3.2.3 Lists of Commands

A list is a sequence of one or more pipelines separated by one of the operators ‘;’, ‘&’, ‘&&’, or ‘||’, and optionally terminated by one of ‘;’, ‘&’, or a newline.

Of these list operators, ‘&&’ and ‘||’ have equal precedence, followed by ‘;’ and ‘&’, which have equal precedence.

A sequence of one or more newlines may appear in a list to delimit commands, equivalent to a semicolon.

If a command is terminated by the control operator ‘&’, the shell executes the command asynchronously in a subshell. This is known as executing the command in the background. The shell does not wait for the command to finish, and the return status is 0 (true). When job control is not active (see Job Control), the standard input for asynchronous commands, in the absence of any explicit redirections, is redirected from /dev/null.

Commands separated by a ‘;’ are executed sequentially; the shell waits for each command to terminate in turn. The return status is the exit status of the last command executed.

The control operators ‘&&’ and ‘||’ denote and lists and or lists, respectively. An and list has the form

     command1 && command2

command2 is executed if, and only if, command1 returns an exit status of zero.

An or list has the form

     command1 || command2

command2 is executed if, and only if, command1 returns a non-zero exit status.

The return status of and and or lists is the exit status of the last command executed in the list.