1 Understand and User Essential Commands

RHCSA Notes
===========


1 - Understand and Use Essential Tools

1.1 : Access a shell prompt and issue commands with correct syntax

Connection modes:
    local text mode
    Local graphcal mode
    remote text mode
    graphical mode

ssh aaron@192.168.0.17


1.2 : Log in and switch users in multiuser targets
-------------------------------------------

Linux Security :
    Access Controls
    PAM
    Network Security
    SSH Hardening
    SELinux
    Many More

Linux Accounts
    User Accounts (/etc/passwd)
    Group Accounts(/etc/group)

Account Types
    User Accounts
    Superuser Account (UID=0)
    System Accounts (UID<100 OR between 500 - 1000)
    Service Accounts

#NB, users can have multiple group IDs. 

$ id
This command gives information about the user

$ who
To see the list of users currently logged in 

$ last
Record of all logged in users, and date+time of reboots


Switching Users

$ su - 
To switch user

SUDO
/etc/sudoers
Defines the default configuration and policies applied by the sudo command. This file can only be edited using the visudo command

Only users listed in the /etc/sudoers file can use the sudo command for privilege escalation.


Sudoer file structure:
1 User or Group - bob %sudo(group)
2 Hosts         - localhost, ALL(default)
3 User          - ALL(default)
4 commands      - /bin/ls, ALL(unrestricted)


1.3 : Read, and use system documentation
-----------------------------------------

--help

$ ls --help

All commands have a manual, that can be accessed using the man command.

$ man man
    1 - Executatble programs or shell commands
    2 - System calls
    3 - Library calls
    4 - Special files
    5 - File formats and conventions
    6 - Games
    7 - Miscelleneous
    8 - SYstem administration command
    9 - Kernel routines

-- $ apropos
    Lets you search through man pages

$ apropose director
All man pages which have director in their man pages

NB : sudo mandb
Launch this command on new machines to initialize the apropos db

$ apropos -s 1,8 director
To filter out man page sections


1.4 : Locate, read and user system documentation
-----------------------------------

$ info bash
 Documentation about shell commands, in a pager formats.
 Formated in a hypertext format.


 - /user/share/doc
 Documentation directory in html format



1.5 : Create, delete, copy and move files and directories
----------------------------------------------

$ ls

$ ls -l
Long listing format

$ ls -lh
File sizes in human readable format


Filesystem Tree

/
|
/home
 | /home/aaron
          | Documents
              | Invoice.pdf
 | /home/jane
/var
 | /var/log


$ pwd
Print Working Directory

$ cd
change directory

$ cd /
Go to root directory

$ cd -
Go to previous directory

$ cd
Takes you back to your home directory 

1.6 : Create and manage hard links
----------------------------------

$ stat
Gives you detailed information about some files

File -> Inodes -> Blocks on the data drive

Links : It is the number of hard links to the file.

$ ln path_to_target_file path_to_link_file

$ ln /home/aaron/Pictures/family_dog.jpg /home/jane/Pictures/family_dog.jpg


Hard links to a file can be deleted, so far as the file has one hard link regerencing it, else it would be erased

You can only hard links to files not to the directories


1.7 : Create and manage soft links
----------------------------------

$ ln -s path_to_target_file path_to_link_file

$ sls -l /home/aaron/Pictures/family_do.jpg family_dog.jpg

$ ln -l

$ readlink family_dog_shortcut.jpg
Gives you detailed information of where the link file points to. Changing the softlink name, breaks the kingk



1.8 : List, set and change standard permissions
---------------------------------------------

$ chgrp group_name file/directory

$ chgrp wheel family_dog.jpg

To see what groups our user belongs to

$ groups

To change the user owner of a file or directory

$ sudo chown user file/directory
only root user can change the user owner


$ sudo chown aaron:family family_dog.jpg

Item identifier when issuing the ls -l command

Directory  d
Regular file  -
Character device  c
link  l
secret file  s
pipe  p
block device  b

For directories, we need
r - to read inside the directory
w - to create something inside the directory
x - to move into the directory


To change permissions we use the chmod command
$ chmod permissions file/directory

$ chmod u+w family_dog.jpg

To remove permissions

$ chmod o-r family_dog.jpg

To set the permissions exactly 

$ chmod g-r family_dog.jpg


1.9 : SUID, SGID and sticky bit
-------------------------------


* Set User Identification Bit (SUID) : When ever the file is executed,
it is executed with the user id of the owner of the file and not the person running the files

$ chmod 4664 suidfile
the execute bit for the user is replaced by S
-rwSrw-r--

if the user already has execute permission S is replaced with suidfile
$ chmod 4764 suidfile
-rwsrw-r--


* Set Group ID (SGID) : 

$ chmod 2664 sgidfile
-rw-rwSr--

Find users with the suid or sgid permission

$ find . -perm /4000  # suidfile

$ find . -perm /2000  # sgidfile


* Sticky bit

Set on directories that are shared with people. 
Allows a file that are shared, only the owner can delete the files

