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(bashref.info)Simple Command Expansion

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Simple Command Expansion
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   When a simple command is executed, the shell performs the following
expansions, assignments, and redirections, from left to right.

  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 (Note: 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 the arguments.

  3. Redirections are performed as described above (Note:
     Redirections).

  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.