cronlinuxunix
What are the differences?
I apologise for directly answering everything, but I don't know any useful tutorials, FAQs, etc. Basically what follows is 8 years of making desktop apps (that I help distribute), frustration and googling:
1. How do I figure out what arguments to pass to ./configure?
Practice really. Autotools is easy enough as it is consistent. But there's plenty of stuff out there using cmake, or custom build scripts. Generally, you shouldn't have to pass anything to configure, it should figure out if your system can build foo-tool or not.
Configure and GNU tools all look in /, /usr and /usr/local for dependencies. If you install anything anywhere else (which makes things painful if the dependency was installed by MacPorts or Fink), you will have to pass a flag to configure or modify the shell's environment to help GNU tools find these dependencies.
2. How shared libraries work under OS X / Linux - where they live on the filesystem, how ./configure && make finds them, what actually happens when they are linked against
On Linux they need to be installed to a path that the dynamic linker can find, this is defined by the LD_LIBRARY_PATH environment variable and the contents of /etc/ld.conf. On Mac it is the same for most open source software almost always (unless it is an Xcode Project). Except the env variable is DYLD_LIBRARY_PATH instead.
There is a default path that the linker searches for libraries. It is /lib:/usr/lib:/usr/local/lib
You can supplement this by using the CPATH variable, or CFLAGS or any number of other environment variables really (conveniently complicated). I suggest CFLAGS like so:
export CFLAGS="$CFLAGS -L/new/path"
The -L parameter adds to the link path.
Modern stuff uses the pkg-config tool. Modern stuff you install also installs a .pc file that describes the library and where it is and how to link to it. This can make life easier. But it doesn't come with OS X 10.5 so you'll have to install that too. Also a lot of basic deps don't support it.
The act of linking is just "resolve this function at runtime", really it's a big string table.
3. What are the actual differences between a shared and a statically linked library? Why can't I just statically link everything (RAM and disk space are cheap these days) and hence avoid weird library version conflicts?
When you link to a static library file the code becomes part of your application. It would be like if there was one giant .c file for that library and you compiled it into your application.
Dynamic libraries have the same code, but when the app is run, the code is loaded into the app at runtime (simplified explanation).
You can statically link to everything, however, sadly hardly any build systems make this easy. You'd have to edit build system files manually (eg. Makefile.am, or CMakeLists.txt). However this is probably worth learning if you regularly install things that require different versions of libraries and you are finding installing dependencies in parallel difficult.
The trick is to change the link line from -lfoo to -l/path/to/static/foo.a
You can probably find and replace. Afterwards check the tool doesn't link to the .so or dylib using ldd foo or otool -L foo
Another problem is not all libraries compile to static libraries. Many do. But then MacPorts or Debian may have decided not to ship it.
4. How can I tell what libraries I have installed, and what versions?
If you have pkg-config files for those libraries it is easy:
pkg-config --list-all
Otherwise you often can't easily. The dylib may have a soname (ie. foo.0.1.dylib, the soname is 0.1) that is the same as the library's version. However this is not required. The soname is a binary computability feature, you have to bump the major part of the soname if you change the format of the functions in the library. So you can get eg. version 14.0.5 soname for a 2.0 library. Although this is not common.
I got frustrated with this sort of thing and have developed a solution for this on Mac, and I'm talking about it next.
5. How can I install more than one version of a library without breaking my normal system?
My solution to this is here: http://github.com/mxcl/homebrew/
I like installing from source, and wanted a tool that made it easy, but with some package management. So with Homebrew I build, eg. wget myself from source, but make sure to install to a special prefix:
/usr/local/Cellar/wget/1.1.4
I then use the homebrew tool to symlink all that into /usr/local, so I still have /usr/local/bin/wget and /usr/local/lib/libwget.dylib
Later if I need a different version of wget I can install it in parallel and just change the version that is linked into the /usr/local tree.
6. If I am installing stuff from source on a system that is otherwise managed using packages, what's the cleanest way of doing so?
I believe the Homebrew way is cleanest, so use it or do the equivalent. Install to /usr/local/pkgs/name/version and symlink or hard link the rest in.
Do use /usr/local. Every build tool that exists searches there for dependencies and headers. Your life will be much easier.
7. Assuming I manage to compile something fiddly from source, how can I then package that up so other people don't have to jump through the same hoops? Particularly on OS X....
If it has no dependencies you can tar up the the build directory and give it to someone else who can then do "make install". However you can only do this reliably for the exact same versions of OS X. On Linux it will probably work for similar Linux (eg. Ubuntu) with the same Kernel version and libc minor version.
The reason it is not easy to distribute binaries on Unix is because of binary compatibility. The GNU people, and everyone else change their binary interfaces often.
Basically don't distribute binaries. Things will probably break in very strange ways.
On Mac, the best option is to make a macports package. Everyone uses macports. On Linux there are so many different build systems and combinations, I don't think there is any better advise than to write a blog entry about how you succeeded building x tool in y strange configuration.
If you make a package description (for macports or homebrew) then anyone can install that package, and it solves the dependency problems too. However this is often not easy, and it also isn't easy to get your macports recipe included in the main macports tree. Also macports doesn't support exotic installation types, they offer one choice for all packages.
One of my future goals with Homebrew is to make it possible to click a link on a website (eg. homebrew://blah and it will download that Ruby script, install the deps for that package and then build the app. But yeah, not yet done, but not too tricky considering the design I chose.
8. What are the command line tools I need to master to get good at this stuff? Stuff like otool, pkg-config etc.
otool is really only useful afterwards. It tells you what the built binary links to. When you are figuring out the dependencies of a tool you have to build, it is useless. The same is true of pkg-config as you will have already installed the dependency before you can use it.
My tool chain is, read the README and INSTALL files, and do a configure --help. Watch the build output to check it is sane. Parse any build errors. Maybe in future, ask on serverfault :)
set -e causes the shell to exit if any subcommand or pipeline returns a non-zero status.
The answer the interviewer was probably looking for is:
It would be dangerous to use "set -e" when creating init.d scripts:
From http://www.debian.org/doc/debian-policy/ch-opersys.html 9.3.2 --
Be careful of using set -e in init.d scripts. Writing correct init.d scripts requires accepting various error exit statuses when daemons are already running or already stopped without aborting the init.d script, and common init.d function libraries are not safe to call with set -e in effect. For init.d scripts, it's often easier to not use set -e and instead check the result of each command separately.
This is a valid question from an interviewer standpoint because it gauges a candidates working knowledge of server-level scripting and automation
Best Answer
Dcron, cronie and vixie-cron (but not fcron) provide a crond daemon binary, but they implement it in their own way. Anacron was originally a standalone application which was meant to run cron jobs in systems not expected to be always on (thus the jobs are run when the system is actually on). Fcron provides anacron-like functionalities to the articulate syntax of its fcrontabs, thus it does need to include a separate anacron binary.
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