Users, Groups, Passwords
A]
To check the users that have been created on the system:
=> cat /etc/passwd
Columns in the /etc/passwd ->
a) username
b) placeholder for passwd
c) user id (uid)
d) group id (gid)
e) comment or description
f) home directory
g) login shell
B]
To check the groups existing on the system:
=> cat /etc/group (contains groupname and gid)
C]
The file that contains encrypted passwords:
=> /etc/shadow
The columns of /etc/shadow are mentioned in "man 5 shadow".
D]
The file that has the password policy settings, the min. and max.
UID/GID setting for users and groups that are created, settings
to prevent a user's home directory from being created.
=> /etc/login.defs
E]
To see the current password policy for a user:
=> chage -l (e.g chage -l abg)
To change the password policy for a user:
=> chage (e.g. chage abg)
F] Troubleshooting:
What if the /etc/shadow file gets deleted?
You won't be able to login, so what is the solution?
=> Boot into runlevel 1 (here password is not reqd. to login)
=> cd /etc
=> pwconv passwd (The /etc/shadow file will get created)
(Although the /etc/shadow file is created, the passwords won't
work)
=> Reset the root password
------------------------------------------------------------------
Login Shells and Startup Scripts
To see the different available shells:
=> cat /etc/shells
------------------------------------------------------------------
Files that print Pre-login Messages before the login prompt
1. /etc/motd (motd => message of the day)
2. /etc/issue
------------------------------------------------------------------
Order of Execution of Startup Scripts
A Login shell first calls the /etc/profile script. The actual
order is as follows:
1. /etc/profile
2. /etc/profile.d where profile.d is a directory
3. ~/.bash_profile
4. ~/.bashrc
5. /etc/bashrc
Almost all the variables that can be seen using "set" and "env"
commands are set in '.bash_profile' and not '.bashrc'
Only aliases are set in '.bashrc'
------------------------------------------------------------------
The kill command signals
To see a list of signals and their values that are to be used with
the "kill" command:
=> man 7 signal
The kernel
To dynamically make changes to the kernel:
1. /etc/sysctl.conf
2. sysctl -p (to update) (sysctl -e for errors)
NB: a) For a complete list of sysctl parameters, use "sysctl -a".
b) Changes to the /proc filesystem are temporary & if you want them to persist
across reboots, put an entry in /etc/sysctl.conf
c) Also, you can check /proc/sys/net ... and its sub-directories.
To see the kernel version:
1. ls /lib/modules (modules is a directory)
2. cat /proc/version
3. uname -r
------------------------------------------------------------------
The Boot Process
Boot/Initialization files
1. /etc/grub.conf (kernel /vmlinuz-.... & initrd lines)
2. /etc/fstab
3. /boot/vmlinuz.... & /lib/modules (Kernel Initialization)
4. /etc/inittab (init process),
(to set default runlevel and increase tty's)
5. /etc/rc.d/rc.sysinit (rc.sysinit calls /etc/sysctl.conf)
6. /etc/rc.d/rc?.d (where ? is from 0 to 6 as per the runlevel &
rc?.d are script directories)
7. /etc/rc.d/rc.local
Summing up the Boot Steps:
1. BIOS Initialization
2. Boot Loader
3. Kernel Initialization
4. Top-most process init
5. Entering the desired runlevel
------------------------------------------------------------------
The mount command
The file used by the mount command:
=> /etc/mtab
Mounting the Filesystem
To mount the different filesystems automatically when the system starts:
This is done by /etc/fstab
Columns of /etc/fstab include:
a) LABEL or device
b) Mountpoint
c) Filesystem type
d) Mount Options (async, rw, nouser, quotas, suid,.....)
e) Dump frequency (0 => no frequency, 1 => dump once every day)
(2 => dump once in two days)
f) Filesystem Checking Order (0 => do not check, 1 => 1st root)
(2 => check next, 2nd = boot, home, usr, ...)
After creating a filesystem, make an entry in the /etc/fstab file
to mount the filesystem every time the system starts.
------------------------------------------------------------------
Partition Information (Hard disk, CPU and RAM)
To see the partition information:
1. cat /proc/partitions
2. fdisk -l (also shows fstype, system_id, start
Impressum
Verlag: BookRix GmbH & Co. KG
Tag der Veröffentlichung: 25.05.2014
ISBN: 978-3-7368-1457-8
Alle Rechte vorbehalten
Widmung:
The book is dedicated to all the Linux users across the world as well as those who are new to Linux.