Bash Script Default Parameter Demo

args_error.sh

#!/bin/bash
# Triggers an error of params are empty.
username=${1:?"You must specify a username"}
password=${2:?"You must specify a password"}
echo "Setting password for ${username} to ${password}"
exit 0;

error2.sh

#!/bin/bash
# One-liner to get the path to a binary, or exit with an error
ifconfig=${IFCONFIG:-$(which ifconfig)} || (echo "No binary found for ifconfig";  exit 1) || exit 1;

example1.sh

#!/bin/bash
# Examples of getting default variable values in Bash
value=${VARIABLE_NAME:-"Default value"}
username=${username:-"Guest"}
greeting=${GREETING:-Hello, ${username}}
echo "$greeting"; # 'Hello, Guest!'

example2.sh

#!/bin/bash
# change_mac.sh
# Allow the user to specify the interface, but default to wlan0
iface=${1:-"wlan0"}

echo "Enter iface name (default '${iface}'):"
read inpt
# If they entered something, use that instead
# (input validation omitted)
if [ ! -z "$inpt" ]; then
    iface="$inpt"
fi

# Pass-through for ifconfig command
# Params:
#  $1 - iface - The interface to use. Defaults to $iface.
#  $2 - command - The command to run
function ifctrl() {
    _iface=${1:-"$iface"}
    _cmd=${2:-""}
    if [ -z "$_cmd" ]; then
        echo -e "\033[91m [!] Specify a command\033[0m"
        exit 1;
    fi
    echo "      [cmd] ifconfig {$_iface} {$_cmd}"
    echo $(ifconfig "$_iface" "$_cmd")
}	

# Prints a message to the screen
# Params:
#  $1 - message - The string to display
#  $2 - color - An ANSI color code
function printmsg() {
    _msg=${1:-""}
    _color=${2:-"92"}
    echo -e "\033[${_color}m {$_msg}\033[0m"
}

# Puts the interface up
# Params:
#   $1 - iface - The interface. Defaults to $iface
function ifup() {
    _iface=${1:-"$iface"}
    printmsg " [+] Putting {$_iface} up..."
    echo $(ifctrl "$_iface" up)
}

# Puts the interface down
# Params:
#   $1 - iface - The interface. Defaults to $iface
function ifdown() {
    _iface=${1:-"$iface"}
    printmsg " [+] Putting {$_iface} down..."
    ifctrl "$_iface" down
}
# Sets the MAC address.
# Params:
#   $1 - iface - The interface. Defaults to $iface
#   $2 - mac - The MAC address to use. Defaults to random. 
function set_mac() {
    _iface=${1:-"$iface"}
    _mac=${2:-"random"}
    if [ "$_mac" == "random" ]; then
        printmsg " [+] Generating random MAC address"
    	## Generate random mac
    fi
    printmsg " [+] Setting MAC to: {$_mac}"
    echo $(ifctrl "$_ifaece" "hw ether {$_mac}")
}

example3.sh

#!/bin/bash
# Generates an htpasswd entry for a user.
# Args:
#  $1 - username (default: current user)
#  $2 - password (default: random)

username=${1:-$(whoami)} ## Replace with command output
passwd=${2:-$(openssl rand -base64 8)} ## Replace with command output

echo "Hello, ${username:-New User}" ## String replacement inside a string
echo "Creating htpasswd entry for ${username}"
result=$(${htpasswd:-$(which htpasswd)} -nb ${username} ${passwd})

if [ "$2" != "$passwd" ]; then
    echo "No password specified!"
    echo "Generated random password: ${passwd}"
fi
echo "Add the following to htpasswd:"
echo "${result}"
exit 0

example4.sh

#!/bin/bash
# filename: test.sh
# Generates an htpasswd entry for a user
# usage: ./test.sh
# If no args are specified, will create a random password for your current user.
# Args:
#  $1 - username (default: current user)
#  $2 - password (default: random)
# ENV vars:
#  $HTPASSWD - Path to htpasswd binary
#  $OPENSSL - Path to openssl binary
#  $MIN_PWD_LEN - Min password length (default 8)

htpasswd=${HTPASSWD:-$(which htpasswd)}
openssl=${OPENSSL:-$(which openssl)}

username=${1:-$(whoami)}
password=${2:-$(openssl rand -base64 ${MIN_PWD_LEN:-8})} ## Nested statements work too

echo $($htpasswd -snb $username $password);

if [ -z "$2" ]; then
    echo -e "Generated random password: \033[1m${password}\033[0m";
fi
exit 0;

example5.sh

#!/bin/bash
# Generates a random password with openssl. 
# Args:
#     $1 - username (default: current user)
#     $2 - password (default: random password)
# ENV vars:
#     $HTPASSWD -- The location to the 'htpasswd' binary (run "sudo apt-get install apache2-utils" if missing)
#     $OPENSSL -- The location to the 'openssl' binary 
#     $MIN_PWD_LEN -- The minimum random password length (default 8)

command="${HTPASSWD:-$(which htpasswd)} -nb ${1:-$(whoami)} ${2-$(${OPENSSL:-$(which openssl)} rand -base64 ${MIN_PWD_LEN-8})}";
[ ! -z "$2" ] || echo -e "Generated random password: \033[1m$(echo $command | cut -d ' ' -f4)\033[0m";
echo "Command: \033[1m${command}\033[0m";
exit 0;