lpicv2.txt

General stuff
-------------
- Linux is a kernel
- distrowatch.com
- less -> paging program

=========================================
Hardware settings
------------
lspci
lspci -v -> verbose
lspci -vv -> very verbose
lsusb
lsusb -v -> verbose
lsblk
major number -> the type of storage device
scsi driver used for IDE, SATA etc drives

/proc -> procfs - representation of the running processes, what kernel we are running
/proc/devices -> what devices and drivers are loaded
/proc/modules -> what kernel modules are loaded
/sys -> sysfs - representation of the devices via loaded module/driver
/sys/module/ -> what modules are loaded for every device
/dev -> devfs - actual device files

cold devices -> designed to be plugged in when the system is powered off
hot devices -> can be detected by the system while it is running
D-Bus/udev -> daemons used to detect hardware
dmesg -> displays messages about hardware changes - reads from /dev/kmsg
dmesg -C -> clear the buffer for hardware changes

Hardware device drivers
-------------
lsmod -> list currently loaded drivers - reads from /proc/modules
modprobe -r *driver* -> unload the driver and its dependencies - rmmod is used in the background
modprobe -rv -> verbose
modprobe *driver* -> load the driver and it dependencies - insmod is used in the background
/sys/module/*module_name*/parameters/ -> loaded module options
/etc/modprobe.d/modesetting.conf -> set module options to be used when loading

==================================================
Linux Boot Process
-------------
boot = POST -> locate boot loader -> load boot loader -> load kernel -> start other services
GRUB -> GRand Unified Bootloader
/boot -> contains the kernel, initial RAM disk and other files for the boot
vmlinux -> normal kernel
vmlinuz -> compressed kernel
/proc/cmdline -> parameters passed through to the kernel for the booting process

System and startup managers:
SysVinit -> used in earlier Debian and RH distros - serial boot process
Upstart -> developed by Ubuntu
systemd -> the latest - start services in paralel

top -p X -> display only the process for PID X

systemd-analyze -> display the boot time
systemd-analyze blame -> display a list of all running units ordered by the time they took to initialize

Kernel Ring Buffer Messages
---------------
dmesg -> reads from /dev/ksmg
dmesg -> also logs can be found in /var/log/dmesg or boot.msg
dmesg -C -> clear the ring buffer
dmesg -c -> display and clear the ring buffer
dmesg -w -> follows the ring buffer - good for monitoring
dmesg -H -> displays in human readable format

================================================
Runlevels
--------------
SysVinit:
0 -> shutdown
1/S -> single user mode - used for maintenance
2-5 -> multi-user modes
6 -> reboot
/etc/inittab -> set the default runlevel in this file - only for SysVinit
/etc/rcX.d -> the default enabled/disabled services for the X runlevel
chkconfig *service* on/off -> enable/disable the service - removes the symlink from the current runlevel directory /etc/rcX.d
telinit X -> change the runlevel to X
who -r = runlevel -> display the current runlevel

systemd
-------------
poweroff.target -> runlevel 0
rescue.target -> runlevel 1
multiuser.target -> runleves 2,3,4
graphical.target -> runlevel 5
reboot.target -> runlevel 6

systemctl get-default -> display the default runlevel
systemctl set-default X -> set the default runlevel
systemctl disable/enable X.service -> enable/disable a service
systemctl isolate X.target -> change the runlevel to X
who -r = runlevel

shutdown/reboot
-------------
shutdown -h now -> halt
shutdown -r now -> reboot
shutdown -h +10 -> shutdown in 10 minutes
shutdown -h +10 "shutting down in 10" -> displays message and halts in 10 min
shutdown -c -> cancel shutdown operation
echo "message" | wall -> displays the message to all the users

========================================
Hard disk layout
----------
- no drive letter, Linux uses mount points
/ -> root file system
swapon -s -> display info on the swap
free -m -> display info on the RAM and swap usage - reads info from /proc/meminfo
df -hT -> display file system disk usage

