Advanced Bash−Scripting Guide

Advanced Bash−Scripting Guide
declare, typeset
The declare and typeset commands specify and/or restrict properties of variables.
readonly
Same as declare −r, sets a variable as read−only, or, in effect, as a constant. Attempts to change the
variable fail with an error message. This is the shell analog of the C language const type qualifier.
getopts
This powerful tool parses command−line arguments passed to the script. This is the Bash analog of
the getopt external command and the getopt library function familiar to C programmers. It permits
passing and concatenating multiple options [25] and associated arguments to a script (for example
scriptname −abc −e /usr/local).
The getopts construct uses two implicit variables. $OPTIND is the argument pointer (OPTion INDex)
and $OPTARG (OPTion ARGument) the (optional) argument attached to an option. A colon following
the option name in the declaration tags that option as having an associated argument.
A getopts construct usually comes packaged in a while loop, which processes the options and
arguments one at a time, then decrements the implicit $OPTIND variable to step to the next.
1. The arguments passed from the command line to the script must be preceded
by a minus (−) or a plus (+). It is the prefixed − or + that lets getopts
recognize command−line arguments as options. In fact, getopts will not
process arguments without the prefixed − or +, and will terminate option
processing at the first argument encountered lacking them.
2. The getopts template differs slightly from the standard while loop, in that it
lacks condition brackets.
3. The getopts construct replaces the obsolete and less powerful getopt external
command.
while getopts ":abcde:fg" Option
# Initial declaration.
# a, b, c, d, e, f, and g are the options (flags) expected.
# The : after option 'e' shows it will have an argument passed with it.
do
case $Option in
a ) # Do something with variable 'a'.
b ) # Do something with variable 'b'.
...
e) # Do something with 'e', and also with $OPTARG,
# which is the associated argument passed with option 'e'.
...
g ) # Do something with variable 'g'.
esac
done
shift $(($OPTIND − 1))
# Move argument pointer to next.
# All this is not nearly as complicated as it looks .
Example 11−17. Using getopts to read the options/arguments passed to a script
Chapter 11. Internal Commands and Builtins 138