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The commands of a rule consist of shell command lines to be executed one by one. Each command line must start with a tab, except that the first command line may be attached to the target-and-dependencies line with a semicolon in between. Blank lines and lines of just comments may appear among the command lines; they are ignored. (But beware, an apparently "blank" line that begins with a tab is not blank! It is an empty command; see section Using Empty Commands.)
Users use many different shell programs, but commands in makefiles are always interpreted by `/bin/sh' unless the makefile specifies otherwise. See section Command Execution.
The shell that is in use determines whether comments can be written on command lines, and what syntax they use. When the shell is `/bin/sh', a `#' starts a comment that extends to the end of the line. The `#' does not have to be at the beginning of a line. Text on a line before a `#' is not part of the comment.
Normally make prints each command line before it is executed.
We call this echoing because it gives the appearance that you
are typing the commands yourself.
When a line starts with `@', the echoing of that line is suppressed.
The `@' is discarded before the command is passed to the shell.
Typically you would use this for a command whose only effect is to print
something, such as an echo command to indicate progress through
the makefile:
@echo About to make distribution files
When make is given the flag `-n' or `--just-print',
echoing is all that happens, no execution. See section Summary of Options. In this case and only this case, even the
commands starting with `@' are printed. This flag is useful for
finding out which commands make thinks are necessary without
actually doing them.
The `-s' or `--silent'
flag to make prevents all echoing, as if all commands
started with `@'. A rule in the makefile for the special target
.SILENT without dependencies has the same effect
(see section Special Built-in Target Names).
.SILENT is essentially obsolete since `@' is more flexible.
When it is time to execute commands to update a target, they are executed
by making a new subshell for each line. (In practice, make may
take shortcuts that do not affect the results.)
Please note: this implies that shell commands such as
cd that set variables local to each process will not affect the
following command lines. If you want to use cd to affect the
next command, put the two on a single line with a semicolon between
them. Then make will consider them a single command and pass
them, together, to a shell which will execute them in sequence. For
example:
foo : bar/lose
cd bar; gobble lose > ../foo
If you would like to split a single shell command into multiple lines of text, you must use a backslash at the end of all but the last subline. Such a sequence of lines is combined into a single line, by deleting the backslash-newline sequences, before passing it to the shell. Thus, the following is equivalent to the preceding example:
foo : bar/lose
cd bar; \
gobble lose > ../foo
The program used as the shell is taken from the variable SHELL.
By default, the program `/bin/sh' is used.
Unlike most variables, the variable SHELL is never set from the
environment. This is because the SHELL environment variable is
used to specify your personal choice of shell program for interactive
use. It would be very bad for personal choices like this to affect
the functioning of makefiles. See section Variables from the Environment.
GNU make knows how to execute several commands at once.
Normally, make will execute only one command at a time, waiting
for it to finish before executing the next. However, the `-j' or
`--jobs' option tells make to execute many commands
simultaneously.
If the `-j' option is followed by an integer, this is the number of commands to execute at once; this is called the number of job slots. If there is nothing looking like an integer after the `-j' option, there is no limit on the number of job slots. The default number of job slots is one, which means serial execution (one thing at a time).
One unpleasant consequence of running several commands simultaneously is that output from all of the commands comes when the commands send it, so messages from different commands may be interspersed.
Another problem is that two processes cannot both take input from the
same device; so to make sure that only one command tries to take input
from the terminal at once, make will invalidate the standard
input streams of all but one running command. This means that
attempting to read from standard input will usually be a fatal error (a
`Broken pipe' signal) for most child processes if there are
several.
It is unpredictable which command will have a valid standard input stream
(which will come from the terminal, or wherever you redirect the standard
input of make). The first command run will always get it first, and
the first command started after that one finishes will get it next, and so
on.
We will change how this aspect of make works if we find a better
alternative. In the mean time, you should not rely on any command using
standard input at all if you are using the parallel execution feature; but
if you are not using this feature, then standard input works normally in
all commands.
If a command fails (is killed by a signal or exits with a nonzero
status), and errors are not ignored for that command
(see section Errors in Commands),
the remaining command lines to remake the same target will not be run.