=======================================
Boot Managers
----------
- is located in the MBR (first 512 bytes of the disk)
- the boot loader location is chosen during the installation
MBR = 446 bytes (bootloader stored in assembly code) + 4 x 16 bytes (partition table for up to 4 primary partitions) + 2 bytes (Sentinel for the bootable flag = 0xAA55)
dd if=/dev/sda of=/root/sda.mbr.bak count=1 bs=512 -> backup the MBR

GRUB Legacy
----------
/boot/grub/menu.lst -> config file for GRUB Legacy
/etc/grub.conf -> usually a symlink to the above
- stanza = menu entry in grub
- a stanza starts with the word title and the next one will start with the same word
grub-install /dev/sda -> install GRUB in the MBR
grub-install /dev/sda1 -> install GRUB in the first sector of the partition

GRUB2
----------
/boot/grub/grub.cfg -> config file for GRUB2
- the GRUB2 config file is usually generated rather than editing it
grub-mkconfig -o /boot/grub/grub.cfg
- edits are not made directly to grub.cfg
/etc/default/grub
/etc/grub.d

Reset root password - RH
----------
- boot to the initialization ram disk
- edit the linux16 entry
- remove rhgb and quiet
- add rd.break and enforcing=0
- switch_root# mount -o remount,rw /sysroot
- chroot /sysroot
- passwd
- exit
- switch_root# mount -o remount,ro /sysroot
- exit
- restorecon /etc/shadow
- setenforce 1

Reset root password - Debian
----------
- edit the linux entry
- add init=/bin/bash
- mount -o remount,rw /
- passwd
- mount -o remount,ro /
- reset

=====================================
Shared Libraries
-----------
ldd -> list what shared libraries are used by a program
ldd -> list dependencies for a program
ldd $(which command)
ldconfig -> display the shared libraries from our system
LD_LIBRARY_PATH -> variable containing where the libraries are located
/etc/ld.so.conf
/etc/ld.so.conf.d

- code that other applications can use
- shared libraries have the .so ending (so = shared objects)

-> where are share libs located?
- Trusted directories:
	/lib
	/usr/lib
- we can read from paths specified in /etc/ld.so.conf
- we can specify a path using
	ldconfig -n <path>
- use the LD_LIBRARY_PATH variable

/usr/local/lib

ldconfig -p -> display the library cache
ldconfig -> rebuild the lib cache after adding a new lib

Adding a shared library
-----------
- mkdir /usr/local/lib/new_folder_for_the_new_library
- cp library.so !$
- echo !$ > /etc/ld.so.conf.d/new_library.conf
- ldconfig
- ldconfig -p | grep new_library

=========================================
Debian Package Management
-----------
dpkg -> the most basic command in managing software on Debian based systems
dpkg -i -> install
dpkg -r -> remove
dpkg -P -> purge (remove the app and the file and the config files)
dpkg --get-selections -> list what is installed
dpkg -l 'vim*' -> list all packages that start with vim
dpkg -S /bin/ls -> display to what package the file belongs
dpkg -L -> list all the files that belong to the package
dpkg-reconfigure -> re-run a config wizard

Repositories
-----------
/etc/apt/sources.list -> file where repos are configured
- aptitude -> can be run interactively
- apt-get
- apt-cache

aptitude search 'zsh.*'
aptitude search '~nzsh.*'
aptitude install
aptitude purge

apt-get update -> update the local db about the info from the repos
apt-cache search -> display basic info about the packege
apt-cache show -> display detailed info about the package
apt-get install
apt-get purge 
apt-get autoremove -> remove dependencies that are not used
apt-get dist-upgrade

==========================================
RPM and YUM Package Management
------------
rpm -qa -> list all the installed apps
rpm -i -> install
rpm -qpi -> display info about the rpm package we want to install (package is not installed yet)
rpm -qpl -> list everything included in the package
rpm -iv -> install and show what is happening
rpm -ivh -> also get a progress bar
rpm -qi -> display info about an installed package
rpm -U -> update a package - if the package is not installed, it will install it
rpm -F -> only update if the package exists
rpm -e -> remove
rpm -K -> verify the signature of the rpm file
rpm -qf -> display to which package a file belongs to
rpm -ql -> list everything that belongs to a package
rpm -V -> verify/check the integrity of the package
rpm -Va -> verify/check the integrity of all packages
rpm -qpR -> display the dependencies of a package

