Shell – Meaning of “${-#*i}” != “$-“

Let's break this down into pieces.

This code runs the command : with some arguments. The command : does nothing and ignores its arguments. Therefore the whole command line does nothing, except whatever side effects happen in the arguments.

The syntax ${parameter_name:=value} exists in all non-antique Bourne-style shells, including ash, bash, ksh and zsh. It sets the parameter to a default if necessary. It is equivalent to

if [ -z "$parameter_name" ]; then parameter_name=value; fi
… ${parameter_name}

In other words, if parameter_name is not set or is set to an empty value, then set it to the indicated value; and then run the command, using the new parameter value. There is a variant, ${parameter_name=value}, which leaves the parameter empty if it was empty, only using the indicated value if the parameter was unset.

You'll find this syntax documented under “parameter expansion” in the POSIX spec, and the dash, bash, ksh and zsh manuals.

There are variations on this syntax, in particular ${parameter_name:-value} which let you use a default value for this expansion only, without assigning to the parameter.

In summary, : ${parameter_name:=value} is a concise way of writing

if [ -z "$parameter_name" ]; then parameter_name=value; fi

Early shells had only a single data type: strings. But it is common to manipulate lists of strings, typically when passing multiple file names as arguments to a program. Another common use case for splitting is when a command outputs a list of results: the command's output is a string, but the desired data is a list of strings. To store a list of file names in a variable, you would put spaces between them. Then a shell script like this

files="foo bar qux"
myprogram $files

called myprogram with three arguments, as the shell split the string $files into words. At the time, spaces in file names were either forbidden or widely considered Not Done.

The Korn shell introduced arrays: you could store a list of strings in a variable. The Korn shell remained compatible with the then-established Bourne shell, so bare variable expansions kept undergoing word splitting, and using arrays required some syntactic overhead. You would write the snippet above

files=(foo bar qux)
myprogram "${files[@]}"

Zsh had arrays from the start, and its author opted for a saner language design at the expense of backward compatibility. In zsh (under the default expansion rules) $var does not perfom word splitting; if you want to store a list of words in a variable, you are meant to use an array; and if you really want word splitting, you can write $=var.

files=(foo bar qux)
myprogram $files

These days, spaces in file names are something you need to cope with, both because many users expect them to work and because many scripts are executed in security-sensitive contexts where an attacker may be in control of file names. So automatic word splitting is often a nuisance; hence my general advice to always use double quotes, i.e. write "$foo", unless you understand why you need word splitting in a particular use case. (Note that bare variable expansions undergo globbing as well.)