$1

https://stackoverflow.com/questions/3601515/how-to-check-if-a-variable-is-set-in-bash

if [ -n "$1" ]; then
  echo "You supplied the first parameter!"
else
  echo "First parameter not supplied."
fi

See also

• Bash [ -c ], alias, &&, Bash history, Autocompletion, set, unset, export, .bash_aliases, .bashrc and .bash profile, PS1, echo, brace expansion, Function, shopt, trap, set -e, source, PATH, complete, disown, .hushlogin, .bashrc, builtin, eval, set, $PATH, $HOME, TZ, .profile, -e, -i, Builtin (Bash), mapfile, ~, return (Bash), for, until, declare, dirname, basename, functions.sh, Bash arithmetic expansion, shift, bash --rcfile, readonly, $1, $@