Fedora – Why does the rpm command have a ‘–pipe’ option

The command man -k queries against a pre-compiled database and not the manual pages themselves.

I suspect that entries may have been made in the database (see man mandb for details) for pages that don't actually exist. I am not familiar enough with the RPM mechanisms to know how this could have happened.

In a similar vein, there is considerable flexibility in what section a given manual page may claim to live. For example, on my system man Carp claims to be in section "3perl" where the underlying file is stored in .../man3/Carp.3perl.gz. The commands

man Carp
man -s 3 Carp
man -s 3perl Carp

all yield the same page while man -s 3junk Carp complains that there is no such entry.

You might find mlocate (a.k.a. locate) to be useful for hunting files by name. I presume it is available for RedHat since redacted@redhat.com is credited as the author.

Named pipes (fifo) have four three advantages I can think of:

• you don't have to start the reading/writing processes at the same time
• you can have multiple readers/writers which do not need common ancestry
• as a file you can control ownership and permissions

• they are bi-directional, unnamed pipes may be unidirectional *

  ^{*) Think of a standard shell | pipeline which is unidirectional, several shells (ksh, zsh, and bash) also offer coprocesses which allow bi-directional communication. POSIX treats pipes as half-duplex (i.e. each side can only read or write), the pipe() system call returns two file handles and you may be required to treat one as read-only and the other as write-only. Some (BSD) systems support read and write simultaneously (not forbidden by POSIX), on others you would need two pipes, one for each direction. Check your pipe(), popen() and possibly popen2() man pages. The undirectionality may not be dependent on whether the pipe is named or not, though on Linux 2.6 it is dependent.}

(Updated, thanks to feedback from Stephane Chazelas)

So one immediately obvious task you cannot achieve with an unnamed pipe is a conventional client/server application.

The last (stricken) point above about unidirectional pipes is relevant on Linux, POSIX (see popen()) says that a pipe need only be readable or writeable, on Linux they are unidirectional. See Understanding The Linux Kernel (3rd Ed. O'Reilly) for Linux-specific details (p787). Other OS's offer bidirectional (unnamed) pipes.

As an example, Nagios uses a fifo for its command file. Various external processes (CGI scripts, external checks, NRPE etc) write commands/updates to this fifo and these are processed by the persistent Nagios process.

Named pipes have features not unlike TCP connections, but there are important differences. Because a fifo has a persistent filesystem name you can write to it even when there is no reader, admittedly the writes will block (without async or non-blocking I/O), though you won't loose data if the receiver isn't started (or is being restarted).

For reference, see also Unix domain sockets, and the answer to this Stackoverflow question which summarises the main IPC methods, and this one which talks about popen()