Linux handbook

What is a Terminal ?

A Terminal is a program that emulates a physical Terminal (Console). The Terminal interacts with the Shell (the Command Line Interface).

What is a Shell ?

The Shell is a command-line interpreter, that is to say, it is a program that processes and executes commands.

A few simple commands

date is a command that prints the current date and time

cal display the calendar of the current month

lscpu command description

edo@ubuntu ~/Playground  lscpu

Architecture:                    x86_64
CPU op-mode(s):                  32-bit, 64-bit
Byte Order:                      Little Endian
Address sizes:                   36 bits physical, 48 bits virtual
CPU(s):                          4
On-line CPU(s) list:             0-3
Thread(s) per core:              2
Core(s) per socket:              2
Socket(s):                       1
NUMA node(s):                    1
Vendor ID:                       GenuineIntel
CPU family:                      6
Model:                           58
Model name:                      Intel(R) Core(TM) i3-3120M CPU @ 2.50GHz
Stepping:                        9
CPU MHz:                         1200.000
CPU max MHz:                     2500,0000
CPU min MHz:                     1200,0000
BogoMIPS:                        4988.79
Virtualization:                  VT-x
L1d cache:                       64 KiB
L1i cache:                       64 KiB
L2 cache:                        512 KiB
L3 cache:                        3 MiB
NUMA node0 CPU(s):               0-3

uptime command to check how long your system has been running

clear command to clear the Terminal screen

reboot command to restart your system

pwd command to print the name of your current working directory

edo@ubuntu ~/Playground pwd
/home/edo/Playground

ls command to list the contents of your current working directory

passwd command to change your password

hostname command to display your system's hostname

free command to display the amount of free and used memory on your system

edo@ubuntu  ~/Playground  free
              total        used        free      shared  buff/cache   available
Mem:        5960092     2623072      519340      476752     2817680     2583180
Swap:             0           0           0

command with the -h option

edo@ubuntu  ~/Playground  free -h
              total        used        free      shared  buff/cache   available
Mem:          5,7Gi       2,5Gi       481Mi       462Mi       2,7Gi       2,4Gi
Swap:            0B          0B          0B

df command to display the amount of disk space available on your system (-h to display a nicer format)

history command display all the commands that you ran in chronological order

uname command to display your system's kernel information:

• -r option to print the current kernel release information
• -a option to print all the information of your current kernel at once

lsb_release -a command to display the Ubuntu version you are currently running

Filesystem

cd command is short for Change Directory and is one of the most used commands in Linux. You can't move around in Linux without it.

ls command to view the contents of the current directory:

• -l Long and detailed listing of files.
• -a List the hidden files.
• -d List directories themselves, not their contents.
• -t Sort files by modification times.
• -u When used with -l, it shows access times instead of modification times. When used with -lt, it will sort by, and show, access times.
• -r Will reverse listing order.
• -R List subdirectories recursively.

man hier command to read more about the Linux filesystem hierarchy

NAME
       hier - description of the filesystem hierarchy

DESCRIPTION
       A typical Linux system has, among others, the following directories:

       /      This is the root directory.  This is where the whole tree starts.

       /bin   This directory contains executable programs which are needed in single user mode
              and to bring the system up or repair it.

       /boot  Contains static files for the boot loader.  This directory holds only the  files
              which  are  needed during the boot process.  The map installer and configuration
              files should go to /sbin and /etc.  The operating system kernel (initrd for  ex‐
              ample) must be located in either / or /boot.

       /dev   Special or device files, which refer to physical devices.  See mknod(1).

