• bin
• boot
• dev
• etc
• home
• lib
• lib64
• local
• media
• mnt
• opt
• proc
• root
• run
• sbin
• srv
• sys
• tmp
• usr
• Var
• It is the home directory for the root user (Super User).
• It provides the working environment for the root user.
• It is the home directory for other users.
• It provides a working environment for other users (excluding root).
• By default, all softwares are installed in this directory.
• It contains commands used by all users except root.
• It contains commands used by only super user (root).
• It is containing variables data like mails, log files of services.
| Command | Usage |
|---|---|
| date | Show the current date and time |
| cal | Show this month's calendar |
| uptime | Show current uptime (how much time you are used) |
| whoami | It will show the current user |
| finger | Display information about user |
| Users/id | Show on which user you logged in/ Current user info |
| man | Shows manual of command |
| username | Shows your user name |
| who/w | It shows the login user details IP, IDLE time…etc (not table format) |
| Command | Usage |
|---|---|
| touch | Creates a 0-byte file |
| cat > filename | Create a file and allow to write |
| nano | Create a file if filename doesn't exist |
| vi | Create a file if filename doesn't exist |
| rm | Remove a file |
| mkdir | Create a directory |
| rmdir | Remove an empty directory |
| rm -rf | Remove a directory if it has any files or directories |
| Command | Description |
|---|---|
| cp | Copy a file |
| mv | Move a file |
| find | Find a file |
| grep | Search for a pattern in a file |
| cd | Switch between directories |
| diff | Find content difference in two files |
| sed | Search and replace a particular pattern |
| chmod | Change file permissions |
| chown | Change ownership of a file |
| file | Show what kind of file it is |
cp <source file> <destination file>
cp <source path> <destination path>
cp file1 file2
cp file1 /home/file2
cp /home/file2 /etc/file1
mv <source file> <destination file>
mv <source path> <destination path>
mv file1 file2
mv file1 /home/file2
mv /home/file1 /etc/file200
Moving a file is used to either rename the file or relocate it from one place to another. If the file already exists in the destination, it will overwrite the data. If not, a new file will be created, and the data will be written into it.
find / -option filename
| Option | Usage |
|---|---|
| -name | For searching a file with its name |
| -user | For files whose owner is a particular user |
| -group | For files belonging to a particular group |
find / -name file1 --->It is searching for file1 in the entire Linux.
diff <filename> <filename>
diff file1 file2 --->It shows the difference between two files.
grep <patron> filename
grep Hi file1 ---> If the word is available with case-sensitive in the file then it is print.
grep Gateway /etc/ssh/sshd_config
grep -i gateway /etc/ssh/sshd_config ---> It is printthe word and i means ignore the case-sensitive.
Grep command will only work files only not for directories.
ls -l | grep dir1 --->It will search for dir1 from the output.
ls -l | grep ^d --->It will show only words that start with 'd'.
Which is used to search a word in the file and replace it with the word required to be in the output.
It will modify the output, but there will no change in the original file
sed ‘s/old_text/new_text/’ file_name ---> Replace the text case-sensitive if the text is a seperate word in the output not in the file
sed ‘s/old_text/new_text/g’ file_name ---> Replace the text case-sensitive if the text is part of the word or a seperate word in the output not in the file.
sed ‘s/old_text/new_text/ig’ file_name ---> Replace the text and ignore the case-sensitive if the text is part of the word or a seperate word in the output not in the file.
sed -i ‘s/old_text/new_text/’ file_name --->Replace the text in the output and update in the file.
sed -n ‘5,10p, file_name --->It will display the lines from 5-10 only
sed ’10,20d’ file_name --->It will delete the lines from 10,20 and print not delete in actual file.
