5. Writing the Commands in Rules

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-prerequisites 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 5.8 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.

5.1 Command Echoing		How to control when commands are echoed.
5.2 Command Execution		How commands are executed.
5.3 Parallel Execution		How commands can be executed in parallel.
5.4 Errors in Commands		What happens after a command execution error.
5.5 Interrupting or Killing make		What happens when a command is interrupted.
5.6 Recursive Use of make		Invoking make from makefiles.
5.7 Defining Canned Command Sequences		Defining canned sequences of commands.
5.8 Using Empty Commands		Defining useful, do-nothing commands.

5.1 Command Echoing

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' it only echoes commands, it won't execute them. 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 prerequisites has the same effect (see section Special Built-in Target Names). .SILENT is essentially obsolete since `@' is more flexible.

5.2 Command Execution

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. (2) 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.

On MS-DOS, if SHELL is not set, the value of the variable COMSPEC (which is always set) is used instead.

The processing of lines that set the variable SHELL in Makefiles is different on MS-DOS. The stock shell, `command.com', is ridiculously limited in its functionality and many users of make tend to install a replacement shell. Therefore, on MS-DOS, make examines the value of SHELL, and changes its behavior based on whether it points to a Unix-style or DOS-style shell. This allows reasonable functionality even if SHELL points to `command.com'.

If SHELL points to a Unix-style shell, make on MS-DOS additionally checks whether that shell can indeed be found; if not, it ignores the line that sets SHELL. In MS-DOS, GNU make searches for the shell in the following places:

1. In the precise place pointed to by the value of SHELL. For example, if the makefile specifies `SHELL = /bin/sh', make will look in the directory `/bin' on the current drive.

2. In the current directory.

3. In each of the directories in the PATH variable, in order.

In every directory it examines, make will first look for the specific file (`sh' in the example above). If this is not found, it will also look in that directory for that file with one of the known extensions which identify executable files. For example `.exe', `.com', `.bat', `.btm', `.sh', and some others.

If any of these attempts is successful, the value of SHELL will be set to the full pathname of the shell as found. However, if none of these is found, the value of SHELL will not be changed, and thus the line that sets it will be effectively ignored. This is so make will only support features specific to a Unix-style shell if such a shell is actually installed on the system where make runs.

Note that this extended search for the shell is limited to the cases where SHELL is set from the Makefile; if it is set in the environment or command line, you are expected to set it to the full pathname of the shell, exactly as things are on Unix.

The effect of the above DOS-specific processing is that a Makefile that says `SHELL = /bin/sh' (as many Unix makefiles do), will work on MS-DOS unaltered if you have e.g. `sh.exe' installed in some directory along your PATH.

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. However, on MS-DOS and MS-Windows the value of SHELL in the environment is used, since on those systems most users do not set this variable, and therefore it is most likely set specifically to be used by make. On MS-DOS, if the setting of SHELL is not suitable for make, you can set the variable MAKESHELL to the shell that make should use; this will override the value of SHELL.

5.3 Parallel Execution

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.

On MS-DOS, the `-j' option has no effect, since that system doesn't support multi-processing.

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 generated by the commands appears whenever each command sends 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.

Finally, handling recursive make invocations raises issues. For more information on this, see Communicating Options to a Sub-make.

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.

5.4 Errors in Commands

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 prerequisites. 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 prerequisites 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 5.5 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.

5.5 Interrupting or Killing 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 prerequisites 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.

5.6 Recursive Use of 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.

For your convenience, GNU make sets the variable CURDIR to the pathname of the current working directory for you. If -C is in effect, it will contain the path of the new directory, not the original. The value has the same precedence it would have if it were set in the makefile (by default, an environment variable CURDIR will not override this value). Note that setting this variable has no effect on the operation of make

5.6.1 How the MAKE Variable Works		The special effects of using `$(MAKE)'.
5.6.2 Communicating Variables to a Sub-make		How to communicate variables to a sub-make.
5.6.3 Communicating Options to a Sub-make		How to communicate options to a sub-make.
5.6.4 The `--print-directory' Option		How the `-w' or `--print-directory' option helps debug use of recursive make commands.

5.6.1 How the 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 9.3 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.

5.6.2 Communicating Variables to a Sub-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 section 5.6.3 Communicating Options to a Sub-make.

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.

5.6.3 Communicating Options to a Sub-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', `-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 `N' and your operating system supports it (most any UNIX system will; others typically won't), the parent make and all the sub-makes will communicate to ensure that there are only `N' jobs running at the same time between them all. Note that any job that is marked recursive (see section Instead of Executing the Commands) doesn't count against the total jobs (otherwise we could get `N' sub-makes running and have no slots left over for any real work!)

If your operating system doesn't support the above communication, then `-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:

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 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.

5.6.4 The `--print-directory' Option

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'.

when processing is completed.

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.

5.7 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:

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 prerequisite 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 `@'.)

5.8 Using Empty Commands

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 10. 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 prerequisites can be remade. However, this is not the best way to do that, because the prerequisites may not be remade properly if the target file actually does exist. See section Phony Targets, for a better way to do this.