Shell POSIX – Field Splitting and Filename Expansion in RHS of Variable Assignment

1. Yes, they are the same.
2. Yes.
3. In bash, and similar shells, you could do something like IFS=$' \t\n'. Otherwise, you could insert the literal control codes by using [space] CTRL+V [tab] CTRL+V [enter]. If you are planning to do this, however, it's better to use another variable to temporarily store the old IFS value, and then restore it afterwards (or temporarily override it for one command by using the var=foo command syntax).
4. • The first code snippet will put the entire line read, verbatim, into $line, as there are no field separators to perform word splitting for. Bear in mind however that since many shells use cstrings to store strings, the first instance of a NUL may still cause the appearance of it being prematurely terminated.
   • The second code snippet may not put an exact copy of the input into $line. For example, if there are multiple consecutive field separators, they will be made into a single instance of the first element. This is often recognised as loss of surrounding whitespace.
   • The third code snippet will do the same as the second, except it will only split on a space (not the usual space, tab, or newline).

As long as you're using a POSIX compliant shell, yes.

From the POSIX definition of shell command language: (relevant points in bold)

A "simple command" is a sequence of optional variable assignments and redirections, in any sequence, optionally followed by words and redirections, terminated by a control operator.

When a given simple command is required to be executed (that is, when any conditional construct such as an AND-OR list or a case statement has not bypassed the simple command), the following expansions, assignments, and redirections shall all be performed from the beginning of the command text to the end:

1. The words that are recognized as variable assignments or redirections according to Shell Grammar Rules are saved for processing in steps 3 and 4.

2. The words that are not variable assignments or redirections shall be expanded. If any fields remain following their expansion, the first field shall be considered the command name and remaining fields are the arguments for the command.

3. Redirections shall be performed as described in Redirection.

4. Each variable assignment shall be expanded for tilde expansion, parameter expansion, command substitution, arithmetic expansion, and quote removal prior to assigning the value.

In the preceding list, the order of steps 3 and 4 may be reversed for the processing of special built-in utilities; see Special Built-In Utilities.

If no command name results, variable assignments shall affect the current execution environment. Otherwise, the variable assignments shall be exported for the execution environment of the command and shall not affect the current execution environment (except for special built-ins).

Also, yes #!/bin/sh matters. From the POSIX definition of sh:

The sh utility is a command language interpreter that shall execute commands read from a command line string, the standard input, or a specified file. The application shall ensure that the commands to be executed are expressed in the language described in Shell Command Language.

So basically it says that sh must follow the rules we covered above.

So as long as you're on a POSIX compliant OS, you're good.