) # carry out action associated with any other selection
.
.
.
;;
esac
|
|
In explaining the large example program, we have so far ignored lines 82 to 91 These contain a structure designed to choose between several different options: a case statement.
81 : case $result in 82 : h|H) help=yes ;; 83 : v) verbose=yes ;; 84 : l) record=yes 85 : log=off 86 : LOG=ON ;; 87 : b) batch=yes ;; 88 : f) file=yes 89 : fname=$OPTARG ;; 90 : *) help=yes ;; 91 : esacThe case command is followed by a variable. This is tested against each of the options in turn, until the esac statement (signifying end of case) is reached.
In addition to setting variables, you can use branches of a case construct to call functions or exit. (An exit statement is used to exit from the current script.)
case statements are not essential to writing scripts that can handle multiway choices, but they make things easier. Consider the following alternative:
if [ ${result} = "h" ]
then
help=TRUE
else
if [ ${result} = "H" ]
then
help=TRUE
else
if [ ${result} = "v" ]
then
verbose=TRUE
else
if [ ${result} = "l" ]
then
record=TRUE
log=off
LOG=ON
else
if [ ${result} = "b" ]
then
batch="yes"
else
if [ ${result} = "f" ]
then
file=TRUE
fname=${OPTARG:-unset}
else
help=TRUE;;
fi
fi
fi
fi
fi
fi
This compound if statement does exactly the same thing as
the earlier case statement, but is much harder to read and debug.
The general format of a case construct is as follows:
case $choice in 1) # carry out action associated with selection 1 . . . ;; 2) # carry out action associated with selection 2 . . . ;; 3) # carry out action associated with selection 3 . . . ;; 4) # carry out action associated with selection 4 . . .The case command evaluates its argument, then selects the matching option from the list and executes the commands between the closing parenthesis following the option and the next double semicolon. In this way, only one out of several possible courses of action can be taken. case tests the argument against its options in order, from top to bottom, and once it has executed the commands associated with an option it skips all the subsequent possibilities and the script continues running on the line after the esac command.
To trap any possible selection use an option like:
The
option matches any possible argument to the
case construct; if no prior option has matched the
argument, the commands associated with the option are
automatically carried out. For this reason, the )
option should be placed at the bottom of the case
construct; if you place it at the top of the construct, the
option will always be executed before the shell has a
chance to check any other options.
There is no effective size limit to a case construct, and unlike an if ... then ... elseif cascade the construct is ``flat''; that is, it is an indivisible structure, and there is consequently no difficulty in working out which construct is being evaluated.