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# 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-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.

## 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'`, 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.

## 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`.

## 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 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.

## 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 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.

## 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 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.

## 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`

### 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 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.

### 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 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 `make`s, 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`.

### 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, ``-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-`make`s, 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 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.

### 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-`make`s. `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.

## 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 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 ``@'`.)

## 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 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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