If a command fails and the `-k' or `--keep-going'
option was not given
(see section Summary of Options),
make aborts execution. If make
terminates for any reason (including a signal) with child processes
running, it waits for them to finish before actually exiting.
When the system is heavily loaded, you will probably want to run fewer jobs
than when it is lightly loaded. You can use the `-l' option to tell
make to limit the number of jobs to run at once, based on the load
average. The `-l' or `--max-load'
option is followed by a floating-point number. For
example,
-l 2.5
will not let make start more than one job if the load average is
above 2.5. The `-l' option with no following number removes the
load limit, if one was given with a previous `-l' option.
More precisely, when make goes to start up a job, and it already has
at least one job running, it checks the current load average; if it is not
lower than the limit given with `-l', make waits until the load
average goes below that limit, or until all the other jobs finish.
By default, there is no load limit.
After each shell command returns, make looks at its exit status.
If the command completed successfully, the next command line is executed
in a new shell; after the last command line is finished, the rule is
finished.
If there is an error (the exit status is nonzero), make gives up on
the current rule, and perhaps on all rules.
Sometimes the failure of a certain command does not indicate a problem.
For example, you may use the mkdir command to ensure that a
directory exists. If the directory already exists, mkdir will
report an error, but you probably want make to continue regardless.
To ignore errors in a command line, write a `-' at the beginning of the line's text (after the initial tab). The `-' is discarded before the command is passed to the shell for execution.
For example,
clean:
-rm -f *.o
This causes rm to continue even if it is unable to remove a file.
When you run make with the `-i' or `--ignore-errors'
flag, errors are ignored in all commands of all rules. A rule in the
makefile for the special target .IGNORE has the same effect, if
there are no dependencies. These ways of ignoring errors are obsolete
because `-' is more flexible.
When errors are to be ignored, because of either a `-' or the
`-i' flag, make treats an error return just like success,
except that it prints out a message that tells you the status code
the command exited with, and says that the error has been ignored.
When an error happens that make has not been told to ignore,
it implies that the current target cannot be correctly remade, and neither
can any other that depends on it either directly or indirectly. No further
commands will be executed for these targets, since their preconditions
have not been achieved.
Normally make gives up immediately in this circumstance, returning a
nonzero status. However, if the `-k' or `--keep-going'
flag is specified, make
continues to consider the other dependencies of the pending targets,
remaking them if necessary, before it gives up and returns nonzero status.
For example, after an error in compiling one object file, `make -k'
will continue compiling other object files even though it already knows
that linking them will be impossible. See section Summary of Options.
The usual behavior assumes that your purpose is to get the specified
targets up to date; once make learns that this is impossible, it
might as well report the failure immediately. The `-k' option says
that the real purpose is to test as many of the changes made in the
program as possible, perhaps to find several independent problems so
that you can correct them all before the next attempt to compile. This
is why Emacs' compile command passes the `-k' flag by
default.
Usually when a command fails, if it has changed the target file at all,
the file is corrupted and cannot be used--or at least it is not
completely updated. Yet the file's timestamp says that it is now up to
date, so the next time make runs, it will not try to update that
file. The situation is just the same as when the command is killed by a
signal; see section Interrupting or Killing make. So generally the right thing to do is to
delete the target file if the command fails after beginning to change
the file. make will do this if .DELETE_ON_ERROR appears
as a target. This is almost always what you want make to do, but
it is not historical practice; so for compatibility, you must explicitly
request it.
make
If make gets a fatal signal while a command is executing, it may
delete the target file that the command was supposed to update. This is
done if the target file's last-modification time has changed since
make first checked it.
The purpose of deleting the target is to make sure that it is remade from
scratch when make is next run. Why is this? Suppose you type
Ctrl-c while a compiler is running, and it has begun to write an
object file `foo.o'. The Ctrl-c kills the compiler, resulting
in an incomplete file whose last-modification time is newer than the source
file `foo.c'. But make also receives the Ctrl-c signal
and deletes this incomplete file. If make did not do this, the next
invocation of make would think that `foo.o' did not require
updating--resulting in a strange error message from the linker when it
tries to link an object file half of which is missing.
