Small getopts tutorial

Description When you want to parse command line arguments in a professional way, getopts is the tool of choice. Unlike its older brother getopt (note the missing s!), it's a shell builtin command. The advantages are:

  • No need to pass the positional parameters through to an external program.
  • Being a builtin, getopts can set shell variables to use for parsing (impossible for an external process!)
  • There's no need to argue with several getopt implementations which had buggy concepts in the past (whitespace, …)
  • getopts is defined in POSIX®.

Some other methods to parse positional parameters (without getopt(s)) are described in: How to handle positional parameters.

Note that getopts is not able to parse GNU-style long options (--myoption) or XF86-style long options (-myoption)!

Terminology

It's useful to know what we're talking about here, so let's see… Consider the following command line:

mybackup -x -f /etc/mybackup.conf -r ./foo.txt ./bar.txt

These are all positional parameters, but they can be divided into several logical groups:

  • -x is an option (aka flag or switch). It consists of a dash (-) followed by one character.
  • -f is also an option, but this option has an associated option argument (an argument to the option -f): /etc/mybackup.conf. The option argument is usually the argument following the option itself, but that isn't mandatory. Joining the option and option argument into a single argument -f/etc/mybackup.conf is valid.
  • -r depends on the configuration. In this example, -r doesn't take arguments so it's a standalone option like -x.
  • ./foo.txt and ./bar.txt are remaining arguments without any associated options. These are often used as mass-arguments. For example, the filenames specified for cp(1), or arguments that don't need an option to be recognized because of the intended behavior of the program. POSIX® calls them operands. To give you an idea about why getopts is useful, The above command line is equivalent to:

mybackup -xrf /etc/mybackup.conf ./foo.txt ./bar.txt

which is complex to parse without the help of getopts.

The option flags can be upper- and lowercase characters, or digits. It may recognize other characters, but that's not recommended (usability and maybe problems with special characters).

How it works

In general you need to call getopts several times. Each time it will use the next positional parameter and a possible argument, if parsable, and provide it to you. getopts will not change the set of positional parameters. If you want to shift them, it must be done manually:

shift $((OPTIND-1))
# now do something with [email protected]

Since getopts sets an exit status of FALSE when there's nothing left to parse, it's easy to use in a while-loop:

while getopts ...; do
  ...
done

getopts will parse options and their possible arguments. It will stop parsing on the first non-option argument (a string that doesn't begin with a hyphen (-) that isn't an argument for any option in front of it). It will also stop parsing when it sees the -- (double-hyphen), which means end of options.

Used variables

variable description
OPTIND Holds the index to the next argument to be processed. This is how getopts "remembers" its own status between invocations. Also useful to shift the positional parameters after processing with getopts. OPTIND is initially set to 1, and needs to be re-set to 1 if you want to parse anything again with getopts
OPTARG This variable is set to any argument for an option found by getopts. It also contains the option flag of an unknown option.
OPTERR (Values 0 or 1) Indicates if Bash should display error messages generated by the getopts builtin. The value is initialized to 1 on every shell startup - so be sure to always set it to 0 if you don't want to see annoying messages! OPTERR is not specified by POSIX for the getopts builtin utility — only for the C getopt() function in unistd.h (opterr). OPTERR is bash-specific and not supported by shells such as ksh93, mksh, zsh, or dash.

getopts also uses these variables for error reporting (they're set to value-combinations which arent possible in normal operation).

Specify what you want

The base-syntax for getopts is:

getopts OPTSTRING VARNAME [ARGS...]

where:

OPTSTRING tells getopts which options to expect and where to expect arguments (see below)
VARNAME tells getopts which shell-variable to use for option reporting
ARGS tells getopts to parse these optional words instead of the positional parameters

The option-string

The option-string tells getopts which options to expect and which of them must have an argument. The syntax is very simple — every option character is simply named as is, this example-string would tell getopts to look for -f, -A and -x:

getopts fAx VARNAME

When you want getopts to expect an argument for an option, just place a : (colon) after the proper option flag. If you want -A to expect an argument (i.e. to become -A SOMETHING) just do:

getopts fA:x VARNAME

If the very first character of the option-string is a : (colon), which would normally be nonsense because there's no option letter preceding it, getopts switches to "silent error reporting mode". In productive scripts, this is usually what you want because it allows you to handle errors yourself without being disturbed by annoying messages.

