Bash – What does “rm is hashed” mean

No, < < and << are not the same thing.

The first is composed of the common < redirection character combined with the first character of the <(command) syntax. This is a ksh construct (also found in bash and zsh) known as process substitution that takes the output of command and provides it in a file whose name refers to the other end of the pipe command is writing to.

In other word you can think of < <(command) as < file, where file contains the output of command.

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