The feature sets of many of the latest Linux products are often so vast that it may take four to six CDs to hold them all. But a number of developers are going the other direction, slimming Linux down to the bare minimum to meet the needs of the latest embedded Linux systems. Embedded Linux systems are growing in number, popularity and usefulness. Witness the launch of Embedded Linux Journal as a sign of the times.

TinyLogin is a utility intended for embedded systems. It provides the functionality of ten login and user-management utilities in a single tiny binary. TinyLogin was started by Sean Bastille back in 1998. It is now maintained as an open-source project by Erik Andersen, sponsored by his employer, Lineo, Inc. Erik also maintains the BusyBox utility; it's no coincidence that the two are designed to work well together.

Why have a single binary for these ten login utilities? The standard utilities that TinyLogin replaces occupy 140K. That isn't much space, but the attitude of an embedded Linux developer is generally to avoid wasting any space on things that are not needed. The TinyLogin binary is 40K. It doesn't have all the options of the programs it replaces, but most embedded systems don't need them all.

Furthermore, since many developers use GNU libc, TinyLogin allows embedded Linux systems the option to bypass the GNU Name Service Switch (NSS) libraries. These libraries allow GNU libc to use a number of alternative methods for obtaining information such as user names and passwords. Most embedded systems have no need for this functionality, so by eliminating these libraries from their target systems, developers can save another 200K.

Linux distributions normally include packages with many separate utilities, often in sets named initscripts, textutils, shellutils or something similar. These utilities and scripts are generally quite feature ladened. The command to list files, ls, supports over 50 command-line options. Because these utilities are so full-featured, they are sometimes larger than expected. For example, on my system the ls command (dynamically linked) occupies 48KB of disk space. While you wouldn't notice that on a standard PC, many of the 50 options are just wasted space on an embedded system or boot disk. TinyLogin combines numerous utilities in a single binary, limiting the features of each utility to those most commonly needed. As another example, the adduser command in TinyLogin supports only three options rather than the 20-plus options of a typical adduser command.

When you enter the name of a utility in a shell it attempts to locate a binary with that name and then execute it. When you install TinyLogin, it creates symbolic links in your filesystem so that instead of having a real adduser command, you have a symbolic link that refers to the TinyLogin binary. A directory listing of adduser would then appear like this:

lrwxrwxrwx 1 nwells users 12 Jan 17 14:47
<@cont_arrow><\#229><@$p>adduser -> /bin/tinylogin

Typing adduser at the command prompt causes the shell to launch the TinyLogin program, which in turn examines the text that you entered to launch it. The program does this by examining the argv programming variable (similar to the positional variables in shell scripting). Seeing that you entered adduser, the TinyLogin program proceeds to act like adduser, interpreting any additional items on the command line as adduser would.

TinyLogin includes the following ten utilities:

You'll have to judge for yourself if the features included with each of these commands will meet the needs of your embedded-system project. For example, the getty command implemented in TinyLogin doesn't use the /etc/gettydefs or /etc/gettytab configuration files. Instead it relies on a number of command-line options. Fortunately, TinyLogin is completely modular. If you need a more full-featured utility, you can always disable any of the TinyLogin utilities that you prefer not to use and use the standard Linux tools to provide those features.

Compiling TinyLogin is simple because no special libraries or configuration steps are necessary. First obtain the source code by visiting http://tinylogin.lineo.com or ftp://opensource.lineo.com/tinylogin and download the most recent version of the compressed tar file. At the time of this writing, the most recent version is the file tinylogin-0.80.tar.gz, but a more recent version may be available soon. Work on TinyLogin seems to progress at a steady pace.

After you've download the tar file, place it in a working directory and use this command to untar the file:

tar -xvzf tinylogin-0.80.tar.gz

Change to the newly created tinylogin directory and enter the command:

cd tinylogin-0.80
make

TinyLogin is most useful on systems without a set of regular Linux utility programs, but you'll probably be exploring it on a standard Linux system. Because of this, you should install the TinyLogin programs (and the symbolic links that make them work as expected) in a separate directory until you're comfortable with them.

By using a directory name with the make install command you can place the TinyLogin command and matching relative symbolic links in a directory of your choosing. For example, use this command from the tinylogin-0.80 directory after compiling the source code as above:

make PREFIX=/tmp install

The /tmp directory will then contain subdirectories named bin, sbin and usr, each with symbolic links to /tmp/bin/tinylogin.

