How to Prompt for Argument If Not Provided in Command in Bash

The aim of this page📝 is to explain how command line arguments and user prompts work in shell scripts, based on the particular example of my script helping me to create a zettelkasten for VSCode in which I manage my tasks and knowledge.

Pavol Kutaj

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Jan 13, 2024

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• In the provided script, command substitution is used to replace the read and echo commands with the user's input.

make_z4v() {
    # Parameters
    kbType="${1:-$(read -p "Type(s/k): " name && echo "$name")}"
    name="${2:-$(read -p "Name: " name && echo "$name")}"
    extract="${3:-$(read -p "Extract: " extract && echo "$extract")}"
    cat="${4:-$(read -p "Category: " cat && echo "$cat")}"
    open="${5:-$(read -p "Open: " open && echo "$open")}"
    url="${6:-$(read -p "URL: " url && echo "$url")}"
    
    # Get current date
    today=$(date +"%Y-%m-%d")
    #...

• Shell scripts can accept command-line arguments.
• These arguments are accessible inside the script using special variables ($1, $2, etc.).
• The variable $1 refers to the first argument, $2 to the second, and so on.
• In the provided script, several variables (kbType, name, extract, cat, open, URL) are being set based on these arguments.
• If an argument is not provided, the script prompts the user for input.
• This is done using the read command, which reads a line from standard input.
• The -p option allows a prompt string to be specified.
• The entered input is stored in a variable.
• The && operator is used to chain commands together.
• The echo command is used to print the value of the variable.
• The :- operator provides a default value for a variable if it is unset or null.
• This operator is part of parameter expansion in shell scripts.
• Command substitution ($(command)) is used to replace a command with its output.
• Compare that to Powershell from which I am currently switching to bash

function New-Kba {
    [CmdletBinding()]
    param (
        [Parameter(Mandatory = $true)][string]$kbType,
        [Parameter(Mandatory = $true)][string]$name,
        [Parameter(Mandatory = $true)][string]$cat,
        [Parameter(Mandatory = $true)][bool]$open?,
        [Parameter(Mandatory = $true)]$url,
        [Parameter(Mandatory = $true)]$extract
    )