System information

An Introduction to Shells in General
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Revision 1.02 October 2002 69
while [ expression ];
do
statements
done
The until statement
The until statement is very similar in syntax and function to the while statement. The only
real difference between the two is that the until statement executes its code block while its
conditional expression is false, and the while statement executes its code block while its
conditional expression is true. The syntax for the until statement is:
until [ expression ]
do
commands
done
In practice the until statement is not very useful, because any until statement you write can
also be written as a while statement.
The shift command
The shift command moves the current values stored in the positional parameters to the left one
position. For example, if the values of the current positional parameters are
$1 = -r $2 = file1 $3 = file2
and you executed the shift command
shift
the resulting positional parameters would be:
$1 = file1 $2 = file2
You can also move the positional parameters over more than 1 place, by specifying a number
with the shift command. The following would shift the positional parameters two places:
shift 2
This is a very useful command when you have a shell program that needs to parse command line
options. This is true because options are typically preceded by a hyphen and a letter that
indicates what the option is to be used for. Because options are usually processed in a loop of
some kind, you often want to skip to the next positional parameter once you have
identifiedwhich option should be coming next.