Repositories
-------------
/etc/yum.conf -> config file for yum
- repo locations can be configured in /etc/yum.repos.d/
/etc/yum.repos.d/
yum repolist -> display the configured repos
yum install
yum install -y -> install without being prompted to accept the changes
yum update -> update package
yum remove
yum info -> display info about a package from the internet
yum search
yum list -> displays all the installed and available packages
yum list installed -> display everything that is installed
yum provides /etc/hosts.allow -> display to which package the file belongs

yumdownloader -> comes from the yum-utils package
yumdownloader --destdir -> sets the destination folder for the download
yumdownloader --resolve -> also downloads the dependencies

rpm2cpio zsh.rpm > zsh.cpio -> create the cpio archive
cpio -id < zsh.cpio -> extract the files to the current folder

=====================================
CLI
----------
uname -a -> display more info about the OS
uname -o -> display what OS we have
type
which
$PATH -> contains the paths to folder where executables are located
- internal command = shell builtin

Internal vs external commands in RHEL/CentOS 7
-----------
- running pwd as a shell builtin -> pwd
- running pwd as an external command -> cd /bin && ./pwd

Environment variables
----------
- scope -> local (available for the current shell) or global (available for all shells)
export VARIABLE -> makes variable available for all shells
export VARIABLE=1
unset VARIABLE -> delete the variable
env -> display exported variables
set -> display variables and functions
set -o -> display shell options
set -o allexport -> all variable are set as global by default
set +o allexport -> turn off the above setting
alias

CLI History
----------
history -> display the current history - written in /home/user/.bash_history
!101 -> will repeat the 101st command from history
!b -> run the latest command that starts with a 'b'
CTRL+r -> reverse search
CTRL+r -> type something -> hit Enter to run
CTRL+r -> type something -> CTRL+r to go to the next command that matches
!$ -> last used argument
HISTCONTROL=erasedups -> remove duplicates from the history

==========================================
Process text streams using filters
----------
cat
cat -vet -> also display the hidden characters (non printable) from the file
tac
nl -> display output and also number the lines
less
head -> top 10 lines by default
tail -> bottom 10 lines by default
tail -f -> follow the file
- in less use / to search forward and ? to search backward

tabs X -> set the tabs to X spaces
echo -e "file\twith\ttabs" -> \t means one tab
expand file1 > file2 -> convert tabs to spaces
unexpand -a file2 > file1 -> convert back to tabs
od -> octal dump
od -b -> octal bytes
od -a -> display as text
od -x -> display as hex

fmt -> set the width of the output (75 charactes by default)
pr -> format for printing with page numbers (66 line per page by default)
uniq -> remove duplicates
join file1 file2 -> joins related data together - by default join uses the first column as the unique data - the data needs to be presorted
paste file1 file2 -> merges the data line by line

sort -> by default sort based on the first character of each line
sort -r -> reverse sort
sort -t':' -n -k3 -> set the delimiter to : and sort based on the 3rd column
wc -> display the number of lines, words and characters
wc -l -> display the number of lines
split -l 300 -> split a file to 300 lines

tr -> translate
tr [a-z] [A-Z] < file -> translate the lower case to upper case from the file
tr -s -> suppress characters
tr -s [:space:] -> suppress additional spaces

sed -> stream editor
sed 's/bash/sh' -> replace bash with sh
sed 's/bash$/sh' -> replace bash from the end of the line with sh
sed '/^#/d' -> delete commented lines
cut -f1,3 -d':' file -> display fields 1 and 3 delimited by : from a file
cut -f1 -d$'\t' file -> diplay field 1 delimited by a TAB from a file

======================================
File management
----------
-copy, rename, delete and archive
cp, mv, rm
cp -u -> copy only if the source is newer than the destination
cp -i -> interactive mode - always ask if the file can be overwritten etc
cp -a -> maintain permissions and ownerships of the source
mv/rm -i -> interactive mode
mv -n -> prevents the destination to be overwritten if it already exists

