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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 each
newline or carriage-return / newline pair to a single space. It also
removes the trailing (carriage-return and) newline, if it's the last
thing in the result.
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)'.
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