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Chapter 8: Functions for Transforming Text 97

8.13 The shell Function

The shell function is unlike any other function other than the wildcard function (see

Section 4.3.3 [The Function wildcard], page 24) in that it communicates with the world

outside of make.

The shell function performs the same function that backquotes (‘‘’) perform in most

shells: it does command expansion. This means that it takes as an argument a shell

command and evaluates to the output of the command. The only processing make does on

the result is to convert each newline (or carriage-return / newline pair) to a single space. If

there is a trailing (carriage-return and) newline it will simply be removed.

The commands run by calls to the shell function are run when the function calls are

expanded (see Section 3.7 [How make Reads a Makeﬁle], page 16). Because this function

involves spawning a new shell, you should carefully consider the performance implications

of using the shell function within recursively expanded variables vs. simply expanded

variables (see Section 6.2 [The Two Flavors of Variables], page 60).

Here are some examples of the use of the shell function:

contents := $(shell cat foo)

sets contents to the contents of the ﬁle foo, with a space (rather than a newline) separating

each line.

files := $(shell echo *.c)

sets files to the expansion of ‘*.c’. Unless make is using a very strange shell, this has the

same result as ‘$(wildcard *.c)’ (as long as at least one ‘.c’ ﬁle exists).

8.14 The guile Function

If GNU make is built with support for GNU Guile as an embedded extension language then

the guile function will be available. The guile function takes one argument which is ﬁrst

expanded by make in the normal fashion, then passed to the GNU Guile evaluator. The

result of the evaluator is converted into a string and used as the expansion of the guile

function in the makeﬁle. See Section 12.1 [GNU Guile Integration], page 133 for details on

writing extensions to make in Guile.

You can determine whether GNU Guile support is available by checking the .FEATURES

variable for the word guile.

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