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# How to Use Variables

A variable is a name defined in a makefile to represent a string of text, called the variable's value. These values are substituted by explicit request into targets, dependencies, commands, and other parts of the makefile. (In some other versions of `make`, variables are called macros.)

Variables and functions in all parts of a makefile are expanded when read, except for the shell commands in rules, the right-hand sides of variable definitions using ``='`, and the bodies of variable definitions using the `define` directive.

Variables can represent lists of file names, options to pass to compilers, programs to run, directories to look in for source files, directories to write output in, or anything else you can imagine.

A variable name may be any sequence of characters not containing ``:'`, ``#'`, ``='`, or leading or trailing whitespace. However, variable names containing characters other than letters, numbers, and underscores should be avoided, as they may be given special meanings in the future, and with some shells they cannot be passed through the environment to a sub-`make` (see section Communicating Variables to a Sub-`make`).

Variable names are case-sensitive. The names ``foo'`, ``FOO'`, and ``Foo'` all refer to different variables.

It is traditional to use upper case letters in variable names, but we recommend using lower case letters for variable names that serve internal purposes in the makefile, and reserving upper case for parameters that control implicit rules or for parameters that the user should override with command options (see section Overriding Variables).

A few variables have names that are a single punctuation character or just a few characters. These are the automatic variables, and they have particular specialized uses. See section Automatic Variables.

- Reference: How to use the value of a variable.
- Flavors: Variables come in two flavors.
- Advanced: Advanced features for referencing a variable.
- Values: All the ways variables get their values.
- Setting: How to set a variable in the makefile.
- Appending: How to append more text to the old value of a variable.
- Override Directive: How to set a variable in the makefile even if the user has set it with a command argument.
- Defining: An alternate way to set a variable to a verbatim string.
- Environment: Variable values can come from the environment.
- Target-specific: Variable values can be defined on a per-target basis.
- Pattern-specific: Target-specific variable values can be applied to a group of targets that match a pattern.
- Automatic: Some special variables have predefined meanings for use with implicit rules.

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