$ chmod +t stickydir
$ chmod 1777 stickydir
drwxrwxrwt

it you remove the execute bit
$ chmod 1666 sticky
drw-rw-rwT



1.10 : Search for files
------------------------
$  find [/path/to/directory] [search_parameters]

Find all image files in a website

$ find /usr/share/ -name '*.jpg'

search large files greater than 10MB

$ find /lib64/ -size +10MB

Files changed in less than a 
$ find /dev/ -mmin -1

$ find -mtime 2         # 24-hour periods

Modification means, creation or editing a file

Modified Time != Change Time

Change Time = Meta data about the data

$ find -cmin -5


File sizes :

c bytes
k kilobytes
M megabytes
G gigabytes

$ find -size 512k

Expressions in search expression 

$ find -name "f*"

$ find -size 512k

$ find -name "f*" -size 512k      # AND operator

$ find -name "f*" -o -size 512k   # OR operator

$ find -not -name "f*"            # NOT operator

$ find \! -name "f*"              # NOT operator


Search Files based on permissions :

$ find -perm 664

$ find -perm -664  # Find files with at least 664 permissions

$ find -perm /664  # Find files with any of these permissions

$ find -perm u=rw,g=rw,o=r # Find files with exactly 664 permissions


$ find -perm 600

$ find -perm -100               #Execute Only permission

$ find \! -perm -o=r

$ find -perm /u=r,g=r,o=r


find /var/log/ -perm -g=w ! -perm /o=rw > /home/bob/data.txt 
Group can write, but others cannot read or write


1.11 : Compare and manipulate file content
-------------------------------------------


$ tac 
$ cat

$ tail , by default gives you the last 10 lines

$ head , view the first 10 lines

> Transforming Test: Sed

Used for search and replace

$ sed 's/canda/canada/g' userinfo.txt

$ sed -i 's/canda/canada/g' userinfo.text
-i inplace, to save the changes

> Extracting columns : cut

$ cut -d ' ' -f 1 userinfo.txt


> Uniqe and sort

uniq commands removes adjancent repeated lines.
Usually, you need to sort before uniq

$ sort countries.txt | uniq


> Comparing Files : diff

diff file1 file2

Identical lines are not shown. 
Using context:

$ diff -c file1 file2


$ diff -y file1 file2
Side by side comparson

$ sdiff file1 file2
Side by side diff


1.12 : Pagers and vi demo
-------------------------


:/This\c - case sensitive search
:yy - copy
:p  - paste
:dd - cut


1.13 : Search using grep
-------------------------

$ grep 'CentOS' /etc/os-release
$ grep -i 'CentOS' /etc/os-release  - -i ignore case

Search all files under a directory or subdirectory

$ grep -r 'CentOS' /etc/
$ grep -ri 'CentOS' /etc/

Invert Search results

$ grep -vi 'centos' /etc/os-release

To match only words

$ grep -wi 'red' /etc/os-release

To show only matching options
$ grep -oi 'centos' /etc/os-release


1.14 : Analyzing text With Regular Expressions
--------------------------------------------------

Regex Operators
^ $ . * + {} ? | [] () [^]

$ cat names.txt
adam
adnan
basam
samad
samuel
sheela
ravi
mausami

Names that start with sam, not with same within
$ grep '^sam' names.txt

Names that end with sam
$ grep 'sam$' names.txt

List lines that end with number 7
$ grep '7$' /etc/login.defs

Match any ONE character
$ grep -r 'c.t' /etc/

Match whole words, with one character as any
$ grep -rw 'c.t' /etc/

Escaping For Special Characters
$ grep '\.' /etc/login.defs

Match The Previous Element 0 or More Times
let* == letttt

$ grep -r 'let*' /etc/
$ grep -r '/.*/' /etc/

All sequences of caracters where 0 appears one or more times
0+ == 000
$ grep -r '0\+' /etc/
Since + is an operator, we use \ - back slash

1.15 : Extended Regular Expressions
-----------------------------------
We user -E not to backslash the regex operator +
All sequences of caracters where 0 appears one or more times

$ grep -Er '0+' /etc/
$ egrep -r '0+' /etc/

{}: Previous Element Can Exist "this many" Times
0{min,max}

This will match strings with 3,4 or 5 0s
$ egrep -r '0{3,5}' /etc/

This will match three 0s at least
$ egrep -r '0{3,}' /etc/

To find all string that contains a 1, then at most three 0s
$ egrep -r '10{,3}' /etc/

To find all string that contain exactly 3 0s
$ egrep -r '0{3}' /etc/

?: Make The previous element optional
$ egrep -r 'disabled?' /etc/

|: Match One Thing Or The Other
$ egrep -r 'enabled?|disabled?' /etc/

[]: Ranges Or Sets
[a-z][0-9]

