Adding New Users, Storage, File System

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• Unit-VI
• Adding New Users, Storage, File System

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/ETC/PASSWD FILE

• The system consults /etc/passwd at login time to
  determine a users UID and home directory
• Each line in the file represents one user and
  contains seven fields separated by colons
• Login name
• Encrypted password placeholder
• UID (user ID) number
• Default GID (group ID) number
• GECOS information full name, office,
  extension, home phone
• Home directory
• Login shell

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• passwd file contains an x in the encrypted
  password field on Linux
• The actual encrypted passwords are stored in
  etc/shadow on Linux
• - Login Name Login names (also known as
  usernames) must be unique and, depending on the
  operating system
• - Encrypted password Encryption algorithms
  traditional
• crypt (based on DES), MD5, Blowfish, and an
  iterative version of MD5
• - UID (user ID) number UID identifies the user
  to the system. Login names are provided for the
  convenience of users, but software and the
  filesystem use UIDs internally. UIDs are usually
  unsigned 32-bit integers

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• Default GID number Like a UID, a group ID number
  is a 32-bit integer. GID 0 is reserved for the
  group called root or system. /etc/group file
  defines the groups, with the GID field in
  /etc/passwd providing a default (or effective)
  GID at login time
• - GECOS field GECOS field is sometimes used to
  record personal information about each user. The
  finger command interprets comma-separated GECOS
  entries in the following order
• Full name (often the only field used)
• Office number and building
• Office telephone extension
• Home phone number

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• - Home directoryusers home directory is
  default directory at login time
• - Login shell The login shell is normally a
  command interpreter such as the Bourne shell or
  the C shell (/bin/sh or /bin/csh), but it can be
  any program. sh is the traditional default for
  UNIX, and bash (the GNU Bourne again shell) is
  the default for Linux

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/ETC/SHADOW AND /ETC/SECURITY/PASSWD FILES

• shadow password file is readable only by the
  superuser and serves to keep encrypted passwords
  safe from password cracking programs
• IBM calls the file that stores the encrypted
  passwords /etc/security/passwd, while the rest of
  the world calls it /etc/shadow
• Like /etc/passwd, /etc/shadow contains one line
  for each user. Each line contains nine fields,
  separated by colons

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• Login name
• Encrypted password
• Date of last password change
• Minimum number of days between password changes
• Maximum number of days between password changes
• Number of days in advance to warn users about
  password expiration
• Linux Days after password expiration that
  account is disabled
• Account expiration date
• A reserved field that is currently always empty

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/ETC/GROUP FILES

• /etc/group file contains the names of UNIX groups
  and a list of each groups members
• Each line represents one group and contains four
  fields
• Name of a group
• Encrypted password or a placeholder
• GID number
• List of members, separated by commas

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ADDING USERSTHE BASIC STEPS

• Basic steps to add user by hand
• Edit the passwd and shadow files to define the
  users account using vipw command.
• Add the user to the /etc/group file.
• Set an initial password.
• Create, chown, and chmod the users home
  directory.

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ADDING USERS USING USERADD

• Command
• sudo useradd user_name g group_name

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Linux Filesystem

• Types of linux filetypes are ext2, ext3, ext4
• Ext3 adds journaling capability to the existing
  ext2 code
• Ext3 sets aside an area of the disk for the
  journal.
• When a filesystem operation occurs, the required
  modifications are first written to the journal.
  When the update is complete, a commit record is
  written to mark the end of the entry.
• If a crash occurs during the update, the
  filesystem uses the journal log to reconstruct a
  perfect consistent filesystem.
• Journaling reduces the time needed to perform
  filesystem consistency checks
• To add journal to the existing filesystem
• tune2fs j /dev/sda1

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Mkfs,fsck

• Mkfs- format filsystems
• General syntax for creating a new filesystem is
• mkfs T fstype -o options rawdevice
• fsck ckeck and repair filesystems
• Filesystem mounting
• sudo mount /dev/sda1 /mnt/temp

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Filesystem Terminology

• Inodes are fixed length table entries that each
  hold information about one file
• ls i
• Superblock is a record that describes the
  characteristics of the filesystems.
• It contains information about the length of a
  disk block, the size and location of the inode
  tables, the disk block map and usage information,
  the size of the block groups
• Filesystems cache disk blocks to increase
  efficiency
• Sync system call flushes modified blocks to their
  permanent homes on disk