PWB1/usr/man/man1/while.1

.th WHILE I 5/31/77
.sh NAME
while \*- shell iteration command
.sh SYNOPSIS
.bd while
expr
.br
	commands... (may include
.it break
or
.it continue\c
)
.br
.bd end
.sh DESCRIPTION
.it While
evaluates the expression
.it expr,
which is similar to (and a superset of) the expression described in
.it "if(I)."
If the expression is true,
.it while
does nothing, permitting the command(s) on following lines to be
read and executed by the Shell.
If the expression is false, the input file is effectively searched
for the matching
.it end
command, and the Shell resumes execution of the command(s) on the line
following the
.it end.
The
.it while-end
grouping may be nested up to three levels deep.
.s3
In addition to the type of expression permitted by
.it if,
.it while
treats a single, nonnull argument as a true expression,
and treats a single null argument or lack of arguments as a false
expression.
.s3
The
.it break
command terminates the nearest enclosing
.it while-end
group, causing execution to resume after the nearest succeeding
unmatched
.it end.
Exit from
.it n
levels is obtained by writing
.it n
.it break
commands on the same line.
.s3
The
.it continue
command causes execution to resume at a preceding
.it while,
i.e., the
.it while
that begins the smallest loop
containing the
.it continue.
.s3
A common loop is that of processing arguments one at a time: see
.it "shift(I)."
.s3
The following is a shell procedure that is also a filter.
It reads a line at a time from the standard input
that existed when the procedure was invoked, exiting on end-of-file.
.s3
.nf
while 1
	= a <\-\- \*v\*v exit
	commands using $a ...
end
.fi
.sh "SEE ALSO"
goto(I), if(I), onintr(I), sh(I), shift(I), switch(I)
.sh DIAGNOSTICS
.nf
while: missing end
while: >3 levels
while: syntax errors like those of if.
break: missing end
break: used outside loop
continue: used outside loop
end: used outside loop
.fi
.sh BUGS
A
.it goto
may be used to terminate one or more
.it while-end
groupings.
Those who use it to branch into a loop will
receive appropriately peculiar results.
When an interrupt is caught and transfer to a label caused by use of
.it "onintr(I),"
all currently effective
.it while-end
loops are cancelled, i.e., the
.it onintr
performs a
.it goto
that breaks all loops.
Neither
.it while
nor
.it end
may be hidden behind \%semicolons or used within other commands.