2.1 Common Commands

Following are lists of commonly used system administration commands.

2.1.1 Clocks

hwclock	Manage hardware clock.
rdate	Get time from network time server.
zdump	Print list of time zones.
zic	Create time conversion information files.

2.1.2 Daemons

apmd	Advanced Power Management daemon.
atd	Queue commands for later execution.
bootpd	Internet Boot Protocol daemon.
fingerd	Finger daemon.
ftpd	File Transfer Protocol daemon.
identd	Identify user running TCP/IP process.
imapd	IMAP mailbox server daemon.
inetd	Internet services daemon.
klogd	Manage syslogd.
lpd	Printer daemon.
mountd	NFS mount request server.
named	Internet domain name server.
nfsd	NFS daemon.
pppd	Maintain Point-to-Point Protocol (PPP) network connections.
rdistd	Remote file distribution server.
rexecd	Remote execution server.
rlogind	rlogin server.
routed	Routing daemon.
rpc.rusersd	Remote users server.
rpc.statd	NFS status daemon.
rshd	Remote shell server.
rwhod	Remote who server.
sshd	Secure shell daemon.
syslogd	System logging daemon.
talkd	Talk daemon.
tcpd	TCP network daemon.
tftpd	Trivial File Transfer Protocol daemon.
update	Buffer flush daemon.
ypbind	NIS binder process.
yppasswdd	NIS password modification server.
ypserv	NIS server process.

2.1.3 Hardware

agetty	Start user session at terminal.
arp	Manage the ARP cache.
cardctl	Control PCMCIA cards.
cardmgr	PCMCIA card manager daemon.
cfdisk	Maintain disk partitions (graphical interface).
fdisk	Maintain disk partitions.
hdparm	Get and set hard drive parameters.
kbdrate	Manage the keyboard's repeat rate.
ramsize	Print information about RAM disk.
setkeycodes	Change keyboard scancode-to-keycode mappings.
setserial	Set serial port information.
slattach	Attach serial lines as network interfaces.

2.1.4 Host Information

arch	Print machine architecture.
dig	Query Internet domain name servers. Replaces nslookup.
dnsdomainname	Print DNS domain name.
domainname	Print NIS domain name.
free	Print memory usage.
host	Print host and zone information.
hostname	Print or set hostname.
nslookup	Query Internet domain name servers. (Deprecated)
uname	Print host information.

2.1.5 Installation

cpio	Copy file archives.
install	Copy files into locations providing user access and set permissions.
rdist	Distribute files to remote systems.
tar	Copy files to or restore files from an archive medium.

2.1.6 Mail

fetchmail	Retrieve mail from remote servers.
formail	Convert input to mail format.
mailq	Print a summary of the mail queue.
makemap	Update sendmail's database maps.
rmail	Handle uucp mail.
sendmail	Send and receive mail.

2.1.7 Managing Filesystems

To Unix systems, a filesystem is a device (such as a partition) that is formatted to store files. Filesystems can be found on hard drives, floppies, CD-ROMs, or other storage media that permit random access.

The exact format and means by which the files are stored are not important; the system provides a common interface for all filesystem types that it recognizes. Under Linux, filesystem types include the Second Extended (ext2) Filesystem, which you probably use to store Linux files. This filesystem was developed primarily for Linux and supports 256-character filenames and 4-terabyte maximum filesystem size. (It is "second" because it is the successor to the extended filesystem type.) Other common filesystem types include the MS-DOS filesystem, which allows files on MS-DOS partitions and floppies to be accessed under Linux, and the ISO 9660 filesystem used by CD-ROMs.

The 2.4 kernel adds optional support for an enhanced version of the ext2 filesystem, the Third Extended (ext3) Filesystem. Many Linux distributions ship kernels with this support preconfigured. The ext3 filesystem is essentially an ext2 filesystem with an added journal. Since it is in all other ways identical to the ext2 system, it is both forward and backward compatible with ext2—all ext2 utilities work with ext3 filesystems. When the kernel interacts with an ext3 filesystem, writes to disk are first written to a log or journal before they are written to disk. This slows down writes to the filesystem, but reduces the risk of data corruption in the event of a power outage. It also speeds up reboots after a system unexpectedly loses power. When rebooting with an ext2 filesystem, the fsck utility scans the entire disk to ensure that all data blocks are listed as either used or free, that each data block is claimed by only one file or directory, and that all files are available in the directory tree. When rebooting an ext3 filesystem using a supported kernel, it need only check the journal and complete any tasks that were left uncompleted.

To change an ext2 filesystem into an ext3 filesystem, simply add a journal using the tune2fs utility with the -j option on the unmounted device. If the filesystem is listed in /etc/fstab, change its specified filesystem from ext2 to ext3, or auto (mountd will automatically detect the right system).

You can specify the level of journaling the kernel should use in the mount options field (or using mount -o). There are three data options:

data=journal

Log all filesystem data and metadata changes. All changes to the filesystem are written twice, once to the journal and once to the filesystem itself. This is the slowest but safest mode.

data=ordered

Log filesystem metadata. Flush data updates to disk before changing metadata. This is the default mode. This slows writes a small amount, but ensures the filesystem is always in sync with changes to its metadata.

data=writeback

Log filesystem metadata. Use the ext2 write process to write data changes. This is the fastest journaling mode. While it maintains filestructure integrity, the contents of files may contain old, stale data when the filesystem is restarted from an unexpected shutdown.

