the function does:

Bash

dedup_env_var () {
# Declares a local variable 'var_name' and assigns it the value of the
# first argument passed to the function.
# For example, if you run dedup_env_var PATH, then $var_name becomes "PATH".
local var_name="$1"

# Declares a local array variable named 'array'.
local array

# This 'if' block checks if the target environment variable is empty or not set.
# `${(P)var_name}` is a Zsh-specific feature called "parameter indirection".
# It takes the value of var_name (e.g., "PATH") and uses it as the name of
# another variable to expand (effectively becoming $PATH).
# `[ -z "..." ]` tests if the resulting string is empty.
if [ -z "${(P)var_name}" ]
then
    # If the variable is empty, it prints an error and exits with a non-zero status.
    echo "Environment variable '$var_name' is not set."
    return 1
fi

# This is the core of the de-duplication logic.
# `typeset -U array` declares 'array' as an array that holds only unique elements.
# Any duplicate values assigned to it will be automatically discarded.
typeset -U array

# This line populates the array.
# 1. `(P)var_name`: Gets the value of the environment variable (e.g., the content of $PATH).
# 2. `(s/:/)`: This is a Zsh expansion flag that splits the string by a delimiter,
#    in this case, the colon ':', creating an array of paths.
# 3. `array=(...)`: The resulting array of paths is assigned to the `array` variable.
#    Because `array` was declared as unique (`-U`), this step automatically removes all duplicates.
array=(${(s/:/)Pvar_name})

# This line rebuilds the environment variable from the cleaned-up array.
# 1. `${(j/:/)array}`: This joins the elements of the `array` back into a single
#    string, using a colon ':' as the separator between them.
# 2. `export "$var_name"=...`: Using parameter indirection again, it assigns the
#    newly joined, de-duplicated string back to the original environment variable
#    (e.g., `PATH=...`) and exports it, making it available to subprocesses.
export "$var_name"="${(j/:/)array}"

}
Explanation of the Commands in the Screenshot
which dedup_env_var: The user runs which to find the location of the dedup_env_var command. Since it is a shell function and not an executable file in the system's PATH, the shell responds by printing the function's source code.

dedup_env_var PATH: The user then calls the function, passing PATH as the argument. This triggers the function to:

Read the current value of the $PATH environment variable.

Split it into an array, removing any duplicate path entries in the process.

Join the unique paths back together into a single, colon-delimited string.

Export the cleaned-up string as the new value for the $PATH variable.

Summary
In essence, dedup_env_var is an elegant and efficient Zsh script that leverages Zsh's advanced parameter expansion and unique array features to clean up environment variables.