Introduction

Many applications use command-line options and environment variables as a way to override configuration values. In Elektra this can be automated by providing a specification that maps command-line options and environment variables to keys in the KDB.

The function elektraGetOpts uses this specification together with argv and envp and creates new keys in the proc namespace for any command-line option or environment variable it finds. Because the proc namespace is used, these values will be preferred over any stored values in a cascading lookup.

There is also the gopts plugin. It is a global plugin and should be mounted via kdbEnsure. This plugin extracts argv and envp via OS specific APIs and then calls elektraGetOpts. The advantage of using gopts is that it retrieves command-line options before any validation plugins are called. This means that values passed on the command-line can be validated.

Setup

To use elektraGetOpts you need to link against elektra-opts, elektra-meta and elektra-ease.

Options

To define a command-line option either set the opt metakey to the short option you want to use, or set opt/long to the long option you want to use. For short options, only the first character of the given value will be used ('\0' is ignored). Short and long options can be used simultaneously.

Additionally a key can also be associated with multiple short/long options. To achieve this treat opt as an array. For example for two options -a and -b you would set opt=#1, opt/#0=a and opt/#1=b. If not explicitly stated otherwise, you can replace opt with any opt/# array element in all meta-keys mentioned in this document. This of course includes long options (i.e. opt/#0/long, etc.).

While you can specify multiple options (or environment variables, see below) for a single key, only one of them can be used at the same time. Using two or more options (or variables) that are all linked to the same key, will result in an error.

Option Arguments

Per default an option is expected to have an argument. Arguments to short and long options are given in the same way as with getopt_long(3) (i.e. -oarg, -o arg, --option arg or --option=arg).

To change whether an option expects an argument set opt/arg to either "none" or "optional" (the default is "required").

If you choose "none", the corresponding key will be set to "1", if the option is used. This value can be changed by setting opt/flagvalue.
An option that is set to "optional" is treated the same as with "none", except that you can also set the value with the long option form --option=value. This also means that opt/flagvalue is used, if no argument is given. Contrary to getopt_long(3) options with optional arguments can still have short forms. They just cannot have an argument in this form.

Environment Variables

Elektra also supports parsing environment variables in a similar manner. For these there are however, less configuration options. You can simply specify one or more environment variables for a key using the env metakey (or env/# meta-array for multiple).

Arrays

Both options and environment variables expose special behavior, if used in combination with arrays.

If an option is specified on a key with basename #, the option can be used repeatedly. All occurrences will be collected into the array.

Environment variables obviously cannot be repeated, instead a behavior similar that used for PATH is adopted. On Windows the variable will be split at each ';' character. On all other systems ':' is used as a separator.

Parameter Arguments

All unused elements of argv are be collected into an array. You can access this array by specifying args=remaining on a key with basename #. The array will be copied into this key. As is the case with getopt(3) processing of options will stop, if -- is encountered in argv.

If elektraGetOpts is called with a parentKey that has posixly = 1 set, then processing of options will also stop, when a non-option argument is encountered. This is similar to the POSIX version of getopt(3). NOTE: posixly = 1 must be set on the parent key passed to elektraGetOpts, not in the specification itself.

Additionally, there is args=indexed. If it is specified on a key, the key must also have the metakey args/index=N set to an integer N. Such a key will be set to the unused element at index N. If a key has args=indexed and args/index=N, then there must also be keys for all integers 0 <= X < N with args=indexed and args/index=N set. For example, you cannot use args/index=0 and args/index=2 without args/index=1.

Combining args=indexed and args=remaining in the same specification (on different keys) is also possible. The key with args=remaining will only contain those elements not used via args=indexed. E.g. if there are keys with args/index=0 and args/index=1 then the args=remaining array will start with the third (index 2) parameter argument. Note however, the args=remaining array always starts with index #0, even if it doesn't contain the first parameter argument.

Example

[from]
args = indexed
args/index = 0
 
[to]
args = indexed
args/index = 1
 
[more/#]
args = remaining

If an application app with the specification above is called as ./app apple banana cherry date, then the keys will be assigned as follows:

from = apple
to = banana
more/#0 = cherry
more/#1 = date

Sub-Commands

elektraGetOpts also supports sub-commands. Explaining sub-commands is easiest through the help of an example: add and commit are both sub-commands of git, since we can call git add and git commit and they do entirely different things. The most important impact of using sub-commands is their effect on option arguments. For example calling git -p add and git add -p result in different behavior, since the -p option is interpreted differently. The options that git understands are separate from the options that its sub-command add knows. However, the option -p is understood by both. In git it is short for --paginate and in add it is short for --patch.

