lsg15.htm

linux-unix130.linux.and.unix.ebooks130 linux and unix ebookslinuxLearning Linux - Collection of 12 E

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<FONT SIZE=5 COLOR="#FF0000"><B>Using the rdev Family</B></FONT></CENTER></H3>



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<P>The rdev command is a utility not just for identifying the root device, as shown earlier in this chapter (it's used when creating a boot floppy disk), but for obtaining all kinds of information about your Linux system and making some configuration changes. The rdev utility can be cumbersome to use, and many administrators ignore it and its companion utilities completely.



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<NOTE>If you use LILO to boot Linux, you can ignore all the rdev commands as these parameters are set in the LILO configuration. The only times you will need rdev is when you change kernels and want to make a boot floppy disk for emergency use, or you want to change the RAM disk size. If you don't use LILO, you may occasionally need to use the rdev commands, although it is rare that they will be necessary as the Linux setup procedures define most of these parameters for you. The exception is changes in RAM disk size.</NOTE>



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<P>When run by itself, rdev displays the currently defined root partition and directory:



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<FONT COLOR="#000080">$dev



/dev/sda3 /</FONT></PRE>



<P>In this example, /dev/sda3 (third partition on the first SCSI hard disk) is the current root partition. You can use rdev to change the root partition and point to the kernel image to be used by Linux by providing both parameters as arguments:



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<FONT COLOR="#000080">rdev /vmlinuz /dev/sda3</FONT></PRE>



<P>This command changes the kernel image used to vmlinuz in the root directory of the third partition. You usually perform this command only when you create an emergency floppy disk.



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<P>The rdev command has several options for changing the way it acts, as shown in the following list:



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-h



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Displays help</FONT>



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-r



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Makes rdev act like the command ramsize (see below)</FONT>



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-R



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Makes rdev act like the rootflags command (see below)</FONT>



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-s



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Makes rdev act like the swapdev command (see below)</FONT>



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-v



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Makes rdev act like the vidmode command (see below)</FONT>



</TABLE><P>Although you can use these options to alter rdev's behavior, you can also use the following commands directly:



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<LI>The ramsize command specifies the size of the RAM disk in kilobytes.



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<LI>The rootflags command enables you to mount the root directory as read-only.



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<LI>The swapdev command identifies the swap device.



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<LI>The vidmode command specifies the video mode.



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<P>In order to change many of the parameters, you must specify an offset that indicates the decimal value of the kernel location with the rdev command, which is why many administrators don't like the command. To use rdev or one of the utilities in its family, you must calculate the offsets according to the following rules:



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Offset 498



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Root flags</FONT>



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Offset 504



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RAM disk size</FONT>



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Offset 506



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VGA mode</FONT>



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Offset 508



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Root device</FONT>



</TABLE><P>The rootflags command has many options, only one of which really works to enable you to mount the root directory as read-only. Because this feature is seldom (if ever) necessary, most administrators can effectively ignore the rootflags command. (If you are running off a CD-ROM or from a small hard drive that contains only the binaries, you may want to consider using rootflags, but because you can accomplish the same task using file permissions, there's not much need to use rootflags.)



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<P>The vidmode command (or rdev -v) lets you change the video mode. If you issue the vidmode command by itself with some versions of Linux, it displays the current settings. More recent versions (including the one on this book's CD-ROM) show a help screen:



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<FONT COLOR="#000080">$ vidmode



usage: rdev [ -rsv ] [ -o OFFSET ] [ IMAGE [ VALUE [ OFFSET ] ] ]



 rdev /dev/fd0 (or rdev /linux, etc.) displays the current ROOT device



 rdev /dev/fd0 /dev/hda2 sets ROOT to /dev/hda2



 rdev -R /dev/fd0 1 set the ROOTFLAGS (readonly status)



 rdev -s /dev/fd0 /dev/hda2 set the SWAP device



 rdev -r /dev/fd0 627 set the RAMDISK size



 rdev -v /dev/fd0 1 set the bootup VIDEOMODE



 rdev -o N ... use the byte offset N



 rootflags ... same as rdev -R



 swapdev ... same as rdev -s



 ramsize ... same as rdev -r



 vidmode ... same as rdev -v



Note: video modes are: -3=Ask, -2=Extended, -1=NormalVga, 1=key1, 2=key2,...



 use -R 1 to mount root readonly, -R 0 for read/write.</FONT></PRE>



<P>The legal values for vidmode are as follows:



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-3



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Prompt</FONT>



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-2



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Extended VGA</FONT>



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-1



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Normal VGA</FONT>



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0



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The same as pressing 0 at the prompt</FONT>



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1



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The same as pressing 1 at the prompt</FONT>



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2



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The same as pressing 2 at the prompt</FONT>



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<A NAME="I2"></A>n



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The same as pressing n at the prompt</FONT>



</TABLE><P>You can change the video mode using one of these values on the command line or using a number or letter to emulate pressing a value at the prompt.



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<FONT SIZE=5 COLOR="#FF0000"><B> Summary</B></FONT></CENTER></H3>



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<P>This chapter looked at the proper procedures for starting and stopping a Linux system. As you have seen, shutting down the system properly is vitally important. This chapter has also looked at the init process, an important aspect of the Linux system. Now that these basic procedures are out of the way, you are ready to look at some of the important aspects of system administration.



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