Linux-Commands

Linux fundamentals:
***********

to see the linux version:

$ lsb_release -a   : ubuntu machine

$ cat /etc/os-release : centos machine

2. To check which shell you are woring on:

echo $shell

3. Commands are nothing but binaries stored on your OS
for eg: you execute pwd --> your shell goes and looks for pwd binary on your OS and exeuctes it
for eg : you give wrong command --> your shell will not be able to find the binary and you get error command not found
****************************
4. print the current date

# date

- prints the current date with time and timezone

Examples:
Convert seconds since the epoch (1970-01-01 UTC) to a date
  $ date --date='@2147483647'

Show the time on the west coast of the US (use tzselect(1) to find TZ)
  $ TZ='America/Los_Angeles' date

Show the local time for 9AM next Friday on the west coast of the US
  $ date --date='TZ="America/Los_Angeles" 09:00 next Fri'
*************************

5. print current month calender

# cal
**************************
6. Manual or man

it is binary or command that displays documentation of other commands

# man date

gives documentation of date commanduse up and down arror to scroll
press q to comoe out of it


for some commands we can also give 

commandname --help

date --help ===> doesnt work for pwd

********************************
List the file in a directory

# ls ==> list files in the directory

# ls / ==> all files in root directory

# ls -l / ==> all files in long format in root directory - more details

# ls -lS / ===> sort all the files by size and list them

# ls -lt / ==> ===> sort all the files by time and list them

# ls -al ==> list all files which are hidden ==> the .files
*******************************
Create a directory

# mkdir myproject

# ls  --> lists directory

GO inside the directory

# cd directory name

Come out of directory

cd ..  ==> come out of current directory

Go to specific directory

cd /home
or
cd /tmp


***********************************

go inside the directory and create a file

using Vim editor

vim file1

press i

enter data

press esc --> :wq!

give ls

file will be in directory

Create one more file

Creta file using nano editor

nano file1

just enter data ==> new file

ctl x ==> exit at bottom

presss y to give yes

press enter

out of file

ls

Touch comand to create empty file
**************************

Cat comand

list contents of file1

cat file1

*****************************

Make a copy of an exisitng file

cp sourcefile newfilename

cp file1 copiedfile

ls

cat copiedfile

file content will be same

************************************

Move file form 1 directory to another

Lets move file1 from myprojetc directory to another directory

comeout of myproject directory

create a new directory

mkdir newprojectfiles

cd newprojectfiles

pwd  ==> copy the path

go back to old directory

cd myproject

mv file1 /copiedpath/file1

eg : mv file1 /home/ec2-user/mynewfiles/file1

files will be not copied but moved to new directory or location

**********************************

Echo command:

print/display a line of text/string by passing it as an argument

# echo hello world

echo -e 'hello \bworld' ==> will print the text without space

here -e : enable interpretation of backslash escapes

echo -e 'hello \nworld'  ===> will seperate words and prints eachword in a new line


ehco can be used to store values

a=45
ehco $a ==> wil give 45

*************************************

Remove a file:

rm filename

rm copiedfile 

ls

removes the files

Remove directories

rm -rf mynewprojects

       -r, -R, --recursive
              remove directories and their contents recursively

       -d, --dir
              remove empty directories

  -f, --force
              ignore nonexistent files and arguments, never prompt

*********************

less command

create a file

vim file1

press i

instert lots of data in sepeerate lines

abc
12
ewe
asrwr
qwrwr
qwrwr
qrwr
..
...


save the file and comout

now give cat file1  ==> wil display content but you cannot scroll up and down the page

instead give less command

less file1  ==. will displayed the content page wise
use up and down arrow to scroll

more file1 ==> also shows how much more % data is left to be dipslayed

*********************

tail command

tail file1

will print only last 10 lines of the file

or

tail -n 5 file1 ==> print only last 5 lines of the file

*******************

head command

head filename

will print only first 10 lines of the file

head -n 5 filename

will print only first 5 lines of the file

*********************

Grep command

Used to search for characters in a file and present it to you

create afile 

vim filenew

This is my file for devops
created by sonal for devops
sonal is teaching Linux
We are learning Linux commands for devops

:wq!

grep devops filenew  ==> give only that content of file highlighting devops

This is my file for devops
created by sonal for devops
We are learning Linux commands for devops

grep devops filenew -c ==> gives count of lines that have matchin string/character

3

grep devops filenew -v ==> gives those lines that dont have the matching string.

sonal is teaching Linux

grep devops file2 -n ===> gives the line numbers along with lines with matching string

[ec2-user@ip-172-31-15-184 ~]$ grep devops file2 -n
4:this file for devops
5:created by sonal for devops
7:we will need it for devops

************************

History command

************************

Sort commad

this command will print the content of the file on the screen in a sorted manner

cat command just displays the content of the file

but sort command will print the content in sorted manner

# sort file1

# sort file2

*************************

execute 2 commands together 

use | symbol  == this is pipeing symbol

# sort file1 | less

sort file content and displayes data as pages

# sort file1 | grep sonal file2

sort the contnet of the file and give lines that have above string

example2:

[ec2-user@ip-172-31-15-184 ~]$ sort file2
created by sonal for devops
ssh -i "02dec.pem" ec2-user@ec2-3-145-61-249.us-east-2.compute.amazonaws.com
this file for devops
we are learning linux
we will need it for devops

Example 3:===> 2 commands together
[ec2-user@ip-172-31-15-184 ~]$ sort file2 | grep devops file2
this file for devops
created by sonal for devops
we will need it for devops

example 4: execute 2 commands 

[ec2-user@ip-172-31-15-184 ~]$ sort file2 | grep devops file2 -n
4:this file for devops
5:created by sonal for devops
7:we will need it for devops

