Location of ini/config files in linux/unix?

You should adhere your application to the XDG Base Directory Specification. Most answers here are either obsolete or wrong.

Your application should store and load data and configuration files to/from the directories pointed by the following environment variables:

• $XDG_DATA_HOME (default: "$HOME/.local/share"): user-specific data files.
• $XDG_CONFIG_HOME (default: "$HOME/.config"): user-specific configuration files.
• $XDG_DATA_DIRS (default: "/usr/local/share/:/usr/share/"): precedence-ordered set of system data directories.
• $XDG_CONFIG_DIRS (default: "/etc/xdg"): precedence-ordered set of system configuration directories.
• $XDG_CACHE_HOME (default: "$HOME/.cache"): user-specific non-essential data files.

You should first determine if the file in question is:

1. A configuration file ($XDG_CONFIG_HOME:$XDG_CONFIG_DIRS);
2. A data file ($XDG_DATA_HOME:$XDG_DATA_DIRS); or
3. A non-essential (cache) file ($XDG_CACHE_HOME).

It is recommended that your application put its files in a subdirectory of the above directories. Usually, something like $XDG_DATA_DIRS/<application>/filename or $XDG_DATA_DIRS/<vendor>/<application>/filename.

When loading, you first try to load the file from the user-specific directories ($XDG_*_HOME) and, if failed, from system directories ($XDG_*_DIRS). When saving, save to user-specific directories only (since the user probably won't have write access to system directories).

For other, mo

Please do also consult the Filesystem Hierarchy Standard.

(1) No (unfortunately). Edit: The other answers are right, per-user configuration is usually stored in dot-files or dot-directories in the users home directory. Anything above user level often is a lot of guesswork.

(2) System-wide ini file -> user ini file -> environment -> command line options (going from lowest to highest precedence)