shell
Function
The shell
function is unlike any other function except the
wildcard
function
(see section The Function wildcard
) 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 an argument that is a shell command and returns the
output of the command. The only processing make
does on the result,
before substituting it into the surrounding text, is to convert newlines to
spaces.
The commands run by calls to the shell
function are run when the
function calls are expanded. In most cases, this is when the makefile is
read in. The exception is that function calls in the commands of the rules
are expanded when the commands are run, and this applies to shell
function calls like all others.
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)'.