Linux Important files

/etc:

/etc/hosts
- Stores IP addresses and their corresponding hostname plus any aliases.

/etc/hosts.allow and /etc/hosts.deny
- list machines which are allowed or denied use of network services.

/etc/HOSTNAME or /etc/hostname
- contains the hostname for the local system.

/etc/aliases
- used to create mail aliases. Mail for root can be forwarded to another mail account.

/etc/auto.*
- database files for the automounter daemon.

/etc/cron.*
- store daily, weekly, etc. cron jobs.

/etc/ftp*
- allows system administrator to define who can ftp to and from machine. Also define which machines can be used to access local ftp server.

/etc/httpd
– contain configuration and log directories for the Apache Web Server.

/etc/inittab
– define run levels; defines the default runlevel as 3.

/etc/dumpdates
– contains information on when each partition was backed up using the dump command.

/etc/lilo.conf
– Boot loader configuration file.

/etc/motd
– Message Of The Day – message which is displayed when user logs into the system. Maybe a warning message on usage or a funny message.

/etc/passwd
– contains user account information. Readable by everyone.

/etc/shadow
– contains actually passwords if using shadow passwords. Readable only by root.

/etc/sendmail.*
- sendmail configuration files.

/etc/sysconfig
– system configuration files, i.e. pcmcia, network, etc.

/etc/fstab
– file system table – define partitions and their mount points.

/etc/rc.d
– directory which contains system startup scripts.

/etc/services
– enable/disable network services. Can be used to disable ftp services or finger services, etc.

/var:

/var/log/messages
– System message files. View using any method used to view ascii files.

/var/log/lastlog
- Stores information on the last time each user logged into the system. Only viewable using the `last command`.

/var/log/httpd
- Stores log files for web server

/var/log/htmlaccess.log
- Stores information on who is accessing your server.

/var/log/dmesg
- System information. Info on hardware errors and messages seen during boot. View using the `dmesg` command.

/var/log/samba
- Directory containing log files for the Samba Server (Allows Window based machines to access data on UNIX machines).

/var/spool/mail
- contains mailbox files for each user. Created automatically when user account is created.

/var/spool/lpd/
- Printer spooler directory.
is created when you define your printer. Printers can be local or remote.

/var/spool/cron
- Directory contains crontab entries when created by users.

Mounting and Unmounting File Systems

- To mount a file system in the Linux directory tree, you must have a physical disk partition, CD-ROM, or floppy that you want to mount.
- You must also make sure that the directory to which you want to attach the file system, known as the mount point, actually exists.
- The mount point must exist before you try to mount the file system.
- Lets say you want to mount your Red Hat 6.2 CD:
- first, insert the CD into the CD-ROM drive
- then, as root, use the command: mount /dev/hdc /mnt/cdrom
- /dev/hdc = the device name for your CD-ROM drive
- /mnt/cdrom = the mount point. It is an empty directory that was created just for mounting your CD-ROM.
- Now when you want to read files from your CD-ROM, you will access it via the mount point, /mnt/cdrom. The system actually reads the date from /dev/hdc.
- Notice how /mnt/cdrom appears to be a part of the / file system. However, / is a hard disk partition and /mnt/cdrom is on your CD-ROM drive.
- The mount command is used to mount a disk and make it available for use.
- The umount command is used to dismount or un-mount a disk partition or CD-ROM. For example, to unmount the CD-ROM so you can eject the disk: umount /mnt/cdrom
- mount command options:
- -t type: specify the type of file system used on the device we are about to mount. Types include ext2 (linux native), msdos (MS-DOS), vfat (Windows 9x), ntfs (New Technology file system for WindowsNT), and iso9660(CD-ROM)just to name a few.
- Linux attempts to automatically determine the file system type. But it can not, then you must specify the type.
- -o list_of_options: These options allow you to mount the disk as read-only, read/write, whether root can access the data from a remote system(Not secure) and other things. Use the man command to find out more about mount’s options.

Mounting systems as boot time

- File systems that will be used all of the time should be mounted at boot time. For example, the / directory must be mounted when every we boot the system. If /home is on a separate disk, then it must be mounted before users log into the system. Or else, people can log in, but they have no home directory.
- /etc/fstab is the configuration file that defines which disk should be mounted and where (mount point).
- Fields in /etc/fstab:
- File System: Specifies the block device to be mounted from local or remote system.
- Mount Point: Specifies the mount point directory. For swap, this would be the word none.
- Type: Specify the type of file system being mounted.
- Mount options: comma-separated list of mount options.
- Dump Frequency: how often should the file system be backed up by the dump command.
- Pass Number: Specifies the order in which file systems should be checked by the fsck command when the system is booted. For / the value is 1, for all others (local only) the value is 2. NEVER check remote file systems.
- There is no mount point for swap partitions and the type is swap.
- The umount command unmounts a disk partition or CD-ROM. You can not remove, removable media until the drive has been unmounted. If you mount your CD-ROM, then push the button on the door, nothing happens. After unmounting the drive, then you can open the drive door.