File Globbing
----------
* -> 0 or more characters
? -> exactly 1 character
[Aa] -> A or a
ls /dev/sd[ab]?

ls -a -> list all
ls -A -> list almost all - excludes the . and .. directories
ls -lha
ls -ltr -> reverse sort listing on the timestamp
ls -R -> recursive
ls -ld -> long listing on a directory

cp -R -> recursive
mkdir -p -> create parent directory structure
mkdir -m 700 -> create a directory and set the default permissions to 700
rmdir -> can only be used if the folder is empty
rm -rf

touch
touch -a -> change the access time 
touch -m -> change the modified time
stat -> display multiple details about a file
file -> display what type of file we have
find -type f -> look for files
find -type d -> look for folders
find -mmin 5 -> look for files modified in the last 5 minutes
find -mtime 0 -> look for files modified in the last 24h
find -newer -> useful with a baseline file
find -empty -> empty files
find -size +10M -> look for file with the size greated than 10MB

Archiving
---------
tar -c -> create
tar -v -> verbose
tar -f -> destination file
tar -t -> test
tar -x -> expand/extract
gzip file.tar -> zip up a tar file (also compress it)
gunzip
tar -z -> compress using gzip
find /var/log -type f -mtime +1 | cpio -o > log.cpio
cpio -id --no-absolute-filenames < file.cpio -> expand the cpio archive
bzip2
bunzip2
tar -j -> compress using bzip2
xz
unxz
tar -J -> compress using xz

=======================================
Streams, pipes and redirects
----------
< -> standard input
> -> standard output
1> -> standard output
2> -> standard error
>> -> append
2>&1 -> redirect standard error to the same place as the standard output
tee -> redirect standard output to a file and also to the screen

=======================================
Processes
----------
& -> run process in the background
bg -> start a stopped process in the background

Screen
----------
screen sleep 100 -> create a new screen where sleep will run
CTRL+A+D -> detach the screen and return to the previous
screen -ls -> list screens
screen -r ID -> re-attach screen

chmod u+s $(which screen) -> set SUID in order to share screen
chmod 755 /var/run/screen -> ensure correct permissions
screen -S *name* -> start screen with easy name
CTRL+A - :multiuser on -> configure setting for multiple users
CTRL+A - :acladd *user* -> configure ACL
screen -x *user that set up the screen*/*name of the screen* -> connect to screen

nohup *command* -> allow the command to run also after log out
ps -l -> long listing for the processes from the current shell
ps -f -> full listing for the processes from the current shell
ps -elf -> list all processes that run on the system
pgrep
pkill -> terminate the processes
killall -u *user* -> terminate all processes for a user

kill -l -> list all the signals
kill -15 | -term | -sigterm
kill -9 | -kill | -sigkill

top -> sort by CPU usage by default
- press f to toggle the columns from top
- press s to sort the current sort field
top -p PID -> display on the info for the selected PID
- press k to kill the selected process
uptime
free

=============================================
Process execution priority
-----------
nice -> display the default niceness (how much CPU time the application is going to take from the CPU)
ps -l -> look for the NI column in order to view the niceness
-20 to 19 -> values for the niceness
niceness 0 -> default niceness
niceness 19 -> the apps will take the least CPU time - lowest priority
niceness -20 -> the apps will take the most CPU time - highest priority

nice -n 5 *command* -> set the niceness to 5 for the command
renice -n 15 -p PID -> set the niceness to a higher value - lower priority
renice -n -5 -p PID -> set the niceness to a lower value - higher priority - only root can do it
top
- press r for renice

============================================
Search text files using regular expressions
-----------
grep
egrep = grep -E -> enhanced regular expressions
fgrep = grep -F
sed

^ -> beginning of the line
$ -> end of the line
[A-Za-z] -> any letter, uppercase or lowercase
[0-9] -> any digit
[a-z_] -> any lowercase letter or underscore
[349] -> 3, 4 or 9
\s -> whitespace (tab, space, eol character, new line character)
\S -> non-whitespace
\b -> word boundary (spaces, eol, hyphen etc)
\B -> non-word boundary
u* -> will match u for 0 or more times
u? -> will match u 0 or once - optional character
u+ -> will match one or more occurences of u
u{3} -> will match exactly 3 occurences - need to use grep -E