An important thing to know about sub-commands in elektraGetOpts is that they turn on posixly = 1 mode. This means all options for a specific sub-command must be given before any non-option arguments (such as parameters or sub-commands). Otherwise, we couldn't distinguish between git -p add and git add -p. In other words an option argument is always assigned to the first sub-command to its left. Any element of argv that is not the argument for an option argument, either switches to a new sub-command or is the start of the parameter arguments.

A sub-command is created by specifying command on a key. To enable sub-command processing the parent key of the whole specification must have command set to an empty string. All keys marked with command directly below another key K marked with command (e.g. the parent key) are sub-commands of K. It is an error, if the immediate parent of a key X marked with command is not marked with command and X is not the parent of the whole specification.

To inform the application about the invoked sub-commands, elektraGetOpts sets each command key to one of two values:

The basename of the key, whose command was invoked.
An empty string otherwise.

For example consider ./app add more: The parent key will be set to the basename of whatever key command=add was specified on, the key for add will be set to the basename corresponding to more and the key for more is set to an empty string, because none of its sub-commands were invoked. A more detailed example is shown below.

Every key considered by elektraGetOpts is assigned either to the root command, or a single sub-command. Specifically, each key is assigned to the command of its immediate parent. If sub-commands are used and the immediate parent of an opt or args key has no command metadata an error occurs. The value of a key will only be set by elektraGetOpts, if the corresponding sub-command was invoked.

Lastly, it is allowed to have keys with args and command below the same parent. If a matching sub-command is found among the command keys, processing will continue there. Otherwise, the args keys will be considered. This allows an application to implement dynamic commands (like git or kdb) by using the args=remaining array to invoke another application.

If an unknown sub-command is encountered without an args key, an error is returned.

All of this is best understood with an example:

[kdb]
command = ""
 
[kdb/printversion]
description = "print version information and exit (ignoring all other options/commands/parameters)"
opt = v
opt/long = version
opt/arg = none
 
[kdb/getter]
description = "get a key's value"
command = get
 
[kdb/getter/verbose]
description = "print additional information about where the value comes from"
opt = v
opt/long = verbose
opt/arg = none
 
[kdb/getter/keyname]
description = "name of the key to read"
args = indexed
args/index = 0
 
[kdb/setter]
description = "print additional information about where the value will be stored"
command = set
 
[kdb/setter/verbose]
description = "name of the key to write"
opt = v
opt/long = verbose
opt/arg = none
 
[kdb/setter/keyname]
description = "name of the key to write"
args = indexed
args/index = 0
 
[kdb/setter/value]
description = "value to be written"
args = indexed
args/index = 1
 
[kdb/dynamic/#]
description = "dynamically call a user-supplied command"
args = remaining

If we invoke kdb -v, keys below kdb/getter, kdb/setter and kdb/dynamic are not touched. The result is:
- kdb = ""
- kdb/printversion = 1
- kdb/getter = ""
- kdb/setter = ""
If we invoke kdb get -v name, keys below kdb/setter and kdb/dynamic are not touched. The result is:
- kdb = getter
- kdb/getter = ""
- kdb/getter/verbose = 1
- kdb/getter/keyname = name
- kdb/setter = ""
If we invoke kdb -v set -v, keys below kdb/getter and kdb/dynamic are not touched. The result is:
- kdb = setter
- kdb/printversion = 1
- kdb/setter = ""
- kdb/setter/verbose = 1
- kdb/getter = ""
If we invoke kdb -v custom -v -x z, keys below kdb/getter and kdb/setter are not touched. The result is:
- kdb = ""
- kdb/printversion = 1
- kdb/getter = ""
- kdb/setter = ""
- kdb/dynamic/#0 = custom
- kdb/dynamic/#1 = -v
- kdb/dynamic/#2 = -x
- kdb/dynamic/#3 = z

To determine what code to execute, an application would just start with the parent key kdb in the example above. It would then repeatedly look at the current key's value, append that to the current key and continue, until the current key's value was the empty string "". Each of the examined keys corresponds to one of the sub-commands in the invocation and the keys directly below those, contain the relevant options (and for the last sub-command also parameters).

The C code for this example is located in examples/optsCommands.c.

Help Message

When the help option --help is encountered in argv, elektraGetOpts only reads the specification, but does not create any keys in the proc namespace. It will however, generate a help message that can be accessed with elektraGetOptsHelpMessage.

The help message consists of a usage line and an options list. The program name for the usage line is taken from argv[0]. If the value contains a slash (/) it will be considered a path and only the part after the last slash will be used.

