Writing Bash scripts that process required and optional arguments has been tricky. The solution I came upon is to use the number of arguments, $#, instead of their existence, -z “$1”.

#!/usr/bin/env bash

# Process 2 required and other optional arguments

if [[ $# -eq 0 ]]            # Both required arguments are missing.
then
    echo "twenty-three: Missing required arguments 1–2, one and two"
    exit 1
fi
if [[ $# -eq 1 ]]            # Second required argument is missing.
then
    echo "twenty-three: Missing required argument 2, two"
    exit 1
fi
required1="$1"
required2="$2"
shift 2                      # Strip off the required arguments on the left.
if [[ $# -ge 1 ]]            # Are there optional arguemnts?
then
    optional="$*"            # Capture remaining arguments in optional.
else
    optional=""              # Set optional to the null string.
fi

echo "twenty-three: required1: '$required1'"
echo "twenty-three: required2: '$required2'"
echo "twenty-three:  optional: '$optional'"

exit 0

Figure 1. Sample Bash script, twenty-three.sh, for processing two required arguments and multiple trailing arguments.

Of course, you can do the same processing within a function.

function f23()
{
if [[ $# -eq 0 ]]            # Both required arguments are missing.
then
    echo "f23: Missing required arguments 1–2, one and two"
    exit 1
fi
if [[ $# -eq 1 ]]            # Second required argument is missing.
then
    echo "f23: Missing required argument 2, two"
    exit 1
fi
local required1="$1"
local required2="$2"
shift 2                      # Strip off the required arguments on the left.
if [[ $# -ge 1 ]]            # Are there optional arguemnts?
then
    local optional="$*"      # Capture remaining arguments in optional.
else
    local optional=""        # Set optional to the null string.
fi

.
.
.

} # f23

Figure 2. Sample Bash function, f23, for processing two required arguments and multiple trailing arguments.