\ssystem -> will match 'file system'
\bsystem -> will match 'file system' and 'file-system'

grep -v -> won't display matches of the regular expressions
^\s*# -> lines that start with a whitespace or are commented

sed -i.bak '/^\s*#/d;/^$/d' -> delete commented and empty lines

=================================================
VIM
---------
vimtutor
ESC :q -> exit
ESC :q! -> force exit
k -> up
j -> down
h -> left
l -> right
/*text* -> search forward
?*text* -> search backwards

3 modes in vim:
- command
- edit
- lastline (using :)

command mode:
i -> insert text before the current character
I -> insert text at the beginning of the current line
a -> append after the current character
A -> append after the current line
o -> insert a new line below the current one
O -> insert a new line above the current one
G -> navigate to the last line
u -> undo

d for delete
dw -> delete to the end of the current word
3dw -> delete 3 words
d0 -> delete up to the start of the line
d$ -> delete to the end of the line
dG -> delete to the end of the document
dd -> delete current line

y (yank) for copy
yw -> copy current word
yy -> copy current line

c (clear) for cut
cw -> cuts current word

p -> paste deleted/copied text starting from the current position
P -> paste at the beginning of the line

lastline mode
:w -> save
:wq -> save and exit
:x -> save and exit
:e -> revert to last saved version
:wq! -> force save and exit
:q! -> quit without saving
ZZ -> save and exit

============================================
Partitions and File Systems
----------
MBR:
- maximum size of 2TB per partition
- maximum 4 primary partitions (out of which 1 can be an extended partition)
- Linux limits up to 15 partitions for SCSI devices

GPT:
- maximum size of 8ZB per partition
- maximum 128 primary partitions
- same limitation from Linux for SCSI devices

File Systems: ext2, ext3, ext4, ReiserFS, XFS, Btrfs etc

fdisk -> create partitions with MBR
parted -> create partitions with MBR/GPT
gdisk -> create partitions with GPT

fdisk /dev/sdX -> start an interactive session for creating partitions
fdisk -l /dev/sdX -> check the partition table for sdX

mkfs -t ext4 /dev/sdXY -> create an ext4 file system on the sdXY partition
mkfs.ext4 /dev/sdXY
mke2fs -t ext4 /dev/sdXY
mkswap /dev/sdXY -> create swap partition

mkfs -t vfat /dev/sdXY -> create a VFAT file systems for removable devices
mkfs.vfat /dev/sdXY
mkdosfs /dev/sdXY

xfsprogs -> for mkfs.xfs
mkfs.xfs /dev/sdXY
mkfs.xfs -f /dev/sdXY -> force create a XFS file system
mkswap /dev/sdXY -> create swap space

====================================
Maintaining the integrity of the file systems
----------
fsck /dev/sdXY -> check and repair file system inconsistencies
fsck -> file systems should not be mounted before starting fsck
fsck -> is calling other utilities for checking the file systems:
	- e2fsck for ext2
	- xfs_repair for xfs
	- btrfsck for btrfs
fsck -A -> check all file systems from fstab with fsckpass>0
fsck -AR -> as above, but exclude root file system
fsck -ARM -> as above, but exclude all mounted file systems
fsck -f /dev/sdXY -> force a full check even if a file system was marked as clean

xfs_check
xfs_repair /dev/sdXY -> check and repair
xfs_repair -n /dev/sdXY -> just check the file system

df -T -> display file system type
df -h -> display disk space info
df -i -> display inode utilization
du -sh /home -> display the size of the folder - summary
du -h /home -> display size of each file

btrfsck /dev/sdXY -> for BTR file systems

display file system metadata:
- tune2fs for ext
- dumpe2fs for ext
- xfs_info for xfs
- xfs_metadump is more detailed than xfs_info

debugfs
tune2fs -l /dev/sdXY -> display metadata
dumpe2fs /dev/sdXY -> display detailed metadata
xfs_info /dev/sdXY
/etc/mke2fs.conf -> config file for mke2fs

