About BASH getopts

getopts is the bash version of another system tool, getopt. Notice that the bash command has an s at the end, to differentiate it from the system command.

Advantages

• It can handle things like -xvf filename in the expected Unix way, automatically.
• It makes sure options are parsed like any standard command (lowest common denominator), avoiding surprises.
• Disadvantages
• It cannot handle options with optional arguments à la GNU.
• Options are coded in at least 2, probably 3 places -- in the call to getopts, in the case statement that processes them, and in the help/usage message that documents them.
• Note that getopts is neither able to parse GNU-style long options (--myoption) nor XF86-style long options (-myoption). So, when you want to parse command line arguments in a

Syntax

getopts optstring optname [ arg ]

Usage

- if a character is followed by a colon, the option is expected to have an argument, which should be separated from it by whitespace. The colon (‘:’) and question mark (‘?’) may not be used as option characters

Note

There are two reserved characters which cannot be used as options: the colon (":") and the question mark ("?").

How it Works

getopts processes the positional parameters of the parent command. In bash, this is stored in the shell variable "$@". So, if you run this command:

mycmd -a argument1 -b argument2

During the time that mycmd is running, the variable $@ contains the string "-a argument1 -b argument2". You can use getopts to parse this string for options and arguments.

Every time you run getopts, it looks for one of the options defined in optstring. If it finds one, it places the option letter in a variable named optname. If the option does not match those defined in optstring, getopts sets variable optname to a question mark ("?").

If the option is expecting an argument, getopts gets that argument, and places it in $OPTARG. If an expected argument is not found, the variable optname is set to a colon (":"). Getopts then increments the positional index, $OPTIND, that indicates the next option to be processed.

The special option of two dashes ("--") is interpreted by getopts as the end of options.

getopts is designed to run multiple times in your script, in a loop, for example. It processes one option per loop iteration. When there are no more options to be processed, getopts returns false, which automatically terminates a while loop. For this reason, getopts and while are frequently used together.

Specifying the optstring

optstring is a string which defines what options and arguments getopts look for. For instance, in this call to getopts:
getopts "apZ" optname
The options expected by getopts are -a, -p, and -Z, with no arguments. These options can be combined in any order as -aZ, -pa, -Zap, etc.

Let's say that you'd like the -a and -Z options to take arguments. You can specify this by putting a colon (":") after that option in optstring. For example:
getopts "a:pZ:" optname

Now you can specify arguments to the -a and -Z options such as -a argument1 -pZ argument2. The -p option cannot take arguments, because there is no colon after the p in optstring.

Error checking

By default, getopts will report a verbose error if it finds an unknown option or a misplaced argument. It also sets the value of optname to a question mark ("?"). It does not assign a value to $OPTARG.

If the option is valid but an expected argument is not found, optname is set to "?", $OPTARG is unset, and a verbose error message is printed.
Silent error checking

However, if you put a colon at the beginning of the optstring, getopts runs in "silent error checking mode." It will not report any verbose errors about options or arguments, and you need to perform error checking in your script.

In silent mode, if an option is unexpected, getopts sets optname to "?" and $OPTARG to the unknown option character.

If the option is OK but an expected argument is not found, optname is set to a colon (":") and $OPTARG is set to the unknown option character.
Specifying custom arguments
You will usually want getopts to process the arguments in $@, but in some cases, you may want to manually provide arguments for getopts to parse. If so, you can specify these args as the final argument of the getopts command.

Examples

Here is a bash script using getopts. The script prints a greeting, with an optional name, a variable number of times. It takes two possible options: -n NAME and -t TIMES.

removes n strings from the positional parameters list. Thus shift $((OPTIND-1)) removes all the options that have been parsed by getopts from the parameters list, and so after that point, $1 will refer to the first non-option argument passed to the script.
Update

As mikeserv mentions in the comment, shift $((OPTIND-1)) can be unsafe. To prevent unwanted word-splitting etc, all parameter expansions should be double-quoted. So the safe form for the command is

Note that $OPTIND is a shell variable managed by the getopts builtin command of the shell. When the shell starts, $OPTIND equals 1 and is incremented every time a getopts call discoveres options or option arguments.

he exact use of shift $(($OPTIND-1)) is to skip the options parsed and standardized by getopts (i.e. -uvh will become -u -v -h), so by doing so, you will have your first "non-option" argument in the $0 position of the argument array.

shift "$((OPTIND-1))"
getopts is used by shell scripts to parse positional parameters. optstring contains the option characters to be recognized; if a character is followed by a colon, the option is expected to have an argument, which should be separated from it by whitespace. The colon (‘:’) and question mark (‘?’) may not be used as option characters. Each time it is invoked, getopts places the next option in the shell variable name, initializing name if it does not exist, and the index of the next argument to be processed into the variable OPTIND. OPTIND is initialized to 1 each time the shell or a shell script is invoked. When an option requires an argument, getopts places that argument into the variable OPTARG. The shell does not reset OPTIND automatically; it must be manually reset between multiple calls to getopts within the same shell invocation if a new set of parameters is to be used.

When the end of options is encountered, getopts exits with a return value greater than zero. OPTIND is set to the index of the first non-option argument, and name is set to ‘?’.
shift n

removes n strings from the positional parameters list. Thus shift $((OPTIND-1)) removes all the options that have been parsed by getopts from the parameters list, and so after that point, $1 will refer to the first non-option argument passed to the script.

As mikeserv mentions in the comment, shift $((OPTIND-1)) can be unsafe. To prevent unwanted word-splitting etc, all parameter expansions should be double-quoted. So the safe form for the command is
shift "$((OPTIND-1))"

getopts normally parses the positional parameters, but if more arguments are given in args, getopts parses those instead.
OPTIND is initialized to 1 each time the shell or a shell script is invoked. When an option requires an argument, getopts places that argument into the variable OPTARG
While the getopt system tool can vary from system to system, bash getopts is defined by the POSIX standard. So if you write a script using getopts, you can be sure that it runs on any system running bash in POSIX mode (e.g., set -o posix).
getopts parses short options, which are a single dash ("-") and a letter or digit. Examples of short options are -2, -d, and -D. It can also parse short options in combination, for instance -2dD.
However, getopts cannot parse options with long names. If you want options like --verbose or --help, use getopt instead.

Difference with getopt
Never use getopt(1). getopt cannot handle empty arguments strings, or arguments with embedded whitespace. Please forget that it ever existed.

Positional And Optional Argument

Positional And Optional Argument

positional and optional arguments. The positional arguments must occur at the location specified (ex: $1, $2) while the optional arguments can occur at any position denoted by their command line flag