- Linux is a free, open source operating system that enables the communication between computer hardware and software. The Linux OS was developed by Linus Torvalds in 1991.
- Open source
- community Support
- Support wide variety of hardware
- Most servers runs on Linux
- Secure compare to Windows
There are many Linux distributions available in the market. It provides a different flavor of the Linux operating system to the users. Popular distros are :
- Ubuntu.
- Fedora.
- Debian.
- Linux Mintand so on.
Linux provides a dedicated set of file systems that stores every bit of data required for booting up the Linux systems. The file system holds the collection of data or files stored within the computer's hard disk or storage device.
Use "man heir" to find more insights on linux files system
ls /etc/
- This directory contains all the configuration file of your application. if something goes wrong you should be looking in this directory.
ls /bin
- This directory contains binaries for use by all users and also contains executable files, Linux commands that are used in single user mode, and common commands that are used by all the users, like cat, cp, cd, ls, etc
ls /sbin
- This directory contains binaries to configure the operating system and executable files. It only contains system binaries which require root privilege to perform certain tasks and are helpful for system maintenance purpose. e.g. fsck, root, init, ifconfig, etc
ls /lib
- This directory contains shared libraries which are often used by the ‘/bin’ and ‘/sbin’ directories. It also contains kernel module. These filenames are identable as ld* or lib*.so.*
ls /opt
- This directory's main purpose is to store optional application software packages. In many cases this is software from outside the distribution repository. Add-on applications from individual vendors should be installed in ‘/opt’. In some systems ‘/opt’ is empty as they may not have any add-on application.
ls /tmp
- This directory in Linux based systems contains necessary files that are temporarily required by the system as well as other software and applications running on the machine.
For example, when you are writing a document, all the content inside that document is saved as a temporary file inside the /tmp directory. After you have saved it, it gets stored in your preferred location, and the temporary file gets removed once you exit the document.
ls /boot
- This directory contains contains the linux boot configuration files. This is one of the MOST important folder. Removing anything from this directory or a file getting corrupted will result in a OS crash after reboot. You system won't be able to boot without files in the /boot directory.
ls /dev
- This directory contain files that represent devices that are attached to the local system. However, these are not regular files that a user can read and write to; these files are called devices files or special files. Device files are abstractions of standard devices that applications interact with via I/O system calls.
ls /media
- The /media directory contains subdirectories where removable media devices inserted into the computer are mounted. For example, when you insert a CD into your Linux system, a directory will automatically be created inside the /media directory. You can access the contents of the CD inside this directory.
ls /mnt
- This directory and its subdirectories are intended for use as the temporary mount points for mounting storage devices, such as CDROMs, floppy disks and USB (universal serial bus) key drives. /mnt is a standard subdirectory of the root directory on Linux
ls /proc
- This directory is for each processes running on our system. It also contains some configuration files
ls /var
- This directory contains variable data files. This includes spool directories and files, administrative and logging data, and transient and temporary files.
- Normal files
- Directories
- References to files
- Character device files
- Block device files
- Symbolic links
- Local domain sockets/named pipes)
Find more info about files in the directory using below command
ls -l
Sort files based on timestamp
ls -lt
Sort files based on timestamp in decending order
ls -lt
Configured in /etc/monitrc
- apt-get update
- apt-get install monit
- systemctl enable monit
- systemctl start monit
Edit /etc/monit/monitrc
systemd is monitoring init system used to manage services
systemctl <command> <unitname>
Make sure this unit always starts at boot.
Opposite of enable
Start this unit now(will not automatically starts at next boot)
Stop a running unit(will not prevent starting at boot, if enabled)
Reread the program configuration files
Kill the process and start again, rereading the configuration files
Check status of unit, show last few lines of log output
- Journalctl
- journactl -u
- journactl --since "2 min ago"
Find current working directory:
pwd
Find currently logged-in user:
whoami
List things in directory:
ls
Change directory:
cd /Dir_name
Clear the terminal:
clear
List hidden files:
ls -a
Create new file in terminal:
touch file.txt
Read file content:
cat file.txt
Create Directory:
mkdir directory_name
Create nested Directory using -p flag:
mkdir -p dir3/dir2/dir1/directory_name
Move File:
mv file_name directory
mv file.txt Downloads/files
Remove File:
rm file_name
Remove Directory:
rm directory_name
To find use of specific command:
man command E.g. man rm
Remove directory and their contents recursively:
rm -r downloads/
Copy file:
cp source destination
Switch to root user:
sudo -i
use w/who to find uptime, how many users has logged in:
w
who
To find and monitor running processes:
top
Network- To find out what ports are open and listening:
sudo netstat -tupln
Create new file using VI editor:
vi test.file
For inserting data press i and write content:
i
To save and quit from vi press esc and then :
:wq
Print "Hello World" message in terminal :
echo "Hello World"
Print std Output to some file :
echo "Hello World" 1> somefile.txt
Print std Output to some file but this override the existing contents :
