When a simple command is executed, the shell performs the
following expansions, assignments, and redirections, from left to
right, in the following order.
1. The words that the parser has marked as variable
assignments (those preceding the command name) and
redirections are saved for later processing.
2. The words that are not variable assignments or
redirections are expanded. If any words remain after
expansion, the first word is taken to be the name of the
command and the remaining words are the arguments.
3. Redirections are performed as described above under
REDIRECTION.
4. The text after the = in each variable assignment undergoes
tilde expansion, parameter expansion, command
substitution, arithmetic expansion, and quote removal
before being assigned to the variable.
If no command name results, the variable assignments affect the
current shell environment. Otherwise, the variables are added to
the environment of the executed command and do not affect the
current shell environment. If any of the assignments attempts to
assign a value to a readonly variable, an error occurs, and the
command exits with a non-zero status.
If no command name results, redirections are performed, but do
not affect the current shell environment. A redirection error
causes the command to exit with a non-zero status.
If there is a command name left after expansion, execution
proceeds as described below. Otherwise, the command exits. If
one of the expansions contained a command substitution, the exit
status of the command is the exit status of the last command
substitution performed. If there were no command substitutions,
the command exits with a status of zero.
