command-line-options.md

Command-line Options

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 uses, these values will be preferred over any stored values in a cascading lookup.

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.

Arguments (Parameters)

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.

Help Message

When one of the help options -h and --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.

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.
• - and h cannot be used as short options, because they would collide with, the "option end marker" and the help option respectively. 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.