==> gives output with line number

************************************

Become a super user to provide privelaged access to create, and installation

by default in AWS root is the previlaged user and you cna become a root user by giving command
sudo su -

suberuser do superuser -

********

create a user in Linux

first become root user and then execute these commands.

useradd alice
useradd bob

cat /etc/passwd ==> users in this system that have been created, at end you will see alice and bob also

Delete a User
********

userdel bob

to check the output

give cat /etc/passwd

or 

tail /etc/passwd ==> will return last lines

you will see bob is deleted

*********

Create a user group

what is a group ==> set of users form a group

commad to create a group

# groupadd purdue  ==> creates a new group

see the group created

# cat /etc/group  ==> shows information about all the groups, at end see you group name
right now there is no user in it

scroll up and see other groups like wheel which has user ec2-user
wheel:x:10:ec2-user

adm group which has user ec2-user


adm:x:4:ec2-user

extreme right is user and left is group name

Also note that, when we just create a user, it also creates a group with same name as user
hence you see alice and bob at the end

****************

Now add users to the created group== command ==> usermod


# usermod -G purdue alice

modify user and add alice user to group purdue

-g ==> group 

# cat /etc/group

purdue:x:1003:alice

***********************************

File permissions:

In linux ever file has 3 permissions : read, write and execute

read : one can view and copy contents of file

Write : one can modify file content

exeucte: one can run the file if it is executable

Permissions to directories will be same

execute commad ls -al

all the files will be displayed including hidden files

take the file that we have created : file1

-rw-rw-r-- 1 ec2-user ec2-user  177 Feb  6 02:20 file1

here :

file1 : name of file

 Feb  6 02:20 : time when the file was last accessed

177 : size of the file in bytes

ec2-user : the right side first one --> is the name of group in which user is created

ec2-user : the left side one --> the name of the user or owner of the file

1 : shows weather file has hard links by default its 1


-rw-rw-r--  : these are permissions : there are 3 sets of permissions

- : in the begingin shows its a file

rw- : permissions given read and write to the owner of the file ec2-user

rw- : permissions given read and write to any user in the group ec2-user

r-- : permissions given read only to any other user who are not owner neither part of ec2-user group


example 2:  take another example from list

-rw-r--r-- 1 ec2-user ec2-user  231 Jul 15  2020 .bashrc

here 

.bashrc : name of the file

Jul 15  2020: date when it was last accessed

231 : size in bytes

ec2-user : the right side first one --> is the name of group in which user is created

ec2-user : the left side one --> the name of the user or owner of the file


-rw-r--r--:


- : in the begingin shows its a file

rw- : permissions given read and write to the owner of the file ec2-user

r- : permissions given read and write to any user in the group ec2-user

r-- : permissions given read only to any other user who are not owner neither part of ec2-user group


example 3:

drwxrwxr-x 2 ec2-user ec2-user   19 Feb  6 01:58 mynewfiles


here 

d : it is a directory

rwx : permissions given read and write, executable to the owner of the file ec2-user

rwx :  permissions given read and write and execute to any user in the group ec2-user

r-x :  permissions given read and exeucte to any other user outside the group ec2-user



*****************************

Change permissions of a file


Change permission of file file1

current permissions : -rw-rw-r-- 1 ec2-user ec2-user  177 Feb  6 02:20 file1


change permissions for all users of the file: add executable permissions to the all users, owner + group + all users other than the group

# chmod a+x file1

here 

a+x ==> a = all users(user, group , all users) , + add permission, x : executable permission

new permission:

-rwxrwxr-x 1 ec2-user ec2-user  177 Feb  6 02:20 file1  : executable added for all users

Example 2:

Now chnage permssion for group.. remove excutable permission for group on the file

ls -al

-rwxrwxr-x 1 ec2-user ec2-user  177 Feb  6 02:20 file1

chmod g-x file1

-rwxrw-r-x 1 ec2-user ec2-user  177 Feb  6 02:20 file1

Example 3:

remove executable for only main user of file

chmod u-x file1

-rw-rw-r-x 1 ec2-user ec2-user  177 Feb  6 02:20 file1


Example 4:

add executable permission to main user

chmod u+x file1

-rwxrw-r-- 1 ec2-user ec2-user  177 Feb  6 02:20 file

Example 5:

remove executable permission for others only

chmod o-x file1

-rwxrw-r-- 1 ec2-user ec2-user  177 Feb  6 02:20 file1

*******************************



list all users account using the /etc/passwd file
$ cat /etc/passwd

Each line in the file has seven fields as follows. For example, consider the following line:
vnstat:x:131:137:vnstat daemon,,,:/var/lib/vnstat:/usr/sbin/nologin

Where,

vnstat – The user name or login name.
x – Encrypted password is stored in the /etc/shadow file.
131 – UID (user ID number)
137 – Primary GID (group ID number)
vnstat daemon – GECOS. It may includes user’s full name (or application name, if the account is for a program), building and room number or contact person, office telephone number, home telephone number and any other contact information.
/var/lib/vnstat – Home directory for the user.
/usr/sbin/nologin – Login shell for the user. Pathnames of valid login shells comes from the /etc/shells file.


********************
Of course we can use pagers such as more/less commands as follows to view the /etc/passwd file:
$ more /etc/passwd

okay, it will show how more pages with content are left

$ less /etc/passwd

you can access the file using up and down arrow

*********************

limit outputs using the head command and tail command as follows:
tail -5 /etc/passwd  -- show last 5 users
head -5 /etc/passwd  -- shows top 5 users


********************

To list only usernames type the following awk command:

$ awk -F':' '{ print $1}' /etc/passwd


Sample outputs:

root
daemon
bin
sys
sync
games
man
lp