More Uses of POSIX Manpages
Besides what Oli said about writing portable scripts (and hacking on coreutils), there are two other situations where the POSIX manual pages may come in handy:
1. You've configured (more) POSIX-compliant behavior.
If you set the
POSIXLY_CORRECT environment variable (to anything, it can even be blank), many GNU utilities and some other programs will behave in the fashion specified by POSIX, even when the developers saw no reason users would likely want this behavior.
This doesn't make your system behave like a truely POSIX-compliant OS. The Linux kernel, GNU libc, and many userlands tools are all deliberately designed to be POSIX-compliant only when doing so is more helpful than harmful. This is one of the reasons GNU/Linux systems like Ubuntu are widely considered not to be Unix systems.
The behavior of
ls is affected by a great many things, but is not affected by whether or not
POSIXLY_CORRECT is set. (You can verify this by checking the source code in, say, 13.04:
ls.c make no reference to that environment variable.)
But some other utilities are affected. For example, the
df utility prints disk usage information for all mounted devices. Normally, Ubuntu's
df (privided by GNU coreutils) shows this in 1 kilobyte blocks. With
POSIXLY_CORRECT set, it shows it in 512 B (i.e., half-kilobyte) blocks. That behavior is required by the POSIX standard, but probably not useful to most users, so it is not the default.
Filesystem 1K-blocks Used Available Use% Mounted on
/dev/sda8 15481840 11816640 2878768 81% /
udev 1020748 12 1020736 1% /dev
tmpfs 412840 5156 407684 2% /run
none 5120 0 5120 0% /run/lock
none 1032100 240 1031860 1% /run/shm
none 102400 32 102368 1% /run/user
/dev/sda6 245679 159043 73529 69% /boot
/dev/sda9 31458256 10024972 19835284 34% /home
/dev/sdd1 1922859824 1687175656 138008496 93% /media/ek/Noether
/dev/sdc1 1922859824 1700447368 124736784 94% /media/ek/Baker
/dev/sdb1 1922859824 1782944724 42239428 98% /media/ek/Spinoza
ek@Kip:~$ POSIXLY_CORRECT= df
Filesystem 512B-blocks Used Available Use% Mounted on
/dev/sda8 30963680 23573440 5817376 81% /
udev 2041496 24 2041472 1% /dev
tmpfs 825680 10312 815368 2% /run
none 10240 0 10240 0% /run/lock
none 2064200 480 2063720 1% /run/shm
none 204800 64 204736 1% /run/user
/dev/sda6 491358 318086 147058 69% /boot
/dev/sda9 62916512 20049944 39670568 34% /home
/dev/sdd1 3845719648 3374351312 276016992 93% /media/ek/Noether
/dev/sdc1 3845719648 3400894736 249473568 94% /media/ek/Baker
/dev/sdb1 3845719648 3565889448 84478856 98% /media/ek/Spinoza
2. There's no "regular" manpage for the command/topic you're interested in.
Sometimes the POSIX manual page is the only one available. For example, the
cd command is a shell builtin only. It's provided by different shells and behaves a little bit differently from shell to shell (in that different shells sometimes make
cd accept different command-line options).
The default interactive shell in Ubuntu is
bash and you can get information about
man bash. But if you want a manual page just for
cd, well, there's no
cd executable (no single globally usable, shell-independent
cd command is a required part of the POSIX standard--shells must implement it, and the POSIX standard "knows" about what it requires. So a POSIX manpage for
cd is possible, and exists.
cd on manpages.ubuntu.com shows the POSIX manual page and two others. This is another kind of example of multiple manual pages with the same name, by the way. What are the others? One is the
cd command in the language Tcl. The other is a CD-ROM driver in the FreeBSD operating system. Manual pages for FreeBSD are sometimes helpful to Ubuntu users, so a whole collection of them can be installed, including
man 4 cd (not one of the FreeBSD manual pages most likely to be helpful to Ubuntu users not also using FreeBSD).
Why Plan 9?
You might be wondering why there are Plan 9 manual pages in Ubuntu at all. After all, unlike Ubuntu (and many other OSes such as FreeBSD), Plan 9 is not even a Unix-style operating system, though as Oli says there are some similarities.
The reason for this is that the Plan 9 userland tools (the basic set of tools corresponding very roughly to coreutils) have been ported to Unix-like systems, so they can be run on OSes such as Ubuntu. They, and their manual pages, are provided by the
Some (not all) of the Plan 9 tools available for Ubuntu have the same name as Ubuntu tools, and perform the same or similar functions.
One of the reasons for having the Plan 9 tools on Ubuntu is that some of them don't correspond directly to any Ubuntu tool (but may still need the tools that do, for interoperability).
Another reason is to support software that depends on Plan 9 tools. For example, the window manager wmii used to be packaged for Ubuntu (and available in official Ubuntu software sources); this
wmii2 package depended on