=============================================
Mounting and Unmounting
-----------
mount
umount /*mount point*

mount -o *options* /dev/sdXY /*mount point*
- options:
rw
noatime -> don't maintain the access time
usrquota -> set user quotas

blkid /dev/sdXY -> get the UUID of the partition
- get UUID from vim:
:r!blkid /dev/sdXY

/etc/mtab -> file containing current mounts - info comes from /proc/mounts
/etc/fstab -> config file for mounts

swapon -s -> display swap partitions
swapon -a -> mount the swap partitions

/dev/cdrom /media/cdrom iso9660 ro,user,noauto 0 0 -> mount config in fstab for cdrom

===============================================
Manage disk quotas
----------
- quotas are applied to a file system as a mount option
- we can apply quotas to a user or to a group

apt-get install quota
yum install quota

who -> check who is loggen on
/etc/nologin -> when created, other users cannot login

rsync -arv /folder1/ /folder2 -> copy contents
-a -> archive, mainain ownerships etc
-r -> recursive
-v -> verbose

Quotas
---------
quotacheck -cmu /dev/sdXY -> create the quota database
-c -> create
-m -> mount
-u -> user
quotaon /dev/sdXY -> turn on quota checking
setquota -u *user* 100000 150000 0 0 /dev/sdXY -> set quota for the user to a soft limit of 100MB (needs to be in 1K blocks) and a hard limit of 150MB and no inode limits
edquota *user* -> interactive form
repquota -au /dev/sdXY -> show quota usage
edquota -p *user1* -u *user2* -> use user1 as prototype and copy details to user2

===========================================
Permissions and Ownership
---------
default permissions:
- 666 for files
- 777 for folders

umask -> set default permissions by removing them
0 or - -> no permissions
4 or r -> read
2 or w -> write
1 or x -> execute

newgrp -> toggle between groups to set the primary group - need to exit after finishing
chmod
- we need execute permissions in order to enter a folder

stat -> display all properties of the file
stat -c %A -> display symbolic permissions (rwx)
stat -c %a -> display octal permissions (644)
stat -c %G -> display GID name
stat -c %g -> display GID number

chown user file
chown user:group file
chgrp group file

Special permissions
-----------
- set user ID bit
- set group ID bit
- sticky bit

- sticky bit - when set on a folder, you can only delete the files you own - indicated by the lowercase t
- uppercase T indicates that the execute permission is not set for others
chmod o+t folder
chmod 1777 folder

- set group ID bit - set on executable -> the executable runs with the group ID of the owner group
- when set on a folder -> all new files are going to be group owned by the group owner of the folder
chmod 2777 folder -> set group ID bit
chmod 3777 folder -> set group ID bit + sticky bit

- set user ID bit - set on executable -> the executable runs with the UID of th owner of the file
chmod u+s file
chmod 4777 file
find /usr/bin/ -perm -u+s -> find executables with SUID bit

==============================================
Hard links
---------
- a hard link is a file with a hard link count greater than 1
- folders have a hard link count greater than 2: the . (dot) folder and the .. (2 dots) folder
ln source destination

Symbolic (soft) links
---------
ln -s source destination
find / -type l -xtype l -> look for broken links

==============================================
System files and their correct location
---------
FHS -> File system Hierachical Standard:
- shareable and static directories - accessible to all users, but not usually written to - often programs or documentation
/usr, /opt etc
- shareable and variable folders - shared folders that will be written to by users
/var/mail, /var/spool/cups
- unshareable and static folders - config and boot folders
/etc, /boot
- unshareable and variable folders - will be written to by root or services
/var/run, /var/lock

Finding files
---------
find -name 'file*' -type f -> the AND between the 2 conditions is implicit
find -name 'file*' -o -type f -> -o for OR
find -type f NOT -iname 'file*' -> find files that don't have 'file' in their name

locate -> search indexed DB
updatedb -> cron job or command to update the DB for locate
/etc/updatedb.conf
- Debian systems have mlocate

whatis command -> display info about the command
whereis command -> display where the program and the documentation are located
which command -> display where the program is located
type command -> display the type of the command (aliased, builtin)