Understanding the Unix and Linux Command: make

Man using laptop
lina aidukaite/Moment/Getty Images

The purpose of the make utility is to determine automatically which pieces of a large program need to be recompiled and issue the commands to recompile them. The manual describes the GNU implementation of make, which was written by Richard Stallman and Roland McGrath. These examples show C programs since they are most common, but you can use make with any programming language whose compiler can be run with a shell command. In fact, make is not limited to programs. You can use it to describe any task where some files must be updated automatically from others whenever the others change.

Note: This page is an extract of the documentation of GNU make. It is updated only occasionally because the GNU project does not use nroff. For complete, current documentation, refer to the info file make.info which is made from the Texinfo source file make.texinfo.  

Syntax and preparation of make utility

make -f makefile ] [ option ] ... target ...  

To prepare to use make, you must write a file called the makefile that describes the relationships among files in your program and states the commands for updating each file. Typically an executable file is updated from object files, which are in turn made by compiling source files.

Once a suitable makefile exists, each time you change some source files this simple shell command make suffices to perform all necessary recompilations. The make program uses the makefile data base and the last-modification times of the files to decide which of the files need to be updated. For each of those files, it issues the commands recorded in the database.

The make utility executes commands in the makefile to update one or more target names, where name is typically a program. If no -f option is present, make will look for the makefiles GNUmakefile, makefile, and Makefile, in that order.

Normally you should call your makefile either makefile or Makefile. (It is recommended to use Makefile because it appears prominently near the beginning of a directory listing, right near other important files such as README.) The first name-checked, GNUmakefile, is not recommended for most makefiles. You should use this name if you have a makefile that is specific to GNU make and will not be understood by other versions of make. If makefile is -, the standard input is read.

The make utility updates a target if it depends on prerequisite files that have been modified since the target was last modified, or if the target does not exist.  

Options

The -b and -m options are ignored for compatibility with other versions of make.

-C dir will change to directory dir before reading the makefiles or doing anything else. If multiple -C options are specified, each is interpreted relative to the previous one: -C -C etc is equivalent to -C /etc. This is typically used with recursive invocations of make.

The -d option will print debugging information in addition to normal processing. The debugging information says which files are being considered for remaking, which file times are being compared and with what results, which files actually need to be remade, which implicit rules are considered and which are applied—everything interesting about how make decides what to do.

The -e option gives variables taken from the environment precedence over variables from makefiles.

The -f file option uses file as a makefile.

The -i option ignores all errors in commands executed to remake files.

The -I dir option specifies a directory dir to search for included makefiles. If several -I options are used to specify several directories, the directories are searched in the order specified. Unlike the arguments to other flags of make, directories given with -I flags may come directly after the flag: -Idir is allowed, as well as -I dir. This syntax is allowed for compatibility with the C preprocessor's -I flag.

The -j jobs option specifies the number of jobs (commands) to run simultaneously. If there is more than one -j option, the last one is effective. If the -j option is given without an argument, make will not limit the number of jobs that can run simultaneously.

The -k option continues as much as possible after an error; while the target that failed, and those that depend on it, cannot be remade, the other dependencies of these targets can be processed all the same.

The -l and -l load options specify that no new jobs (commands) should be started if there are others jobs running, and the load average is at least load (a floating-point number). With no argument, it removes a previous load limit.

The -n option prints the commands that would be executed, but does not execute them.

The -o file option does not remake the file even if it is older than its dependencies, and does not remake anything on account of changes in file. Essentially the file is treated as very old and its rules are ignored.

The -p option prints the database (rules and variable values) that results from reading the makefiles. It then executes as usual or as otherwise specified. This also prints the version information given by the -v switch (see below). To print the database without trying to remake any files, use make -p -f/dev/null.

The -q option sets question mode. Do not run any commands or print anything, just return an exit status that is zero if the specified targets are already up to date, nonzero otherwise.

The -r option eliminates the use of the built-in implicit rules. Also clears out the default list of suffixes for suffix rules.

The -s option silences the operation. It does not print the commands as they are executed.

The -S option cancels the effect of the -k option. This is never necessary except in a recursive make where -k might be inherited from the top-level make via MAKEFLAGS or if you set -k in MAKEFLAGS in your environment.

The -t option touches files (marks them up to date without really changing them) instead of running their commands. This is used to pretend that the commands were done, in order to fool future invocations of make.

The -v option prints the version of the make program plus a copyright, a list of authors, and a notice that there is no warranty.

The -w option prints a message containing the working directory before and after other processing. This may be useful for tracking down errors from complicated nests of recursive make commands.

The -W file option pretends that the target file has just been modified. When used with the -n flag, this shows you what would happen if you were to modify that file. Without -n, it is almost the same as running a touch command on the given file before running make, except that the modification time is changed only in the imagination of make.