If you use make install with no PREFIX, the binary and symbolic links are installed into a _install subdirectory of your current directory, which also works fine for testing.

To really use TinyLogin for your system's utilities, you would use the following command:

make PREFIX=/ install

(Don't use this command until you've explored the utilities as I describe in the next section, especially the getty command.)

Now you're ready to test the functions of TinyLogin.

For testing purposes, I'll assume that you've installed TinyLogin in the /tmp directory, so the TinyLogin binary is located in the directory /tmp/bin. As a first test, use TinyLogin with a command name such as adduser as a parameter. For example, the following command will add the user david to your system:

/tmp/bin/tinylogin adduser david

Enter a password for david as prompted. To delete this user with deluser, enter:

/tmp/bin/tinylogin deluser david

/tmp/bin/adduser david
/tmp/bin/deluser david

A special consideration with user and password management is the GNU C Library Name Service Switch system used on all desktop Linux distributions. This system defines where utilities, such as passwd, look for user data. Options like NIS databases, DNS servers, local text files and local database files are all supported to make large networks easier to manage. TinyLogin doesn't use nsswitch by default, though it can. If you want to support this feature, which is useful but requires additional space and configuration effort, be certain that you have /etc/nsswitch installed and configured. Then set the USE_SYSTEM_PWD_GRP setting in the TinyLogin Makefile equal to true. The maintainer of TinyLogin recommends against using nsswitch because of the additional complications, but he does provide some guidelines for preparing libraries and the nsswitch.conf configruation file. See TinyLogin.html in the docs subdirectory for the details.

Unlike the BusyBox program, TinyLogin doesn't provide much in the way of on-line help for its available options. Of course, the utilities in TinyLogin are more straightforward than those in BusyBox. In fact, you don't need any options for most of the TinyLogin commands (su, login, etc.).

It's a good idea, however, to review the man pages for TinyLogin and for the getty command to see exactly how they work. These are contained in the docs subdirectory of the tinylogin-0.80 directory after you untar the source code.

Using the getty feature of TinyLogin on a standard Linux system is the most dangerous part of testing the program. It requires editing the /etc/inittab file. If you make any silly mistakes while doing this, you may spend some time booting from a floppy disk and fixing things, so be very careful.

As a review, this is the standard process for launching a Linux system in text mode (runlevel 3, nongraphical):

To test getty, you can alter the /etc/inittab file so that one of the ttys (say, tty6) uses the TinyLogin version of getty rather than the standard version that comes with your Linux. To do this, log in as root, make a backup copy of /etc/inittab, and then load the original file into a text editor.

Find the line in inittab that looks like the following sample taken from Red Hat Linux 7. The name of the getty program or the parameters after getty may differ on your system:

6:2345:respawn:/sbin/mingetty tty6

Now change that line to look like this for the test, using the location where you have the TinyLogin symbolic link for getty:

6:2345:respawn:/tmp/sbin/getty -i 38400 tty6

init q

When you log in with a valid user name and password, the TinyLogin version of getty still launches the /bin/login program, rather than the /tmp/bin/login program. You'll need to install TinyLogin into the standard system locations or move around the regular login program and add a symbolic link from /bin/login to the tinylogin binary in order to test the login features of TinyLogin. But testing getty as outlined gives a good glimpse of how TinyLogin can operate in a minimal-boot environment, saving space with a single small binary.

Now go back and change the inittab file back to its previous state so that tty6 will still be usable when you move the TinyLogin files from /tmp.

The documentation for TinyLogin mentions a file called tinylogin.def.h. Here, the docs are not quite up to date, since this file is now called config.h. Using this file, you can select which of the utilities described above are included in the single TinyLogin binary file. This is the same process used to control the contents of the BusyBox application.

By making changes to the #define statements in the config.h file, you can determine which capabilities are, and are not included when you compile the TinyLogin binary. For example, if you don't want to include one of the ten utilities, such as deluser, simply comment out the line by means of C++-style, double-forward slashes, something like this:

//#define TLG_DELUSER

By default, the sulogin and vlock commands are disabled in the config.h file. If you want to include these commands as part of TinyLogin, you'll need to remove the comment slashes in config.h before you compile the program.

Several other miniature utilities can help you in this type of project, such as the minimal editors ae, nano and elvis-tiny; the BusyBox single-binary utility set (which I've referred to several times); and a small Bourne-compatible shell called ash. Links to all of these projects are available on the TinyLogin web site.

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