md_src_libgetenv_README(3) README

kdb-elektrify-getenv(1) -- elektrify the environment of applications

kdb elektrify-getenv <application> <options>

DESCRIPTION

When an application is elektrified using libelektragetenv, it does not only request environ, but also Elektra for every getenv(3) and secure_getenv(3) library call.

Its main purpose is to:

  • have standard ways to modify the environment
  • make relogin (or even restart!) of applications unnecessary
  • allow a hierarchical structure for environment
  • allow settings to only apply for individual applications or only in special context
  • still preserve the advantages (inheriting of environment to subprocesses)
  • Availability in at, cron and similar scripts.

It is implemented using a LD_PRELOAD technique, see USAGE below for global activation.

LOOKUPS

The main purpose of this approach is to finally have a well-defined way to set and get environment variables. Elektra's variables will be in use immediately for every newly started application (no relogin necessary).

To do so, getenv(3) will lookup multiple sources next to searching in the environment (environ). As running example will use getenv('HOME') -> /path/to/home:

1.
Given commandline parameters will always be preferred (see OPTIONS below).

E.g. kdb elektrify-getenv <app> --elektra:HOME=/path/to/home

2.
Then /env/override/<key> will be looked up, where <key> is the parameter to getenv. If found, the key will be returned, if it is a null keys, getenv will return NULL.

E.g. kdb set user/env/override/HOME /path/to/home

3.
Then environment will be requested.

E.g. HOME=/path/to/home kdb elektrify-getenv <application>

3.
Then /env/fallback/<key> will be looked up. If found, the key will be returned, if it is a null keys, getenv will return NULL.

E.g. kdb set user/env/fallback/HOME /path/to/home

OPTIONS

When elektrify-getenv is active, every application additionally accepts Elektra's getenv options. Interleaving Elektra's and the application's options is allowed. Elektra will parse its options (starting with --elektra) first and discard them before the other application is started. Therefore the application will not see that they even existed, e.g.: given kdb elektrify-getenv <application> -V --elektra-debug -L the application will be called with <application> -V -L.

Internal Options

  • --elektra-help: Outputs this help.
  • --elektra-version: Gives version information.
  • --elektra-debug=file, ELEKTRA_DEBUG or /env/option/debug: Trace all getenv(3) calls to a file. stderr if no file is given, e.g. kdb set user/env/option/debug ''. Note that null values (no forth argument), will disable debug messages. See examples below.
  • --elektra-clearenv, ELEKTRA_CLEARENV or /env/option/clearenv: Call clearenv(3) before entering main. This is a recommended security feature. Elektra itself, if configured that way, will still be able to use the environment.
  • --elektra-reload-timeout=time_in_ms, ELEKTRA_RELOAD_TIMEOUT or /env/option/reload_timeout: Activate a timeout based feature when a time is given in ms (and is not 0).

Internal Options are available in three different variants:

1.
as commandline parameter: --elektra-<option>, which are not passed through exec(3) calls.

1.
as environment variable: ELEKTRA_<OPTION>. which might be passed through exec(3) calls, but are removed by clearenv(3) calls.

1.
as Elektra KDB entry: /env/option/<option>, which are the way to achieve an option to be enabled for every application.

E.g. kdb set user/env/option/clearenv '' to clear the environment for all applications started by that user (note that at least PATH should to be set using kdb set user/env/fallback/PATH '/bin:/usr/bin' then).

Note, that null keys are equal to non-set options. E.g. kdb set system/env/option/debug '/tmp/elektra.log' and kdb set user/env/option/debug will activate logging for the system, except for the current user.

Contextual Options

  • --elektra%<name>%=<value> or /env/layer/<name>: Add the contextual information (=layer) %<name>% with it's value <value>. Note that name% is predefined with argv[0] and basename% with basename(argv[0]).

Values can contain / to form hierarchies, e.g. --elektraname%=app/profile

Options for Applications

  • --elektra:key=value, /env/override/<key> or /env/fallback/<key>: set a key/value to be preferred, i.e. the first to considered as explained in LOOKUP.

Keys can contain / to form hierarchies, e.g. --elektra:my/HOME=/path/to/home.

USAGE

To always use Elektra's getenv environment, simply add the output to the file: 

kdb elektrify-getenv | tail -1 | sudo tee -a /etc/ld.so.preload

this also can be done using Elektra: 

sudo kdb mount /etc/ld.so.preload system/ld/preload line null
sudo kdb set "system/ld/preload/new"  `kdb elektrify-getenv | tail -1`

CONTEXT

The metadata context in the specification can be used to facilitate a context-dependent lookup. In its metavalue all replacements of %<name>% will be replaced by the given contextual options --elektra%<name>%=<value> and /env/layer/<name> keys.

E.g. to have a different home directory for any user and application: 

kdb set user/env/layer/user markus
kdb set user/users/markus/konqueror/HOME /home/download
kdb setmeta spec/env/override/HOME context  /users/%user%/%name%/HOME

BUGS

Some applications do not use getenv(3) or secure_getenv(3) for requesting the environment, e.g. shells. This approach cannot work for them.

In the startup-phase (before main is even entered), getenv(3) will not consider /env/override/ or /env/fallback.

Elektra internally tries to avoid using the environment. Some resolvers, however, use it to be conform to some specifications, e.g. XDG. Depending on the setup you use, these parameters might be used. For more information see: 

kdb info resolver

For these parameters, /env/override/ or /env/fallback will not be used internally, but will be used if applications request them, too.

If you use the standard resolvers, the bug won't have any effect.

Also note that --elektra-debug or ELEKTRA_DEBUG does not log getenv(3) used by plugins during the startup-phase.

Commandline Arguments are always to the outmost command, e.g. nice ls --elektra:COLUMNS=20 won't have any effect because only for nice COLUMNS will be set.

EXAMPLES

For illustration this section gives some more examples. 

kdb elektrify-getenv man man --elektra:MANWIDTH=40

Will use MANWIDTH 40 for this invocation of man man. This feature is handy, if an option is only available by environment, but not by command-line arguments, because sometimes environment variables are not trivial to set (e.g. in Makefiles).

Debugging: 

# system wide to stderr (not recommended!):
sudo kdb set system/env/option/debug ""
# system wide to /var/log/elektra.log:
sudo kdb set system/env/option/debug "/var/log/error.log"
# but for my user to ~/.elektra.log:
kdb set user/env/option/debug "$HOME/.elektra.log"
# or disable it for my user:
kdb set user/env/option/debug

Some more examples: 

kdb set user/env/override/MANOPT -- "--regex -LC"
kdb elektrify-getenv getenv MANOPT   # to check if it is set as expected
kdb getenv MANOPT   # if /etc/ld.so.preload is active

Will permanently and user-wide change MANOPT to include --regex, and -LC so that regular expressions will be used (note man echo will return many man pages then) and that they will be shown in English. This feature is handy to change the default behaviour of applications (either system, user or directory-wide). 

kdb set system/env/override/HTTP_PROXY http://proxy.hogege.com:8000/

Will permanently and system-wide change the proxy for all applications that honor HTTP_PROXY, e.g. w3m. We can also link http_proxy to the value of HTTP_PROXY: 

kdb setmeta spec/env/override/http_proxy "override/#0" /env/override/HTTP_PROXY
kdb get /env/override/http_proxy


 

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