Friday, 11 December 2009

GNU Make: a very short introduction

I finally got sick and tired of having a slightly different command line environment in each of the Linux/UNIX environments I use. This led me to put some effort into collating my various dotfiles (for bash, vim and screen) and coming up with a consolidated version of each, tracked using git, that I could install on any machine which I had a unique home directory on and not have to remember which of my preferences I've configured on this particular machine. At some point, I may post annotated version of said files here, along with what lead me to implement the various things in each file.

Today's post, however, is to do with a tool called 'make'. Make is most commonly associated with managing software builds on UNIX/Linux platforms, but more generically it can be described as a dependency tracking engine. A 'makefile' consists of a set of rules of the form:
<target1> [target2 ...]: [dependency(ies)] 
    [command 1]
    [command 2]

Where 'targets' are files that are being generated, 'dependencies' are other files (or other make targets) on which the current target depends, and 'commands' are commands to execute in order to generate an up to date version of the current 'target'.

In the case of my dotfiles, I wished to be able to do a few things without retyping excessive numbers of commands:

  1. For each of the three subdirectories in my 'dotfiles' project, create a gzip compressed TAR archive, named <project>.tgz.

  2. Additionally, my 'vim' directory contains several colour scheme definitions. I want two versions of my 'vim' archive, with and without the 'colors' directory.

  3. I would like the ability to automatically copy these files to a directory accessible via HTTP, allowing me easy access to them wherever I have internet access.

  4. I would like the ability to delete all existing archives in the project directory.

The naive way

So, let's assume that I don't have make available - how would I go about each of these tasks?

Creating archives (ignoring any advanced shell features, for simplicity):
[muz@codd:dotfiles]$ l
bash/ screen/ vim/
[muz@codd:dotfiles]$ tar -czf bash.tgz bash/
[muz@codd:dotfiles]$ tar -czf screen.tgz screen/
[muz@codd:dotfiles]$ tar -czf vim.tgz vim/
[muz@codd:dotfiles]$ l 
bash/ bash.tgz screen/ screen.tgz vim/ vim.tgz

Creating separate archives for vim with and without colors:
[muz@codd:dotfiles]$ ls vim/
colors/ vimrc
[muz@codd:dotfiles]$ tar -cvzf vim-no-colors.tgz vim/ --exclude=colors

Copying to a directory:
[muz@codd:dotfiles]$ cp *.tgz ~/public_html/dotfiles/
[muz@codd:dotfiles]$ l !$
l ~/public_html/dotfiles/
bash.tgz  screen.tgz  vim-no-colors.tgz  vim.tgz

Removing all the tars:
[muz@codd:dotfiles]$ l
bash/  bash.tgz  screen/  screen.tgz  vim/  vim-no-colors.tgz  vim.tgz
[muz@codd:dotfiles]$ rm ./*.tgz
[muz@codd:dotfiles]$ l
bash/  screen/  vim/

The make way

Based on my initial requirements, this was my initial makefile:
all: bash.tgz screen.tgz vim.tgz vim-all.tgz
        rm -f *.tgz
bash.tgz: ./bash/*
        tar -czf bash.tgz ./bash/
screen.tgz: ./screen/*
        tar -czf screen.tgz ./screen/
vim.tgz: ./vim/*
        tar -czf vim.tgz ./vim/ --exclude=colors
vim-all.tgz: ./vim/*
        tar -czf vim-all.tgz ./vim/
publish: all
        cp *.tgz ~/public_html/dotfiles/
        chmod go+r ~/public_html/dotfiles/*.tgz

This does everything that I initially specified.

make all

Create four .tgz files; bash, screen, vim, and vim-all, if any of the files that an archive contains have changed. (I changed vim to mean vim without the colors directory.)

make clean

Remove all files ending .tgz. This is an example of a 'phony target'; a make target that does not refer to a specific file, but instead causes a set of commands to be executed whenever it is invoked.

make publish

Copy all *.tgz files in the project directory to a web directory, and make them readable.

However, this is fairly verbose, and there is a lot of repetition. Using some variables/macros (some internal to make, some defined by me) my final make file is a little less redundant:
# Make file for tarring and publishing dotfiles.

TARCMD=tar -czf

all: bash.tgz screen.tgz vim.tgz vim-all.tgz
        rm -f $(ARCHIVES)
bash.tgz: ./bash/*
        $(TARCMD) $@ ./bash/
screen.tgz: ./screen/*
        $(TARCMD) $@ ./screen/
vim.tgz: ./vim/*
        $(TARCMD) $@ ./vim/ --exclude=colors
vim-all.tgz: ./vim/*
        $(TARCMD) $@ ./vim/
publish: all
        cp $(ARCHIVES) $(HTDOCS)
        chmod go+r $(HTDOCS)/$(ARCHIVES)

(The in-built variable `@' refers to the name of the current target.)

Make can be used to simplify any procedure that requires multiple commands to be run as various files change. Software builds, operating system configuration files, document publishing... the list goes on. From a system/infrastructure administration point of view, there is a strong argument to be made that a makefile is the logical endstate of getting your documentation up to date. Rather than having a document that a human operator follows, have a makefile that a system can execute, and remove as much possibility for human error as possible. (Look for another post on the subject of human error soon.)

To quote an anonymous system administrator: "Everything should be a Makefile."

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