The options list will contain exactly one entry for each key that has at least one option. Each entry has to parts. First all the options for the key are listed and then (possibly on the next line, if there are a lot of options), the description for the key is listed. The description is taken from the opt/help or alternatively the description metakey.

Note: opt/help is specified only once per key. That means even if the key uses opt/#0, opt/#1, etc. (unlike most other metadata) the description will always be taken from opt/help directly, because there can only be one description. In general we recommend using description, because it is used by other parts of Elektra as well. opt/help is intended to provide a less verbose description more suitable for the command-line.

The help message can be modified in a few different ways:

The usage argument of elektraGetOptsHelpMessage can be used to replace the default usage line.
The prefix argument of elektraGetOptsHelpMessage can be used to insert text between the usage line and the options list.
An option can can be hidden from the help message by setting opt/hidden to "1". This hides both the long and short form of the option. If you want to hide just one form, use an array of two options an hide just one index.
If the option has an "optional" or "required" argument, the string ARG will be used as a placeholder by default. You can change this, by setting opt/arg/help for the corresponding option.

Sub-Commands

If sub-commands are in use, the help message returned by elektraGetOptsHelpMessage will apply to the invoked sub-command only. For example ./app --help generates the general help message for ./app containing only options valid for the root command. In contrast, ./app more --help generates the help message for the sub-command more.

Precedence

The order of precedence is simple:

If a short option for a key is found, it will always be used.
If none of the short options for a key are found, we look for long options.
Neither short nor long options are found, environment variables are considered.

Limitations

Both options and environment variables can only be specified on a single key. If you need to have the value of one option/environment variable in multiple keys, you may use fallbacks.
- cannot be used as short options, because it would collide with, the "option end marker".
help cannot be used as a long option, because it would collide with the help option.

Examples

The following specification describes the command line interface similar to the one used by rm. (It is based of rm (GNU coreutils) 8.30).

[force]
opt = f
opt/long = force
opt/arg = none
description = ignore nonexistent files and arguments, never prompt
 
[interactive]
opt = #1
opt/#0 = i
opt/#0/long = interactive
opt/#0/arg = optional
opt/#0/flagvalue = always
opt/#1 = I
opt/#1/flagvalue = once
opt/#1/arg = none
opt/arg/name = WHEN
description = prompt according to WHEN: never, once (-I), or always (-i); without WHEN, prompt always
 
[singlefs]
opt/long = one-file-system
opt/arg = none
description = when removing a hierarchy recursively, skip any directory that is on a file system different from that of the corresponding line argument
 
[nopreserve]
opt/long = no-preserve-root
opt/arg = none
description = do not treat '/' specially
 
[preserve]
opt/long = preserve-root
opt/arg = optional
opt/arg/name = all
opt/flagvalue = root
description = do not remove '/' (default); with 'all', reject any command line argument on a separate device from its parent
 
[recursive]
opt = #1
opt/#0 = r
opt/#0/long = recursive
opt/#0/arg = none
opt/#1 = R
opt/#1/arg = none
description = remove directories and their contents recursively
 
[emptydirs]
opt = d
opt/long = dir
opt/arg = none
description = remove empty directories
 
[verbose]
opt = v
opt/long = verbose
opt/arg = none
env = VERBOSE
description = explain what is being done
 
[showversion]
opt/long = version
opt/arg = none
description = output version information and exit
 
[files/#]
args = remaining
env = FILES
description = the files that shall be deleted

If this specification is used in a program called erm (for Elektra rm), which is called like this:

FILES="one.txt:other.log" VERBOSE=1 erm -fi --recursive

The following keys will be created by elektraGetOpts (assuming the specification is mounted at spec/sw/org/erm/#0/current):

proc/sw/org/erm/#0/current/force = "1"
proc/sw/org/erm/#0/current/interactive = "always"
proc/sw/org/erm/#0/current/recursive = "1"
proc/sw/org/erm/#0/current/verbose = "1"
proc/sw/org/erm/#0/current/files [array] = "#1"
proc/sw/org/erm/#0/current/files/#0 = "one.txt"
proc/sw/org/erm/#0/current/files/#1 = "other.log"

Calling FILES="abcd.txt" erm 123.txt 456.txt meanwhile will result in:

proc/sw/org/erm/#0/current/files [array] = "#1"
proc/sw/org/erm/#0/current/files/#0 = "123.txt"
proc/sw/org/erm/#0/current/files/#1 = "456.txt"

NOTE: proc/sw/org/erm/#0/current/files [array] = "#1" means the array metadata of proc/sw/org/erm/#0/current/files is #1.

You can find a full working example here. However, it uses a hard coded specification which is a bit harder to read.