You can prevent the deletion of a target file in this way by making the
special target .PRECIOUS depend on it. Before remaking a target,
make checks to see whether it appears on the dependencies of
.PRECIOUS, and thereby decides whether the target should be deleted
if a signal happens. Some reasons why you might do this are that the
target is updated in some atomic fashion, or exists only to record a
modification-time (its contents do not matter), or must exist at all
times to prevent other sorts of trouble.
make
Recursive use of make means using make as a command in a
makefile. This technique is useful when you want separate makefiles for
various subsystems that compose a larger system. For example, suppose you
have a subdirectory `subdir' which has its own makefile, and you would
like the containing directory's makefile to run make on the
subdirectory. You can do it by writing this:
subsystem:
cd subdir; $(MAKE)
or, equivalently, this (see section Summary of Options):
subsystem:
$(MAKE) -C subdir
You can write recursive make commands just by copying this example,
but there are many things to know about how they work and why, and about
how the sub-make relates to the top-level make.
MAKE Variable Works
Recursive make commands should always use the variable MAKE,
not the explicit command name `make', as shown here:
subsystem:
cd subdir; $(MAKE)
The value of this variable is the file name with which make was
invoked. If this file name was `/bin/make', then the command executed
is `cd subdir; /bin/make'. If you use a special version of
make to run the top-level makefile, the same special version will be
executed for recursive invocations.
As a special feature, using the variable MAKE in the commands of
a rule alters the effects of the `-t' (`--touch'), `-n'
(`--just-print'), or `-q' (`--question') option.
Using the MAKE variable has the same effect as using a `+'
character at the beginning of the command line. See section Instead of Executing the Commands.
Consider the command `make -t' in the above example. (The `-t' option marks targets as up to date without actually running any commands; see section Instead of Executing the Commands.) Following the usual definition of `-t', a `make -t' command in the example would create a file named `subsystem' and do nothing else. What you really want it to do is run `cd subdir; make -t'; but that would require executing the command, and `-t' says not to execute commands.
The special feature makes this do what you want: whenever a command
line of a rule contains the variable MAKE, the flags `-t',
`-n' and `-q' do not apply to that line. Command lines
containing MAKE are executed normally despite the presence of a
flag that causes most commands not to be run. The usual
MAKEFLAGS mechanism passes the flags to the sub-make
(see section Communicating Options to a Sub-make}), so your request to touch the files, or print the
commands, is propagated to the subsystem.
make
Variable values of the top-level make can be passed to the
sub-make through the environment by explicit request. These
variables are defined in the sub-make as defaults, but do not
override what is specified in the makefile used by the sub-make
makefile unless you use the `-e' switch (see section Summary of Options).
To pass down, or export, a variable, make adds the variable
and its value to the environment for running each command. The
sub-make, in turn, uses the environment to initialize its table
of variable values. See section Variables from the Environment.
Except by explicit request, make exports a variable only if it
is either defined in the environment initially or set on the command
line, and if its name consists only of letters, numbers, and underscores.
Some shells cannot cope with environment variable names consisting of
characters other than letters, numbers, and underscores.
The special variables SHELL and MAKEFLAGS are always
exported (unless you unexport them).
MAKEFILES is exported if you set it to anything.
make automatically passes down variable values that were defined
on the command line, by putting them in the MAKEFLAGS variable.
See the next section.
Variables are not normally passed down if they were created by
default by make (see section Variables Used by Implicit Rules). The sub-make will define these for
itself.
If you want to export specific variables to a sub-make, use the
export directive, like this:
export variable ...
If you want to prevent a variable from being exported, use the
unexport directive, like this:
unexport variable ...
As a convenience, you can define a variable and export it at the same time by doing:
export variable = value
has the same result as:
variable = value export variable
and
export variable := value
has the same result as:
variable := value export variable
Likewise,
export variable += value
is just like:
variable += value export variable
See section Appending More Text to Variables.