The last field in the /etc/fstab entry specifies at what point in the boot process the filesystem should be verified with fsck. It should never need to be checked because journaling ensures filesystem integrity. You can set this field to 0. You can also turn off the automatic check of the filesystem that normally occurs every 20th time the system is mounted using tune2fs -i 0 -c 0.

Although not covered in this edition of Linux in a Nutshell, Linux supports three other open source journaling filesystems: IBM's Journaled Filesystem (JFS), SGI's Extensible Filesystem (XFS), and the Naming System Venture's Reiser Filesystem (ReiserFS). In some situations these can be faster than ext3. Some Linux distributions use these alternative filesystems by default.

debugfs	Debug ext2 filesystem.
dosfsck	Check and repair a DOS or VFAT filesystem.
dumpe2fs	Print information about superblock and blocks group.
e2fsck	Check and repair an ext2 filesystem.
e2image	Store disaster recovery data for an ext2 filesystem.
fdformat	Format floppy disk.
fsck	Another name for e2fsck.
fsck.ext2	Check and repair an ext2 filesystem.
fsck.minix	Check and repair a MINIX filesystem.
fuser	List processes using a filesystem.
mke2fs	Make a new ext2 filesystem.
mkfs	Make new filesystem.
mkfs.ext2	Another name for mke2fs.
mkfs.ext3	Yet another name for mke2fs.
mkfs.minix	Make new MINIX filesystem.
mklost+found	Make lost+found directory.
mkraid	Set up a RAID device.
mkswap	Designate swap space.
mount	Mount a filesystem.
raidstart	Activate a RAID device.
raidstop	Turn off a RAID device.
rdev	Describe or change values for root filesystem.
resize2fs	Enlarge or shrink an ext2 filesystem.
rootflags	List or set flags to use in mounting root filesystem.
showmount	List exported directories.
swapoff	Cease using device for swapping.
swapon	Begin using device for swapping.
sync	Write filesystem buffers to disk.
tune2fs	Manage an ext2 filesystem.
umount	Unmount a filesystem.

2.1.8 Managing the Kernel

depmod	Create module dependency listing.
insmod	Install new kernel module.
lsmod	List kernel modules.
modinfo	Print kernel module information.
modprobe	Load new module and its dependent modules.
rmmod	Remove module.
sysctl	Examine or modify kernel parameters at runtime.

2.1.9 Networking

chat	Establish dial-up IP connections.
dip	Establish dial-up IP connections.
gdc	Administer gated routing daemon.
ifconfig	Manage network interfaces.
ipchains	Administer firewall facilities (2.2 kernel).
iptables	Administer firewall facilities (2.4 kernel).
named	Translate between domain names and IP addresses.
nameif	Assign names to network devices.
netstat	Print network status.
nfsstat	Print statistics for NFS and RPC.
nsupdate	Submit dynamic DNS update requests.
portmap	Map daemons to ports.
rarp	Manage RARP table.
route	Manage routing tables.
routed	Dynamically keep routing tables up to date.
rpcinfo	Report RPC information.
ruptime	Check how long remote system has been up.
rwho	Show who is logged into remote system.
traceroute	Trace network route to remote host.

2.1.10 Printing

checkpc	Examine and repair printer settings.
lpc	Control line printer.
tunelp	Tune the printer parameters.

2.1.11 Security and System Integrity

badblocks	Search for bad blocks.
chroot	Change root directory.

2.1.12 Starting and Stopping the System

bootpd	Internet Boot Protocol daemon.
bootpgw	Internet Boot Protocol gateway.
bootptest	Test bootpd.
halt	Stop or shut down system.
init	Change runlevel.
reboot	Shut down, then reboot system.
runlevel	Print system runlevel.
shutdown	Shut down system.
telinit	Change the current runlevel.
uptime	Display uptimes of local machines.

2.1.13 System Activity and Process Management

A number of additional commands in Chapter 3 are particularly useful in controlling processes, including kill, killall, killall5, pidof, ps, and who.

fuser	Identify processes using file or filesystem.
ipcrm	Remove interprocess communication message queue, semaphore array, or shared memory segment.
ipcs	Print interprocess communication information.
renice	Change the priority of running processes.
top	Show most CPU-intensive processes.
vmstat	Print virtual memory statistics and process statistics.

2.1.14 Users

chpasswd	Change multiple passwords.
groupadd	Add a new group.
groupdel	Delete a group
groupmod	Modify groups.
grpck	Check the integrity of group system files.
grpconv	Convert group file to shadow group file.
lastlog	Generate report of last user login times.
newusers	Add new users in a batch.
pwck	Check the integrity of password system files.
pwconv	Convert password file to shadow passwords.
rusers	Print who-style information on remote machines.
rwall	Print a message to remote users.
useradd	Add a new user.
userdel	Delete a user and that user's home directory.
usermod	Modify a user's information.
w	List logged-in users.
wall	Write to all users.
whoami	Show how you are currently logged in.

2.1.15 Miscellaneous

anacron	Schedule commands for periodic execution.
atrun	Schedule commands for later execution.
cron	Schedule commands for specific times.
dmesg	Print bootup messages after the system is up.
ldconfig	Update library links and do caching.
logger	Send messages to the system logger.
logrotate	Compress and rotate system logs.
run-parts	Run all scripts in a directory.