Custom arguments to parse

The getopts utility parses the positional parameters of the current shell or function by default (which means it parses "[email protected]").

You can give your own set of arguments to the utility to parse. Whenever additional arguments are given after the VARNAME parameter, getopts doesn't try to parse the positional parameters, but these given words.

This way, you are able to parse any option set you like, here for example from an array:

while getopts :f:h opt "${MY_OWN_SET[@]}"; do
  ...
done

A call to getopts without these additional arguments is equivalent to explicitly calling it with "[email protected]":

getopts ... "[email protected]"

Error Reporting

Regarding error-reporting, there are two modes getopts can run in:

  • verbose mode
  • silent mode

For productive scripts I recommend to use the silent mode, since everything looks more professional, when you don't see annoying standard messages. Also it's easier to handle, since the failure cases are indicated in an easier way.

Verbose Mode

invalid option VARNAME is set to ? (question-mark) and OPTARG is unset
required argument not found VARNAME is set to ? (question-mark), OPTARG is unset and an error message is printed

Silent Mode

invalid option VARNAME is set to ? (question-mark) and OPTARG is set to the (invalid) option character
required argument not found VARNAME is set to : (colon) and OPTARG contains the option-character in question

Using it

A first example Enough said - action!

Let's play with a very simple case: only one option (-a) expected, without any arguments. Also we disable the verbose error handling by preceding the whole option string with a colon (:):

#!/bin/bash

while getopts ":a" opt; do
  case $opt in
    a)
      echo "-a was triggered!" >&2
      ;;
    \?)
      echo "Invalid option: -$OPTARG" >&2
      ;;
  esac
done

I put that into a file named go_test.sh, which is the name you'll see below in the examples.

Let's do some tests:

Calling it without any arguments

$ ./go_test.sh
$

Nothing happened? Right. getopts didn't see any valid or invalid options (letters preceded by a dash), so it wasn't triggered.

Calling it with non-option arguments

$ ./go_test.sh /etc/passwd
$

Again — nothing happened. The very same case: getopts didn't see any valid or invalid options (letters preceded by a dash), so it wasn't triggered. The arguments given to your script are of course accessible as $1 - ${N}.

Calling it with option-arguments

Now let's trigger getopts: Provide options.

First, an invalid one:

$ ./go_test.sh -b
Invalid option: -b
$

As expected, getopts didn't accept this option and acted like told above: It placed ? into $opt and the invalid option character (b) into $OPTARG. With our case statement, we were able to detect this. Now, a valid one (-a):

$ ./go_test.sh -a
-a was triggered!
$

You see, the detection works perfectly. The a was put into the variable $opt for our case statement. Of course it's possible to mix valid and invalid options when calling:

$ ./go_test.sh -a -x -b -c
-a was triggered!
Invalid option: -x
Invalid option: -b
Invalid option: -c
$

Finally, it's of course possible, to give our option multiple times:

$ ./go_test.sh -a -a -a -a
-a was triggered!
-a was triggered!
-a was triggered!
-a was triggered!
$

The last examples lead us to some points you may consider:

  • invalid options don't stop the processing: If you want to stop the script, you have to do it yourself (exit in the right place)
  • multiple identical options are possible: If you want to disallow these, you have to check manually (e.g. by setting a variable or so)

An option with argument

Let's extend our example from above. Just a little bit:

  • -a now takes an argument
  • on an error, the parsing exits with exit 1
#!/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

Let's do the very same tests we did in the last example:

Calling it without any arguments

$ ./go_test.sh
$

As above, nothing happened. It wasn't triggered.

Calling it with non-option arguments

$ ./go_test.sh /etc/passwd
$

The very same case: It wasn't triggered.

Calling it with option-arguments

Invalid option:

$ ./go_test.sh -b
Invalid option: -b
$

As expected, as above, getopts didn't accept this option and acted like programmed.

Valid option, but without the mandatory argument:

$ ./go_test.sh -a
Option -a requires an argument.
$

The option was okay, but there is an argument missing.

Let's provide the argument:

$ ./go_test.sh -a /etc/passwd
-a was triggered, Parameter: /etc/passwd
$

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