MPE/iX Shell and Utilities Reference Manual, Vol 2

sh(1) MPE/iX Shell and Utilities sh(1)

command && command

command || command

command & command

command &

command |&

command ; command

command ;

command <newline>

The following table gives the order of precedence of the preceding operators. Highest priority

operators are listed ﬁrst and operators on the same line have equal priority.

()

&& ||

& |& ; <newline>

The meaning of these operations is as follows:

(command)

executes command in a subshell. This means that the current shell invokes a second

shell, and this second shell actually executes command. In this way, command exe-

cutes in a completely separate execution environment; it can change working direc-

tories, change variables, open ﬁles, and so on without affecting the ﬁrst shell. The

subshell’s environment begins as a copy of the current environment, so the value of

the

ENV

environment variable is not run when a subshell starts.

| creates a pipe between the two commands that the | operator connects. This means

that the standard output of the ﬁrst command becomes the standard input of the sec-

ond command. A series of commands connected by pipes is called a pipeline. The

exit status is that of the last command in the pipeline.

&& is the logical AND operator. The shell executes the second command if and only if

the ﬁrst command returns a true (zero) exit status.

|| This is the logical OR operator. The shell executes the second command if and only

if the ﬁrst command returns a false (non-zero) exit status.

& executes the command that precedes it asynchronously. This means that the shell just

starts the command running and then immediately goes on take new input, before the

command ﬁnishes execution. On systems where asynchronous execution is not pos-

sible, this operation is effectively equivalent to ;.

1-522 Commands and Utilities