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Defining Canned Command Sequences
When the same sequence of commands is useful in making various
targets, you can define it as a canned sequence with the `define'
directive, and refer to the canned sequence from the rules for those
targets. The canned sequence is actually a variable, so the name must
not conflict with other variable names.
Here is an example of defining a canned sequence of commands:
yacc $(firstword $^)
mv y.tab.c $@
Here `run-yacc' is the name of the variable being defined; `endef'
marks the end of the definition; the lines in between are the commands.
The `define' directive does not expand variable references and
function calls in the canned sequence; the `$' characters, parentheses,
variable names, and so on, all become part of the value of the variable
you are defining. Note: Defining Variables Verbatim, for a
complete explanation of `define'.
The first command in this example runs Yacc on the first dependency
of whichever rule uses the canned sequence. The output file from Yacc
is always named `y.tab.c'. The second command moves the output to the
rule's target file name.
To use the canned sequence, substitute the variable into the
commands of a rule. You can substitute it like any other variable
(Note: Basics of Variable References.). Because variables
defined by `define' are recursively expanded variables, all the
variable references you wrote inside the `define' are expanded now.
foo.c : foo.y
`foo.y' will be substituted for the variable `$^' when it occurs in
`run-yacc''s value, and `foo.c' for `$@'.
This is a realistic example, but this particular one is not needed in
practice because `make' has an implicit rule to figure out these
commands based on the file names involved (*note Using Implicit Rules:
In command execution, each line of a canned sequence is treated just
as if the line appeared on its own in the rule, preceded by a tab. In
particular, `make' invokes a separate subshell for each line. You can
use the special prefix characters that affect command lines (`@', `-',
and `+') on each line of a canned sequence. Note: Writing the Commands
in Rules. For example, using this canned sequence:
@echo "frobnicating target $@"
frob-step-1 $< -o $@-step-1
frob-step-2 $@-step-1 -o $@
`make' will not echo the first line, the `echo' command. But it *will*
echo the following two command lines.
On the other hand, prefix characters on the command line that refers
to a canned sequence apply to every line in the sequence. So the rule:
does not echo *any* commands. (Note: Command Echoing, for a
full explanation of `@'.)
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