Linux and uClinux rc file and inittab file

On a linux system inittab file, rc scripts and relavant config files in /etc play key roles at system startup. Inittab file is read and parsed by init process, to decide what level the system will run at and which processes are started at bootup and at normal operation. Then the rc script will be run at that level.

There are system V and BSD styles to the arrangement of inittab, rc scripts and config files. And there are differences between Linux distributions. But the key point is, the system will startup at a predefined level most likely defined in inittab, and rc scripts will be run for that level to spawn all the necessary processes.

In uClinux there are inittab and rc file as well. But there are no run levels. An embedded system usually runs at just one level. So be carefull, uClinux inittab and rc are different from that of standard Linux.

In uClinux, rc is executed BEFORE inittab! rc file basically contains commands to prepare the system environment such mount /var, make directories, startup network etc. Inittab can be used to startup programs and services, such as serial terminals, IP servers, etc.

Something about the terminal. The line for terminal is usually like this:

ttyS0:vt100:/bin/agetty 9600 ttyS0

agetty is an excutable binary in /bin directory. It opens a tty port, prompt a login name and invokes /bin/login command. The above is the simplest format of an agetty command. It actually has quite a few options to change its behaviors. For example this one:

agetty -L 9600 ttyS1 vt100

This will force the line to be a local line with no need for carrier detect. This can be useful when you have a locally attached terminal where the serial line does not set the carrier detect signal. Try this if your terminal just sleeps instead of giving you a password: prompt.

Another thing is that agetty will optionally display the contents of /etc/issue. You can use a -f to display an alternative file, or you can use -i to ignore any issue file, not to display the contents of /etc/issue (or other) before writing the login prompt.

One more note, if there is no agetty in inittab, and you choose init with "enable console shell" when you configure core applications, then the uClinux will start without login at all.