Editing Make (software) (section)

===Rules===

Each rule begins with a ''dependency line'' which consists of the rule's [[Orientation (graph theory)|target]] name followed by a colon (:), and optionally a list of targets (also known as prerequisites) on which the rule's target depends.<ref>{{cite web|url=https://www.gnu.org/software/make/manual/html_node/Prerequisite-Types.html|title=Prerequisite Types (GNU make)|website=GNU.org|publisher=[[GNU Project]]|access-date=15 December 2020|archive-date=2 December 2020|archive-url=https://web.archive.org/web/20201202083734/https://www.gnu.org/software/make/manual/html_node/Prerequisite-Types.html|url-status=live}}</ref>

 target [target ...]: [component ...]
 {{keypress|TAB}}[command 1]
 	   .
 	   .
 	   .
 {{keypress|TAB}}[command n]

Usually a rule has a single target, rather than multiple.

A dependency line may be followed by a recipe: a series of [[Tab character|TAB]] indented command lines that define how to generate the target from the components (i.e. source files). If any prerequisite has a more recent timestamp than the target file or the target does not exist as a file, the recipe is performed.

The first command may appear on the same line after the prerequisites, separated by a semicolon,
<syntaxhighlight lang="make">
targets: prerequisites ; command
</syntaxhighlight>
for example,
<syntaxhighlight lang="make">
hello: ; @echo "hello"
</syntaxhighlight>

Each command line must begin with a tab character. Even though a [[space character|space]] is also [[Whitespace (computer science)|whitespace]], Make requires tab. Since this often leads to confusion and mistakes, this aspect of makefile syntax is subject to criticism. [[Eric S. Raymond]] describes it as "one of the worst design botches in the history of Unix"<ref name="esr">"Chapter 15. Tools: make: Automating Your Recipes", ''The Art of Unix Programming'', [[Eric S. Raymond]] 2003</ref> and ''[[The Unix-Haters Handbook]]'' said "using tabs as part of the syntax is like one of those pungee [sic] stick traps in [[The Green Berets (film)|''The Green Berets'']]". Feldman explains the choice as caused by a [[Technical debt|workaround]] for an early implementation difficulty, and preserved by a desire for [[backward compatibility]] with the very first users:

{{blockquote|Why the tab in column 1? [[Yacc]] was new, [[Lex (software)|Lex]] was brand new. I hadn't tried either, so I figured this would be a good excuse to learn. After getting myself snarled up with my first stab at Lex, I just did something simple with the pattern newline-tab. It worked, it stayed. And then a few weeks later I had a user population of about a dozen, most of them friends, and I didn't want to screw up my embedded base. The rest, sadly, is history.|Stuart Feldman<ref name="esr"/>}}

GNU Make since version 3.82 allows the choice of any symbol (one character) as the recipe prefix using the .RECIPEPREFIX special variable:
<syntaxhighlight lang="make">
.RECIPEPREFIX := :
all:
:@echo "recipe prefix symbol is set to '$(.RECIPEPREFIX)'"
</syntaxhighlight>

Each command is executed in a separate [[Unix shell|shell]]. Since operating systems use different shells, this can lead to unportable makefiles. For example, GNU Make (all POSIX Makes) executes commands with [[/bin/sh]] by default, where [[Unix]] commands like [[cp (Unix)|cp]] are normally used. In contrast, Microsoft's ''nmake'' executes commands with cmd.exe where [[Batch file|batch]] commands like [[copy (command)|copy]] are available but not necessarily cp.

Since a recipe is optional, the dependency line can consist solely of components that refer to other targets:
<syntaxhighlight lang="make">
realclean: clean distclean
</syntaxhighlight>

The following example rule is evaluated when Make updates target file.txt via {{code|make file.txt}}. If file.html is newer than file.txt or file.txt does not exist, then the command is run to generate file.txt from file.html.

<syntaxhighlight lang="make">
file.txt: file.html
	lynx -dump file.html > file.txt
</syntaxhighlight>

A command can have one or more of the following prefixes (after the tab):
* [[hyphen-minus|minus]] (-) specifies to ignore an error from the command
* [[at sign|at]] (@) specifies to ''not'' output the command before it is executed
* [[plus sign|plus]] (+) specifies to execute the command even if Make is invoked in "do not execute" mode

Ignoring errors and silencing echo can alternatively be obtained via the special targets {{code|.IGNORE}} and {{code|.SILENT}}.<ref name=SUS>{{man|1|make|SUS}}</ref>

Microsoft's NMAKE has predefined rules that can be omitted from these makefiles, e.g. {{code|c.obj   $(CC)$(CFLAGS)|lang=make}}.