Monday, October 11, 2010

parameters and options

In BASH shell script:
$0 : the program itself
$1 : the first parameter
$2 : the second parameter
A string enclosed in single or double quotes will be passed as a single parameter, and the quotes will be stripped.
$# : the number of total parameters, not including the program itself
$@ : the array of parameters. Double-quoted "$@" is equal to "$1", "$2", "$3".... This is the most common usage. But there are other variations, like $*, "$*", $@, or "$@". They can be explained distinctly. For more detail, see examples here.

For options, using
getopt optstring optname

where optstring is a double-quote string of option letters, like ":p:q". A colon (:) after an option letter indicates that the option
requires a value; The leading colon tells getopts to be silent and suppress the
normal error messages.

The second parameter, optname, is the name of a variable which will receive the name of the option found. If an option is expected to have a value, the value, if present, will be placed in the variable OPTARG. In silent mode, either of the following two error conditions may occur.

  1. If an unrecognized option is found, then optname will contain a ? and OPTARG will contain the unknown option.
  2. If an options that requires a value is found but the value is not, then optname will contain a : and OPTARG will contain the name of the option whose argument is missing.
For example, getopt ":p:q" optnames
will require an option (-p) with value, and -q without having value.


echo "OPTIND starts at $OPTIND"
while getopts ":pq:" optname
case "$optname" in
echo "Option $optname is specified, with value"
echo "Option $optname has value $OPTARG"
echo "Unknown option $OPTARG"
echo "No argument value for option $OPTARG"
# Should not occur
echo "Unknown error while processing options"
echo "OPTIND is now $OPTIND"

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