All variables are global and can be used in Sections or Functions. Note that, by default, variables are limited to 1024 characters. To extend this limit, \R{build}{build NSIS} with a bigger value of NSIS_MAX_STRLEN in Source/exehead/config.h or use the \W{http://nsis.sourceforge.net/download/specialbuilds/}{special build}.
Declare a user variable. Allowed characters for variables names: [a-z][A-Z][0-9] and '_'. All defined variables are global, even if defined in a section or a function. To make this clear, variables defined in a section or a function must use the /GLOBAL flag. The /GLOBAL flag is not required outside of sections and functions.
Registers. These variables can be used just like user variables, but are usually used in shared functions or macro's. You don't have to declare these variables, so you won't get any name conflicts when using them in shared code. When using these variables in shared code, it's recommended that you use the stack to save and restore their original values. These variables can also be used for communication with plug-ins, because they can be read and written by the plug-in DLL's.
The installation directory ($INSTDIR is modifiable using \R{StrCpy}{StrCpy}, \R{readregstr}{ReadRegStr}, \R{readinistr}{ReadINIStr}, etc. - This could be used, for example, in the \R{oninit}{.onInit} function to do a more advanced detection of install location).
Note that in uninstaller code, $INSTDIR contains the directory where the uninstaller lies. It does \\<b\\>not\\</b\\> necessarily contain the same value it contained in the installer. For example, if you write the uninstaller to $WINDIR and the user doesn't move it, $INSTDIR will be $WINDIR in the uninstaller. If you write the uninstaller to another location, you should keep the installer's $INSTDIR in the registry or an alternative storing facility and read it in the uninstaller.
The current output directory (set implicitly via \R{setoutpath}{SetOutPath} or explicitly via \R{StrCpy}{StrCpy}, \R{readregstr}{ReadRegStr}, \R{readinistr}{ReadINIStr}, etc)
\b For parsing out the PARAMETER portion, see GetParameters on the \R{getparameters}{useful functions appendix}. If /D= is specified on the command line (to override the install directory) it won't show up in $CMDLINE.
Note that some of the new constants will not work on every OS. For example, $CDBURN_AREA will only work on Windows XP and above. If it's used on Windows 98, it'll be empty. Unless mentioned otherwise, a constant should be available on every OS.
The common files directory. This is a directory for components that are shared across applications (usually C:\\Program Files\\Common Files but detected at runtime).
The windows desktop directory (usually C:\\windows\\desktop but detected at runtime). The context of this constant (All Users or Current user) depends on the \R{setshellvarcontext}{SetShellVarContext} setting. The default is the current user.
A symbol that contains the path where NSIS is installed. Detected at compile time. Useful if you want to call resources that are in NSIS directory e.g. Icons, UIs...
The start menu folder (useful in adding start menu items using \R{createshortcut}{CreateShortCut}). The context of this constant (All Users or Current user) depends on the \R{setshellvarcontext}{SetShellVarContext} setting. The default is the current user.
The start menu programs folder (use this whenever you want $STARTMENU\\Programs). The context of this constant (All Users or Current user) depends on the \R{setshellvarcontext}{SetShellVarContext} setting. The default is the current user.
The start menu programs / startup folder. The context of this constant (All Users or Current user) depends on the \R{setshellvarcontext}{SetShellVarContext} setting. The default is the current user.
The documents directory. A typical path for the current user is C:\\Documents and Settings\\Foo\\My Documents. The context of this constant (All Users or Current user) depends on the \R{setshellvarcontext}{SetShellVarContext} setting. The default is the current user.
The directory that contains shortcuts to the user's recently used documents.
\e{$FAVORITES}
The directory that contains shortcuts to the user's favorite websites, documents, etc. The context of this constant (All Users or Current user) depends on the \R{setshellvarcontext}{SetShellVarContext} setting. The default is the current user.
The user's music files directory. The context of this constant (All Users or Current user) depends on the \R{setshellvarcontext}{SetShellVarContext} setting. The default is the current user.
The user's picture files directory. The context of this constant (All Users or Current user) depends on the \R{setshellvarcontext}{SetShellVarContext} setting. The default is the current user.
The user's video files directory. The context of this constant (All Users or Current user) depends on the \R{setshellvarcontext}{SetShellVarContext} setting. The default is the current user.
The document templates directory. The context of this constant (All Users or Current user) depends on the \R{setshellvarcontext}{SetShellVarContext} setting. The default is the current user.
\e{$APPDATA}
The application data directory. Detection of the current user path requires Internet Explorer 4 and above. Detection of the all users path requires Internet Explorer 5 and above. The context of this constant (All Users or Current user) depends on the \R{setshellvarcontext}{SetShellVarContext} setting. The default is the current user.
A directory where administrative tools are kept. The context of this constant (All Users or Current user) depends on the \R{setshellvarcontext}{SetShellVarContext} setting. The default is the current user.
The path to a temporary folder created upon the first usage of a plug-in or a call to \R{initpluginsdir}{InitPluginsDir}. This folder is automatically deleted when the installer exits. This makes this folder the ideal folder to hold INI files for \L{../Docs/InstallOptions/Readme.html}{InstallOptions}, bitmaps for the splash plug-in, or any other file that a plug-in needs to work.
