8.1.3. Migrating to UTF-8
The generalization of UTF-8 encoding has been a long awaited solution to numerous difficulties with interoperability, since it facilitates international exchange and removes the arbitrary limits on characters that can be used in a document. The one drawback is that it had to go through a rather difficult transition phase. Since it could not be completely transparent (that is, it could not happen at the same time all over the world over), two conversion operations were required: one on file contents, and the other on filenames. Fortunately, the bulk of this migration has been completed and we discuss it largely for reference.
CULTURE Mojibake and interpretation errors
When a text is sent (or stored) without encoding information, it is not always possible for the recipient to know with certainty what convention to use for determining the meaning of a set of bytes. You can usually get an idea by getting statistics on the distribution of values present in the text, but that doesn't always give a definite answer. When the encoding system chosen for reading differs from that used in writing the file, the bytes are mis-interpreted, and you get, at best, errors on some characters, or, at worst, something completely illegible.
Thus, if a French text appears normal with the exception of accented letters and certain symbols which appear to be replaced with sequences of characters like “é” or è” or “ç”, it is probably a file encoded as UTF-8 but interpreted as ISO-8859-1 or ISO-8859-15. This is a sign of a local installation that has not yet been migrated to UTF-8. If, instead, you see question marks instead of accented letters — even if these question marks seem to also replace a character that should have followed the accented letter — it is likely that your installation is already configured for UTF-8 and that you have been sent a document encoded in Western ISO.
So much for “simple” cases. These cases only appear in Western culture, since Unicode (and UTF-8) was designed to maximize the common points with historical encodings for Western languages based on the Latin alphabet, which allows recognition of parts of the text even when some characters are missing.
In more complex configurations, which, for example, involve two environments corresponding to two different languages that do not use the same alphabet, you often get completely illegible results — a series of abstract symbols that have nothing to do with each other. This is especially common with Asian languages due to their numerous languages and writing systems. The Japanese word mojibake has been adopted to describe this phenomenon. When it appears, diagnosis is more complex and the simplest solution is often to simply migrate to UTF-8 on both sides.
As far as file names are concerned, the migration can be relatively simple. The convmv tool (in the package with the same name) was created specifically for this purpose; it allows renaming files from one encoding to another. The use of this tool is relatively simple, but we recommend doing it in two steps to avoid surprises. The following example illustrates a UTF-8 environment containing directory names encoded in ISO-8859-15, and the use of convmv to rename them.
$ ls travail/
Ic?nes ?l?ments graphiques Textes
$ convmv -r -f iso-8859-15 -t utf-8 travail/
Starting a dry run without changes...
mv "travail/�l�ments graphiques" "travail/Éléments graphiques"
mv "travail/Ic�nes" "travail/Icônes"
No changes to your files done. Use --notest to finally rename the files.
$ convmv -r --notest -f iso-8859-15 -t utf-8 travail/
mv "travail/�l�ments graphiques" "travail/Éléments graphiques"
mv "travail/Ic�nes" "travail/Icônes"
Ready!
$ ls travail/
Éléments graphiques Icônes Textes
For the file content, conversion procedures are more complex due to the vast variety of existing file formats. Some file formats include encoding information that facilitates the tasks of the software used to treat them; it is sufficient, then, to open these files and re-save them specifying UTF-8 encoding. In other cases, you have to specify the original encoding (ISO-8859-1 or “Western”, or ISO-8859-15 or “Western (European)”, according to the formulations) when opening the file.
For simple text files, you can use recode (in the package of the same name) which allows automatic recoding. This tool has numerous options so you can play with its behavior. We recommend you consult the documentation, the recode(1) man page, or the recode info page (more complete).
8.2. Configuring the Network
BACK TO BASICS Essential network concepts (Ethernet, IP address, subnet, broadcast).
Most modern local networks use the Ethernet protocol, where data is split into small blocks called frames and transmitted on the wire one frame at a time. Data speeds vary from 10 Mb/s for older Ethernet cards to 10 Gb/s in the newest cards (with the most common rate currently growing from 100 Mb/s to 1 Gb/s). The most widely used cables are called 10BASE-T, 100BASE-T, 1000BASE-T or 10GBASE-T depending on the throughput they can reliably provide (the T stands for “twisted pair”); those cables end in an RJ45 connector. There are other cable types, used mostly for speeds in above 1 Gb/s.
An IP address is a number used to identify a network interface on a computer on a local network or the Internet. In the currently most widespread version of IP (IPv4), this number is encoded in 32 bits, and is usually represented as 4 numbers separated by periods (e.g. 192.168.0.1), each number being between 0 and 255 (inclusive, which corresponds to 8 bits of data). The next version of the protocol, IPv6, extends this addressing space to 128 bits, and the addresses are generally represented as series of hexadecimal numbers separated by colons (e.g., 2002:58bf:13bb:0002:0000:0000:0020, or 2002:58bf:13bb:2::20 for short).
A subnet mask (netmask) defines in its binary code which portion of an IP address corresponds to the network, the remainder specifying the machine. In the example of configuring a static IPv4 address given here, the subnet mask, 255.255.255.0 (24 “1”s followed by 8 “0”s in binary representation) indicates that the first 24 bits of the IP address correspond to the network address, and the other 8 are specific to the machine. In IPv6, for readability, only the number of “1”s is expressed; the netmask for an IPv6 network could, thus, be 64.
The network address is an IP address in which the part describing the machine's number is 0. The range of IPv4 addresses in a complete network is often indicated by the syntax, a.b.c.d/e, in which a.b.c.d is the network address and e is the name of the bits affected by the network part in an IP address. The example network would be written thus: 192.168.0.0/24. The syntax is similar in IPv6: 2002:58bf:13bb::/64.
A router is a machine that connects several networks to each other. All traffic coming through a router is guided to the correct network. To do this, the router analyses incoming packets and redirects them according to the IP address of their destination. The router is often known as a gateway; in this configuration, it works as a machine that helps reach out beyond a local network (towards an extended network, such as the Internet).