Terminfo (formerly Termcap) is a database of terminal capabilities and more. For every (well almost) model of terminal it tells application programs what the terminal is capable of doing. It tells what escape sequences (or control characters) to send to the terminal in order to do things such as move the cursor to a new location, erase part of the screen, scroll the screen, change modes, change appearance (colors, brightness, blinking, underlining, reverse video etc.). After about 1980, many terminals supported over a hundred different commands (some of which take numeric parameters).
One way in which terminfo gives the its information to an application program is via the "ncurses" functions that a programmer may put into a C program. For example, if a program wants to move the cursor to row 3, col 6 it simply calls: move(3,6). The move() function (part of ncurses) knows how to do this for your terminal (it has read terminfo). So it sends the appropriate escape sequence to the terminal to make this particular move for a certain terminal. Some programs get info directly from a terminfo files without using ncurses. Thus a Linux package that doesn't require ncurses may still need a terminfo file for your terminal.
The terminfo abbreviations are usually longer than those of termcap and thus it's easier to guess what they mean. The manual pages for terminfo are more detailed (and include the old termcap abbreviations). Also, the termcap entries had a size limitation which is not present for terminfo. Thus, unless you are already committed to working with termcap, it's suggested you use terminfo.
The terminfo database is compiled and thus has a source part and a compiled part. The old termcap database has only a source part but this source can, by a single command, be both converted to terminfo source and then compiled. Thus you may get by without having any terminfo source since the termcap source can create the compiled terminfo database. To see a display of the database for the terminal you're now using (including a PC monitor) type "infocmp" and you should see the source terminfo "file" for it.
To see if your terminal (say vt100) is in the terminfo data base type "locate vt100". If you need to find the terminfo name for your terminal, explore the listing of files in the compiled database or see What is the terminfo name of my terminal ?
Typing "locate vt100" may show /usr/lib/terminfo/v/vt100, /usr/share/terminfo/v/vt100, /home/.../.terminfo/v/vt100, and/or /etc/terminfo/v/vt100. All these are possible locations of the compiled terminfo files. Although the /etc/terminfo directory is not a standard location for it, having a few terminal types there could be useful in case the /usr directory is not accessible. For example /usr could be on a separate disk or partition that failed to mount. Normally, programs that use your main terminfo data base are able to find it if it's in at least one of the locations mentioned above. Otherwise the environment variable TERMINFO may be set to the path to this database. Example: TERMINFO=/usr/share/terminfo
For the Debian Distribution of Linux, several commonly used terminals (including the monitor-console) are in the ncurses-term package. These are put into /etc/terminfo/. All of the terminals in the database are in the ncurses-bin package and go into /usr/share/terminfo/.
If the compiled terminfo is in more than one location, everything is usually OK until someone installs new terminfo files (from a newer distribution, from the net, by editing the old one, etc.). Each new terminfo file needs replace all the existing older copies of that file (at various locations) unless you abolish redundant locations. If you don't ensure this gets done, then some application programs could wind up still finding and using the old (and possibly buggy) terminfo data that sill exists in a "possible" location. Setting the environment variable TERMINFO to the up-to-date location (as mentioned above) would help avoid this problem.
While the source-code file may not be installed on your computer there's another way to get the source-code if you have the compiled code. Just use the "infocmp" command.
The source code file (for all terminals) may be /etc/termcap and/or terminfo.src (or another name). See the man pages: terminfo(5) or termcap(5) for the format required to create (or modify) these source files. The file terminfo.src may be in various locations on your computer or it may not be included with your Linux distribution. Use the locate command to try to find it. It is available on the web at http://www.tuxedo.org/terminfo or http://download.sourceforge.net/mirrors/OpenBSD/src/share/termtypes/termtypes.master
The data in the source files is compiled with the "tic" program which is capable of converting between termcap format and terminfo format. Thus you can create a compiled terminfo data base from termcap source. The installation program which was used to install Linux probably installed the compiled files on your hard disk so you don't need to compile anything unless you modify /etc/termcap (or terminfo.src ). "tic" will automatically install the resulting compiled files into a terminfo directory ready to be used by application programs. Which location it's installed in depends on ... See "man tic" for the explanation.
It's a good idea to take a look at the terminfo entry for the terminal you are using (source code of course) and read the comments. A quick way to inspect it without comments is to just type "infocmp". But the comments may tell you something special about the terminal such as how you need to set it up so that it will work correctly with the terminfo database.
