3.7.1 Simple Command Expansion
When a simple command is executed, the shell performs the following
expansions, assignments, and redirections, from left to right.
The words that the parser has marked as variable assignments (those
preceding the command name) and redirections are saved for later
The words that are not variable assignments or redirections are
expanded (see section 3.5 Shell Expansions).
If any words remain after expansion, the first word
is taken to be the name of the command and the remaining words are
Redirections are performed as described above (see section 3.6 Redirections).
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.