====== getopts =====

  * [[bash]]
  * [[getopt]]

  * [[http://wiki.bash-hackers.org/howto/getopts_tutorial|Small getopts tutorial]]

''getopts'' is a built-in bash feature.  Comparatively, ''[[getopt]]'' is a separate binary.

==== getopts Syntax ====

<code bash>
getopts optstring name [args]
</code>

==== Setting options ====

Use the ''optstring'' variable to set what arguments can be accepted by the program.  ''getopts'' uses short argument names only (-a, not --arg).

Here's a sample that accepts ''-a'', ''-b'' and ''-c'' as **optional** arguments.  Getopts does not support required arguments.

Strings for passed arguments must occur after a whitespace.

<code bash>
getopts abc opts
</code>

To set an argument as **required**, add a colon to the end of the argument:

<code bash>
getopts ab:c
</code>

==== Invalid Options ====

If an invalid option is passed, getopts will throw an error by default:

<code>
sample.sh -d
sample.sh: illegal option -- d
</code>

When an invalid option is passed, the ''name'' variable is set to ''?'', allowing you to manage the user input.  However, bash will still dump its own error as well.

<code bash>
case ?)
  echo "Invalid argument";;
</code>

There are two ways to silence errors from bash.  One is to set ''OPTERR'' variable to zero, and the other is to add a '':'' to the front of the options variable, before any letters.

Valid syntax for both:

<code>
OPTERR=0
opts=:abc
</code>

Best practice would be to silence the errors using '':abc'' syntax.  This allows the invalid option passed to be set as ''OPTARG'', so that the script can see what option the user passed.

<code bash>
case ?)
  echo "Invalid option: -$OPTARG";;
</code>


==== Option Arguments ====

Arguments following options must have a whitespace between them.

To toggle an option as requiring an argument, add a '':'' to the letter in the options variable.

Require ''-c'' option to have an argument, when used.

<code bash>
getopts abc: opt
</code>

The argument passed to the option is set as ''OPTARG''.

<code bash>
c)
  echo "Passed -c $OPTARG"
</code>

=== Multiple Arguments ===

There are two ways to pass multiple arguments to an option: enclose the argument in quotes, or pass the option more than once and build on the old variable.

** Quoted argument **

<code bash>
bkup.mysql -d "mysql client"
...
d)
  DBS=$OPTARG
</code>

** Multiple options **

<code bash>
bkup.mysql -d mysql -d client
...
d)
  DBS="$DBS $OPTARG"
</code>

==== Missing Argument ====

If error reporting is silenced using '':'' and an option is passed with an argument, the ''name'' variable is set to '':'' and the script can do specific actions based on that input as well.

<code bash>
bkup.mysql -d 
...
getopts abcd:
...
:)
  echo "Option -$OPTARG passed, missing an argument";;
</code>
==== Example ====

<code bash>
#!/bin/bash
 
while getopts ":a:" opt; do
  case $opt in
    a)
      echo "-a was triggered, Parameter: $OPTARG" >&2
      ;;
    \?)
      echo "Invalid option: -$OPTARG" >&2
      exit 1
      ;;
    :)
      echo "Option -$OPTARG requires an argument." >&2
      exit 1
      ;;
  esac
done
</code>