       /etc   Contains  configuration files which are local to the machine.  Some larger soft‐
              ware packages, like X11, can have their own subdirectories  below  /etc.   Site-
              wide  configuration files may be placed here or in /usr/etc.  Nevertheless, pro‐
              grams should always look for these files in /etc and  you  may  have  links  for
              these files to /usr/etc.

whoami command prints the name of the currently logged-in user

su command to switch to any user on your system

cd ~ command will land you straight to your home directory

touch command:

1. You can update the last modification and access times of existing files.
2. You can create new empty files.

mkdir command is short for make directory is used to create directories:

• -p option to create a whole path of directories

Editors

nano editor is a very popular and easy-to-use command-line editor.

shortcuts:

vi editor is a more advanced Linux editor with tons of features and is by far the most popular editor among advanced Linux users.

works in two different modes:

1. insert mode enables you to insert text into a file
2. command mode allows you to do things like copying, pasting, and deleting text

insert mode

To insert text, you need to switch to insert mode. There are several ways you can use to change to insert mode:

command mode

To switch back to command mode, you need to press the Esc key. Most popular commands list:

Tip:

You should know that :%s/old/new will only replace the first occurrence of the word old with new on all the lines. To replace all the occurrences of the word old with new on all the lines, you should use the global option :%s/old/new/g

saving and exiting

achieve the same result:

1. :wq
2. :x
3. ZZ

file viewing commands

cat command is one of the most popular and frequently used commands in Linux. The cat (short for concatenate) command concatenates and prints files to the standard output (terminal).

tac command is basically cat written in reverse.

more command displays the content of a file one page at a time.

• Enter > to scroll down one line.
• Space Bar > to go to the next page.
• b > to go back one page.
• q > to quit.

less command is an improved version of the more. The advantage of less is that you can use the UP/DOWN arrow keys to navigate through the file. Also, less is faster than more.

head command displays the first few lines of a file

• -n option to specify the number of lines you wish to view

tail command displays the last few lines of a file

• -n option to specify the number of lines you wish to view

Copying, Moving, and Deleting Files

Copying

cp command to make a copy

• -r option

cp source_file(s) destination

Moving

mv command to move a file (or a directory) to a different location. You can also use the mv command to rename files.

mv source_file(s) destination

Hiding

You can hide any file by renaming it to a name that starts with a dot.

mv file.txt .file.txt

Removing

rm command to remove (delete) files

• -r option to remove directories

Useful commands

Determining a command's type

type command to determine the type (category) of a command

edo@ubuntu:~$ type pwd 
pwd is a shell builtin

edo@ubuntu:~$ type ls 
ls is an alias for ls --color=tty

Finding a command's location

which command to determine the location of an executable command

edo@ubuntu:~$ which rm
/bin/rm

edo@ubuntu:~$ which reboot
/sbin/reboot

whatis command to get a brief description of what a command does

edo@ubuntu:~$ whatis free
free (3)             - allocate and free dynamic memory
free (1)             - Display amount of free and used memory in the system

edo@ubuntu:~$ whatis df
df (1)               - report file system disk space usage

The man page

man page is a manual page that has proper documentation to help you understand how to use a command

The info page

info pages as an alternative documentation to the man pages

The very helpful apropos command

apropos command is one of the most helpful and yet underrated Linux commands

/usr/share/doc directory is another excellent place to look for help in Linux

Links

File inodes

Every file on Linux has a set of attributes like:

• File type
• File size
• File owner
• File permissions
• Number of hard links
• File timestamp

These attributes are stored in a data structure called the inode (index node), and each inode is identified by a number (inode number).

What is an Inode?

An inode is simply a file data structure that stores file information (attributes), and every inode is uniquely identified by a number (inode number).

There are two commands you can use to view the inode number of a file:

1. ls -i file
2. stat file

soft links

is simply a file that points to another file

ln -s original_file soft_link

a soft link has the following properties:

• The inode of a soft link is different from the original file.
• A soft link becomes useless once the original file is deleted.
• Any change to the soft link is actually a change in the original file.
• You can create soft links to directories.

hard links

is simply an additional name for an existing file. It has the same inode of the original file, and hence, it's indistinguishable from the original file.

ln original_file hard_link

A hard link has the following properties:

• A hard link has (shares) the same inode of the original file.
• A hard link remains intact if the original file gets deleted.
• Any change in the hard link is reflected in the original file.
• You can't create hard links to directories.

Root

root is a Linux user that has permission to do anything on the system. root is also known as the superuser.

todo

TODO...

TODO

todo

TODO...