You may notice that the export and unexport directives
work in make in the same way they work in the shell, sh.
If you want all variables to be exported by default, you can use
export by itself:
export
This tells make that variables which are not explicitly mentioned
in an export or unexport directive should be exported.
Any variable given in an unexport directive will still not
be exported. If you use export by itself to export variables by
default, variables whose names contain characters other than
alphanumerics and underscores will not be exported unless specifically
mentioned in an export directive.
The behavior elicited by an export directive by itself was the
default in older versions of GNU make. If your makefiles depend
on this behavior and you want to be compatible with old versions of
make, you can write a rule for the special target
.EXPORT_ALL_VARIABLES instead of using the export directive.
This will be ignored by old makes, while the export
directive will cause a syntax error.
Likewise, you can use unexport by itself to tell make
not to export variables by default. Since this is the default
behavior, you would only need to do this if export had been used
by itself earlier (in an included makefile, perhaps). You
cannot use export and unexport by themselves to
have variables exported for some commands and not for others. The last
export or unexport directive that appears by itself
determines the behavior for the entire run of make.
As a special feature, the variable MAKELEVEL is changed when it
is passed down from level to level. This variable's value is a string
which is the depth of the level as a decimal number. The value is
`0' for the top-level make; `1' for a sub-make,
`2' for a sub-sub-make, and so on. The incrementation
happens when make sets up the environment for a command.
The main use of MAKELEVEL is to test it in a conditional
directive (see section Conditional Parts of Makefiles); this
way you can write a makefile that behaves one way if run recursively and
another way if run directly by you.
You can use the variable MAKEFILES to cause all sub-make
commands to use additional makefiles. The value of MAKEFILES is
a whitespace-separated list of file names. This variable, if defined in
the outer-level makefile, is passed down through the environment; then
it serves as a list of extra makefiles for the sub-make to read
before the usual or specified ones. See section The Variable MAKEFILES}.
make
Flags such as `-s' and `-k' are passed automatically to the
sub-make through the variable MAKEFLAGS. This variable is
set up automatically by make to contain the flag letters that
make received. Thus, if you do `make -ks' then
MAKEFLAGS gets the value `ks'.
As a consequence, every sub-make gets a value for MAKEFLAGS
in its environment. In response, it takes the flags from that value and
processes them as if they had been given as arguments.
See section Summary of Options.
Likewise variables defined on the command line are passed to the
sub-make through MAKEFLAGS. Words in the value of
MAKEFLAGS that contain `=', make treats as variable
definitions just as if they appeared on the command line.
See section Overriding Variables.
The options `-C', `-f', `-I', `-o', and `-W'
are not put into MAKEFLAGS; these options are not passed down.
The `-j' option is a special case (see section Parallel Execution).
If you set it to some numeric value, `-j 1' is always put into
MAKEFLAGS instead of the value you specified. This is because if
the `-j' option were passed down to sub-makes, you would
get many more jobs running in parallel than you asked for. If you give
`-j' with no numeric argument, meaning to run as many jobs as
possible in parallel, this is passed down, since multiple infinities are
no more than one.
If you do not want to pass the other flags down, you must change the
value of MAKEFLAGS, like this:
MAKEFLAGS=
subsystem:
cd subdir; $(MAKE)
or like this:
subsystem:
cd subdir; $(MAKE) MAKEFLAGS=
The command line variable definitions really appear in the variable
MAKEOVERRIDES, and MAKEFLAGS contains a reference to this
variable. If you do want to pass flags down normally, but don't want to
pass down the command line variable definitions, you can reset
MAKEOVERRIDES to empty, like this:
MAKEOVERRIDES =
This is not usually useful to do. However, some systems have a small
fixed limit on the size of the environment, and putting so much
information in into the value of MAKEFLAGS can exceed it.
If you see the error message `Arg list too long', this may be the problem.
(For strict compliance with POSIX.2, changing MAKEOVERRIDES does
not affect MAKEFLAGS if the special target `.POSIX' appears
in the makefile. You probably do not care about this.)
