Internal and External Commands

Internal commands are buil-in commands.
If you don't know the type of a command, you can use "type" command:


..$ type cd
cd is a shell builtin

..$ type pwd
pwd is a shell builtin

..$ type bash
bash is /bin/bash

There is a trick about "type" command:

..$ type -a pwd
pwd is a shell builtin
pwd is /bin/pwd

"-a" option shows us is the command duplicated by external command or not.
let's understand it with "time" command:

..$ time pwd
/home/user

real 0m0.000s
user 0m0.000s
sys 0m0.000s

..$/usr/bin/time pwd
/home/user
0.00user 0.00system 0:00.04elapsed 0%CPU
(0avgtext+0avgdata 2240maxresident)k
64inputs+0outputs (1major+83minor)pagefaults 0swaps

So what happened up there:
When we typed "time pwd" we used the internal(built-in) time command
Then we typed "/usr/bin/time pwd"    that was the external time command
I hope this is a helpful explanation about difference between internal and external commands.

Appending multiple files into a single file

Let's assume you have file1.txt file2.txt file3.txt
Open the terminal and...

..$ cat file1.txt file2.txt file3.txt > file4.txt

VIM tricks:
*While you are in normal mode you can press "ZZ" to save and exit quickly
*You can go insert mode to below the current line with pressing "o" button
*If you want to delete current line quickly in normal mode, you should press "dd"

Note to myself

/etc/apt/sources.list    holds the archive region

http://unix.stackexchange.com/questions/88644/how-to-check-os-and-version-using-a-linux-command          os information

/dev info
http://linuxboxadmin.com/articles/tools-and-utilities/linux-file-permissions-beyond-rwx.html

char and block file
http://www.linuxnix.com/linuxunix-difference-between-char-and-block-file/

user is not in the sudoers file

sudo vim /etc/sudoers

#

# This file MUST be edited with the 'visudo' command as root.

#

# Please consider adding local content in /etc/sudoers.d/ instead of

# directly modifying this file.

#

# See the man page for details on how to write a sudoers file.

#

Defaults    env_reset

Defaults    mail_badpass

Defaults    secure_path="/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/snap/bin"

# Host alias specification

# User alias specification

# Cmnd alias specification

# User privilege specification

root    ALL=(ALL:ALL) ALL

# Members of the admin group may gain root privileges

you sould add your user below the "root ALL=(ALL:ALL) ALL" like this:

# User privilege specification

root    ALL=(ALL:ALL) ALL

user    ALL=(ALL:ALL) ALL

some important things for me

lsb_release -a

cat /etc/lsb-release 
to learn os


adduser username
usermod -aG sudo username
su - username
to add user and switch to it




userdel mynewuser
userdel -r mynewuser
visudo
mynewuser ALL=(ALL:ALL) ALL
to delete user


ps aux and top



Command 1. Using wget
# wget http://ipecho.net/plain -O - -q ; echo


Command 2: Using curl
# curl ipecho.net/plain; echo


Command 3: Using wget
# wget http://observebox.com/ip -O - -q ; echo


Command 4: Using curl
# curl icanhazip.com


Command 5: Using curl
# curl ifconfig.me
to get public ip

Adding and Removing User in Ubuntu

 First login to root:

..$ su
..root# adduser fly
Adding user `fly' ...
Adding new group `fly' (1001) ...
Adding new user `fly' (1001) with group `fly' ...
Creating home directory `/home/fly' ...
Copying files from `/etc/skel' ...
Enter new UNIX password:

Retype new UNIX password: 

passwd: password updated successfully

Changing the user information for fly

Enter the new value, or press ENTER for the default

Full Name []: 

Room Number []: 

Work Phone []: 

Home Phone []: 

Other []: 

Is the information correct? [Y/n] y

..root#

After adding user fly we can add it to the sudo group:

..root# usermod -aG sudo fly

..root#

Now we should switch the fly:

..root# su - fly

..fly$

If you want to delete it

Close and reopen the terminal

Login as a root:

..root# userdel -r fly

for loop in linux terminal

   I have a folder which has 3 files:

..$ ls
file1 file2 file3

Let's make a for loop:

..$ for x in `ls`
> do
> echo $x
> done;

The output:

file1
file2
file3


The most important thing is the backtick.

This is a backtick:    `  
And when we use the ls in loop, we need double backtick:
 
   `ls`

If you don't know where the backtick button is on your keyboard, you can search the images in google.