In order to save disk space, one may delete all of the terminfo database except for the terminals types that you have (or might need in the future). Don't delete any of the termcaps for a "Linux terminal" (the console) or the xterm ones if you use X Window. The terminal type "dumb" may be needed when an application program can't figure out what type of terminal you are using. It would save disk space if install programs only installed the terminfo for the terminals that you have and if you could get a termcap for a newly installed terminal over the Internet in a few seconds.
Unfortunately, there are a number of bugs in the terminfo and termcap files. In addition, many of these terminfo files are incomplete and do not define certain features available on the terminals. Sometimes you can get by without modifying the terminfo but in other cases you need to modify it or possibly use another emulation that has a good terminfo.
The sad state of the supplied terminfo files is due to a number of reasons. One is that during the 1980's when many of them were written (often in termcap format), application programs did not utilize more advanced terminal features. Thus if such feature were not in the termcap (or terminfo) file, no one complained. Today, programs such as vim use "context highlighting" and minicom uses the terminal's graphics character set. These often need more definitions to be added to the old termcap. This may (or may not) have already been done.
Most terminals had hardware bugs (in their firmware) and sometimes these were "fixed" by modifying the termcap. Then the manufacturer might send out replacement chips which would fix the bug. Not all owners would bother to get the replacement chips. Thus there may be 2 or more terminfos for your terminal, depending on what firmware chips it has in it. This situation was often not noted in the termcap and only one termcap may be supplied with Linux. Some hardware bugs which existed for features that were almost never used in the past likely never did get fixed. Also, some reported hardware bugs may never have been fixed since they were not of much significance at the time or because the company went out of business, etc.
To do this you need a manual for your terminal showing what escape sequences it uses. Newer manuals from the 1990's often don't show this. You also need a terminfo manual (the man page "terminfo" is one). After you edit the terminfo source file you compile it using "tic". "tic" should automatically put the compiled terminfo file in the correct directory reserved for it.
If you would like to find a better terminfo entry for a certain terminal than the one supplied, you might try searching the Internet (but what you find could be worse). If your new terminfo entry is better than the old one and it seems stable (you've used it for a while with no problems) then you should send a copy to the maintainer of terminfo as noted at the start of the source file for terminfo (or termcap).
Included in the terminfo are often a couple of initialization strings which may be sent to the terminal to initialize it. This may change the appearance of the screen, change what mode the terminal is in, and/or make the terminal emulate another terminal. An initialization string is not automatically sent to the terminal to initialize it. One might expect that the getty program should do this but if it did, one could make a change to the set-up at the terminal and this change wouldn't be happen because the init string would automatically cancel it. Application programs don't seem to initialize either. To do this you must use a command given on the command line (or in a shell script such as /etc/profile) to send the init strings. Such commands are: "tset", "tput init", or "setterm -initialize". Sometimes there is no need to send the init strings since the terminal may set itself up correctly when it is powered on (using options/preferences one has set up and saved in the non-volatile memory of the terminal).
The Environment variable TERM should be set to the name of terminal which you are using. If TERM hasn't been set yet and you don't know the name of your terminal see What is the terminfo name of my terminal ?. It is normally set by the terminal_type parameter passed to the getty program (look at it in the /etc/inittab file). This name must be in the Terminfo data base. Just type "set" at the command line to see what TERM is set to (or type: tset -q). At a console (monitor) TERM is set to "linux" which is the PC monitor emulating a fictitious terminal model named "linux". Since "linux" is close to a vt100 terminal and many text terminals are also, the "linux" designation will sometimes work as a temporary expedient with a text terminal.
If more than one type of terminal may be connected to the same port (/dev/tty...) (for example, if there is a switch to permit different terminal types to use the same serial port, or if the port is connected to a modem to which people call in from different types of terminals) then TERM needs to be set each time someone connects to the serial port. There is often a query escape sequence so that the computer may ask the terminal what type it is. Another way is to ask the user to type in (or select) the type of terminal s/he is using. You may need to use tset for this or write a short shell script to handle this.
One way to do this is to use "tset" (see the manual page). tset tries to determine the terminal name of the terminal you are using. Then it looks up the data in terminfo and sends your terminal an init string. It can also set the value of TERM. For example, a user dials in and logs in. The .profile login script is executed which contains within it the following statement: eval `tset -s ?vt100`. This results in: The user is asked if s/he is using a vt100. The user either responds yes or types in the actual terminal type s/he is using. Then tset sends the init string and sets TERM to this terminal name (type).