A similar variable MFLAGS exists also, for historical
compatibility. It has the same value as MAKEFLAGS except that it
does not contain the command line variable definitions, and it always
begins with a hyphen unless it is empty (MAKEFLAGS begins with a
hyphen only when it begins with an option that has no single-letter
version, such as `--warn-undefined-variables'). MFLAGS was
traditionally used explicitly in the recursive make command, like
this:
subsystem:
cd subdir; $(MAKE) $(MFLAGS)
but now MAKEFLAGS makes this usage redundant. If you want your
makefiles to be compatible with old make programs, use this
technique; it will work fine with more modern make versions too.
The MAKEFLAGS variable can also be useful if you want to have
certain options, such as `-k' (see section Summary of Options), set each time you run make. You simply put a value for
MAKEFLAGS in your environment. You can also set MAKEFLAGS in
a makefile, to specify additional flags that should also be in effect for
that makefile. (Note that you cannot use MFLAGS this way. That
variable is set only for compatibility; make does not interpret a
value you set for it in any way.)
When make interprets the value of MAKEFLAGS (either from the
environment or from a makefile), it first prepends a hyphen if the value
does not already begin with one. Then it chops the value into words
separated by blanks, and parses these words as if they were options given
on the command line (except that `-C', `-f', `-h',
`-o', `-W', and their long-named versions are ignored; and there
is no error for an invalid option).
If you do put MAKEFLAGS in your environment, you should be sure not
to include any options that will drastically affect the actions of
make and undermine the purpose of makefiles and of make
itself. For instance, the `-t', `-n', and `-q' options, if
put in one of these variables, could have disastrous consequences and would
certainly have at least surprising and probably annoying effects.
If you use several levels of recursive make invocations, the
`-w' or `--print-directory' option can make the output a
lot easier to understand by showing each directory as make
starts processing it and as make finishes processing it. For
example, if `make -w' is run in the directory `/u/gnu/make',
make will print a line of the form:
make: Entering directory `/u/gnu/make'.
before doing anything else, and a line of the form:
make: Leaving directory `/u/gnu/make'.
Normally, you do not need to specify this option because `make'
does it for you: `-w' is turned on automatically when you use the
`-C' option, and in sub-makes. make will not
automatically turn on `-w' if you also use `-s', which says to
be silent, or if you use `--no-print-directory' to explicitly
disable it.
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:
define run-yacc yacc $(firstword $^) mv y.tab.c $@ endef
Here run-yacc is the name of the variable being defined;
endef marks the end of the definition; the lines in between are the
commands. The define directive does not expand variable references
and function calls in the canned sequence; the `$' characters,
parentheses, variable names, and so on, all become part of the value of the
variable you are defining.
See section 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
(see section 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. For example:
foo.c : foo.y
$(run-yacc)
`foo.y' will be substituted for the variable `$^' 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
(see section 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.
See section Writing the Commands in Rules.
For example, using this canned sequence:
define frobnicate @echo "frobnicating target $@" frob-step-1 $< -o $@-step-1 frob-step-2 $@-step-1 -o $@ endef
make will not echo the first line, the echo command.
But it will echo the following two command lines.
On the other hand, prefix characters on the command line that refers to a canned sequence apply to every line in the sequence. So the rule:
frob.out: frob.in @$(frobnicate)
does not echo any commands. (See section Command Echoing, for a full explanation of `@'.)
It is sometimes useful to define commands which do nothing. This is done simply by giving a command that consists of nothing but whitespace. For example:
target: ;
defines an empty command string for `target'. You could also use a line beginning with a tab character to define an empty command string, but this would be confusing because such a line looks empty.
You may be wondering why you would want to define a command string that
does nothing. The only reason this is useful is to prevent a target
from getting implicit commands (from implicit rules or the
.DEFAULT special target; see section Using Implicit Rules and
see section Defining Last-Resort Default Rules).
You may be inclined to define empty command strings for targets that are not actual files, but only exist so that their dependencies can be remade. However, this is not the best way to do that, because the dependencies may not be remade properly if the target file actually does exist. See section Phony Targets, for a better way to do this.
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