This will match cat or cut
$ egrep -r 'c[au]t' /etc/

To find drive letters in /etc
$ egrep -r '/dev/[a-z]*[0-9]?' /etc/


() : Subexpressions

$ egrep -r '/dev/(([a-z]|[A-Z])*[0-9]?)*' /etc/


[^] : Negated Ranges Or Sets

To make sur http is not followed by letter s
$ egrep -r 'http[^s]' /etc/

No lower-case caracter after the slash
$ egrep -r '/[^a-z]' /etc/


1.16 : Archive, backup, compress, unpack and uncompress files
--------------------------------------------------------------

1. Archive
2. Compress
3. Backup

tar = tape archive

To display the content of an archive
$ tar --list --file archive.tar

$ tar -tf archive.tar

$ tar tf archive.tar

Tar commands:
$ tar --create --file archive.tar file1
$ tar cf archive.tar file1

To add a file to the archive:
$ tar --append --file archive.tar file2
$ tar rf archive.tar file2

To add a directory:
$ tar --create --file archive.tar Pictures/

To extract the files:
$ tar --extract --file archive.tar 
$ tar xf archive.tar

To  extract to another directory:
$ tar --extract --file archive.tar --directory /tmp/
$ tar xf archive.tar -C /tmp/

File archives also stores permissions and ownership information




1.17: Compress and Uncompress files
-----------------------------------

gzip, bzip2, xz

$ gzip file1
$ bzip2 file2
$ xz file3

#All these commands compress the file and deletes the original

To uncompress:
$ gunzip file1.gz       gzip --decompress file1.gz
$ bunzip file2.bz2      bzip2 --decompress file2.bz2
$ unxz file3.xz         xz --decompress file3.xz

To keep original files untoched after compression

$ gzip --keep file1
$ bzip2 --keep file2
$ xz --keep file3



$ zip archive file1

$ zip -r archive.zip Pictures/

$ unzip archive.zip

Generating a tar archives

$ tar --create --gzip --file archive.tar.gz file2

$ tar --create --bzip2 --file archive.tar.bz2 file2

$ tar --create --xz --file archive.tar.xz file2

Autop Compress

$ tar --create --autocompress --file archive.tar.gz


1.18: Archive, compress, unpack and uncompress files using star
-----------------

SYNTAX:
$ star -options file=/path_to_archive.star files

$ star -cv file=/home/aaron/archive2.star file1

TO LIST THE CONTENT:
$ star -tv file=/home/aaron/archive2.star

TO EXTRACT:
$ star -xv file=/home/aaron/archive2.star

TO EXTRACT TO A DIFFERENT DIRECTORY:
$ star -xv file=/home/aaron/archive2.star -C /tmp/

TO ARCHIVE AND COMPRESS FILES
$ star -cv -z file=/home/aaron/archive2.star.gz file1

$ star -cv -bz file=/home/aaron/archive2.star.bz2 file1

To unpack : Same options for different compression types. Star automatically detects the compression format

$ star -xv file=/home/aaron/archive2.star.gz


1.19: Use input-output redirection
-----------------------------------

stdin  <
stdout 1>
stderr 2>

$ grep -r '^The' /etc/ > 2> /dev/null

Redirecting both output and errors in one comman
$ grep -r '^The' /etc/ 1>output.txt 2>errors.txt

$ grep -r '^The' /etc/ 1>>output.txt 2>>errors.txt

Recirecting both output and errors in same file
$ grep -r '^The' /etc/ > all_output.txt 2>&1
or
$ grep -r '^The' /etc/ 1>all_output 2>&1 


Redirecting Input

$ sendemail someon@example.com < emailcontent.txt

Heredoc and Here String

$ sort <<\'EOF''
>6
>3
>2
>5
>1
>4
>EOF
>1
>2
>3
>4
>5
>6

$ bc <<<1+2+3+4

Pipiing :

$ grep -v '^#' /etc/login.defs | sort | column -t
column -t is used to align output into columns

1.20 : Backing files to a Remote System
----------------------------------------

Syntax:
$ rsync <options> local_dir user@host:/remote_dir

$ rsync -a Pictures/ aaron@9.9.9.9:/home/aaron/Pictures

Always have a slash at the end of directory names

You can synch both local and/or remote directories

Disk Imaging:

$ sudo dd if=/dev/vda of=diskimage.raw bs=1M status=progress
First unmount drive before using dd

If you want to restore a disk image from a file
$ sudo dd if=diskimage.raw of=/dev/vda bs=1M status=progress



1.21: Securely transfer files between systems
----------------------------------------------

$ scp aaron@192.168.1.27:/home/aaron/myfile.tgz /home/aaron/myfile.tgz

$ scp /home/aaron/my_archive.tar aaron@192.168.1.27:/home/aaron/my_archive.tar

From remote machine to remote machine

$ scp aaron@192.168.1.27:/home/aaron/familyphoto.jpg aaron@192.168.1.59:/home/aaron/familyphoto.jpg



sftp

$ sftp aaron@192.168.1.27

$ sftp> help == $ sftp> ?

Copy remote file to local machine

$ sftp> get familypicture.jpg

$ sftp> get -r Pictures

Copy local files to remote machine
$ sftp> put myarchive.tgz

$ sftp> put -r my_photos/

$ sftp> bye