I am running the following code when I have two arguments
if (( $# == 2 )); then : ${fdir:="${@:-1}"} pfm -w2 "" "unspecified -d option" echo "use last argument as substitute" printf '%s\n\n' "fdir: ${@:-1}" echo "\$1: $1 \$2: $2" This is the result
pregion --dyn "John" ./01cuneus pregion --dyn John ./01cuneus unspecified -d option use last argument as substitute fdir: John ./01cuneus $1: John $2: ./01cuneus No, "${@:-1}" doesn't. But "${@: -1}" does.
But note that at least in Bash and Ksh, if there are no positional parameters, then "${@: -1}" gives "$0", i.e. the shell name. This is similar to how using ${@:0} gives $0 and all positional the positional parameters. Zsh doesn't seem to give $0 with ${@: -1} or the cleaner ${@[-1]}.
The gotcha here is that ${var:-value} is a standard expansion which expands to the default value value if var is unset or empty. The substring/array slice expansion ${var:p:n}, of which ${@: -1} is a special case, is not standard and the shell interprets ${@:-1} as the default value expansion on $@. The space removes the ambiguity, and so would putting the index in a variable, e.g. i=-1; echo "${@:i}"
set -- a b c echo "${@: -1}" # prints 'c', the last element echo "${@:-1}" # prints 'a b c' set -- echo "${@: -1}" # prints '/bin/bash' or something like that echo "${@:-1}" # prints '1', the default value given echo "${@:-no args}" # prints 'no args' Also as noted in comments, the expansion in : ${fdir:="${@:-1}"} is unquoted, so it may cause the shell to generate an arbitrarily long list of filenames. That could be prevented with : "${fdir:="${@: -1}"}", or the perhaps more readable [ -z "$fdir"] && fdir="${@: -1}"
: ${fdir:="${@: -1}"}, the quotes are in the wrong place as the expansion would be subject to split+glob (and even when passed to : constitute the kind of DoS vulnerability described at Security implications of forgetting to quote a variable in bash/POSIX shells zsh (or fish), you'd rather use $argv[-1]. Also ${@[-1]} in zsh or yash. ${@:$#}. $0 if there were no positional parameters "${@:$#}" as safer option than "${@: -1}", in case one misses the space.