echo "Hello World" > somefile.txt
Redirecting standart output and append data in existing file use ">>" :
echo "Hello World" >> somefile.txt
To Redirect standard error use 2> :
cat nonExistingFile.txt 2> somefile.txt
Redirect input using <, Here we are mailing error.txt to the user palak:
mail -s This is errror file" palak < error.txt
Use for searching patterns in files
cat somefile.txt | grep Thanks
Cut file using delimiter and find fields
cat somefile.txt | cut -d: f1
Sort the file contents in alphabatical orders, ignore the leading whitespace using b case-insesitive f
cat somefile.txt | sort -bf
Search somethig in directory
grep search_term ./*
Install software
sudo apt-get install software-name
Used for searching apt packages on a Ubuntu or Debian based systems
sudo apt-cache search software-name
Change file mode - Read-4, Write-2, Execute-1, no-permissiom-0
chmod 777 testfile.txt -
Change file mode automatically- /etc/login.defs - search UMASK
edit umask 022 --- owner none permissomiom taken, from group read execute permission taken
Show first 10 lines of the file
head somefile.txt
Show last 10 lines of the file
tail somefile.txt
Find user details and user password
sudo -i
tail /etc/passwd
tail /etc/shadow
Add user and create set home directory
useradd -m -d /home/user1 -s /bin/bash user1
cat /etc/shadow user1:!:19275:0:99999:7:::
User can't login as password is not set (you can see the ! mark )
Find which kernel version a system is currently running
uname -a
Find system's current IP address
ifconfig
ip addr show
Check for free disk space
df -ah
Check for openports on linux machine
netstat
netstat -tulpn
Check CPU usage for a process
ps aux | grep nginx
top
htop
- Read (r) - 4
- Write (w) - 2
- Execute (x) - 1
drwxr-xr-x
rwx - Root user permission r-x - Group permission r-x - perimission to others
Check the permission of the directory
ls -ld
Assign all permissiom to root users in this directory
chmod 700 /opt/directory
The crontab is a list of commands that you want to run on a regular schedule, and also the name of the command used to manage that list.
The crontab is a list of commands that you want to run on a regular schedule, and also the name of the command used to manage that list.
List crontab
crontab -l
Edit crontab
crontab -e
MIN HOUR DOM MOY DOW COMMAND 15 10 * * (3-5) ech0 "$(date): checkin in." >> /var/log/chcking
sudo less /var/spool/cron/crontab/user
ls /etc/cron.d/
crontab -e -u user
sudo vi /etc/crotab === system wide crontab
Group details
cat /etc/group
set password for user
passwd user1
Lock user List crontab
usermod -L user1
crontab -l
To persist aliases, add alias in -
vi .bashrc
alias ls -la = l
List open file
lsof
Find who opens this file at the moment
lsof /var/log/nginx/access.log
Find all the files open up by the process
ps aux | grep nginx
What file user has opened right now
lsof -u user_name
Which process is listening on what port
lsof -i :port_name
Which process is listening to tcp protocol
lsof -i tcp
tar -zcvf doc.tar.gz Directory/
-
'f' will specify the filename for the directory (docs.tar.gz in this case)
-
'v' is verbose, means that we will get one line of output for each file we are compressing
-
'c' is for create, we are creating a new archive
-
'z' is for zipping (we are using the g zip program), used to compress archive as well as archiving it
tar -zxf doc.tar.gz
-
'z' shows that it is g zipped
-
'x' means to extract it
-
'f' defines the archive filename (in this case: docs.tar.gz)
- running
- waiting or sleeping
- stopped
- zombie
A process in a computer operating system is an executable program in action. The executable program has machine instructions necessary to carry out a specific task. A corresponding process is born when a program is executed for carrying out a task. A programmer writes a software program using a high-level programming language such as C. This is also called ‘code’ and the programmer compiles it to create an executable program. The compilation process converts the code into a set of machine-level instructions, and it becomes intelligible to the operating system. The compiler for Linux systems is GCC or Gnu C Compiler.
The executable program remains a passive entity, until it is instructed to run or execute by the user. Then it creates a new entity called a process, which is visible by the command ‘ps’. The process is associated with three identifiers – the Process ID or PID, the Parent Process ID or PPID and the Group ID or GID.
In a Linux system, the first process to start is the ‘init’ and it has a PID of 1. All subsequent processes are init’s children, grandchildren and so on. For active process in a Linux system, the command ‘pstree’ will bring up the entire hierarchy, while ‘top’ will show the dynamic view of processes. The Linux kernel uses a scheduler and it controls the execution sequence of all the processes. Linux processes can have one of four states at any given time: running, waiting or sleeping, stopped and zombie.
A process achieves a running state when it is actually executing (running) or waiting for execution in the queue of the scheduler, which means it is ready to run or execute. For this reason, the running state is also known as runnable and is represented by R.
A process is in a waiting or sleeping state if it must wait for an event to occur or some resource-specific operation needs to complete before the process can continue to run. Therefore, depending on the circumstances, the waiting state is further subcategorized into an interruptible or S state and an uninterruptible or D state.
If the scheduler sends a stop signal to a process, the process goes into a stopped state. This might happen, for example, when the process is being debugged or analyzed and this state is represented by T.
When a process has completed its execution, but is waiting to retrieve its exit state, the process is said to be in a zombie state, designated by Z. Once it crosses the zombie state or retrieves its exit status, the process dies or ceases to exist.