sed 's/Hi/Hello/' file2
sed 's/Gold/Silver/g' file2
Sed 's/gold/silver/ig' file2
Sed -i 's/gold/silver/ig' file2
sed -n '1,2p' file2
sed -n '1p' file2
sed '1d' file2 ---> It will delete in the output not actual file
sed ‘1,2d’ file2 ---> It will delete in the output not actual file
| Type | Example | Home Directory | Shell |
|---|---|---|---|
| Super User | Root | /root | /bin/bash |
| System User | ftp, ssh, apache | /var/ftp, /etc | /sbin/nologin |
| Normal User | created, ec2-user | /home/username | /bin/bash |
useradd <option> <username>
| Option | Description |
|---|---|
| -c or --comment | Specify a comment or description for the new user |
| -m or --create-home | Create a home directory for the new user |
| -s or --shell | Specify the login shell for the new user |
| -g or --gid | Specify the primary group for the new user |
| -G or --groups | Specify supplementary groups for the new user (comma-separated group list) |
sudo useradd nani
We created a user named 'nani' is creating automatically, a group with the same name 'nani' is also created, uid=1001(nani) gid=1001(nani) groups=1001(nani).. The user's working directory is created under the '/home' directory, specifically '/home/nani'. All users' working directories are typically created under the '/home' directory.
sudo passwd nani
After setting the password for the user, you need to enable password-based authentication in the /etc/ssh/sshd_config file by setting it to "yes", and then reload or restart the sshd service. To do this, you can edit the /etc/ssh/sshd_config file and change the PasswordAuthentication parameter to "yes".
sudo nano /etc/ssh/sshd_config
PasswordAuthentication yes
sudo systemctl reload sshd
sudo systemctl restart sshd
sudo passwd root now you can create password to root
sudo passwd <user_name> here how you can create password to any user_name you want
you cannot use sudo in all users to give sudo permissions become root and add the particular user to sudoers file using below commands.
<user_name> ALL=(ALL:ALL)ALL: This line grants the ansibleadmin user full sudo privileges on all hosts. Breaking it down:
ansibleadmin ALL=(ALL:ALL) ALL
ansibleadmin: Specifies the username.
ALL: Specifies that the user can run commands as any user.
(ALL:ALL): Specifies that the user can run commands as any group and on any host.
ALL: Specifies that the user can run any command.
ansibleadmin ALL=(ALL) NOPASSWD: ALL: This line grants the ansibleadmin user full sudo privileges on all hosts without requiring a password. Breaking it down:
ansibleadmin: Specifies the username.
ALL: Specifies that the user can run commands as any user.
(ALL): Specifies that the user can run commands on any host.
NOPASSWD: Specifies that no password is required when running sudo commands.
ALL: Specifies that the user can run any command.
sudo usermod <option> username
| Option | Description |
|---|---|
| -u user id | Changes the user ID of the specified user |
| -G Secondary group id | Adds the user to a supplementary group. You need to specify the group ID |
| -g Primary group id | Changes the primary group of the user. You need to specify the group ID |
| -d Home directory | Changes the home directory of the user |
| -c Comment | Adds a comment to the user's account information |
| -s Shell | Changes the login shell of the user |
Change the login shell for a user:
sudo usermod -s /bin/bash john
This command changes the login shell of the user "john" to "/bin/bash".
Change the home directory for a user:
sudo usermod -d /home/newhome john
This command changes the home directory of the user "john" to "/home/newhome".
Add a user to an additional group:
sudo usermod -aG newgroup john
This command adds the user "john" to the supplementary group "newgroup".
Change the primary group of a user:
sudo usermod -g newprimarygroup john
This command changes the primary group of the user "john" to "newprimarygroup".
Change the user ID (UID) of a user:
sudo usermod -u 1001 john
This command changes the UID of the user "john" to 1001.
In Linux, permissions to files and directories are governed by three sets of permissions: read, write, and execute. These permissions are assigned to three classes of users: owner, group, and others.
Here's how permissions are represented and what they mean:
Read (r): If a user has read permission, they can view the contents of a file or list the contents of a directory.
Write (w): If a user has write permission, they can modify the contents of a file or create, delete, or rename files within a directory.
Execute (x): For files, execute permission allows the user to execute the file as a program or script. For directories, execute permission allows the user to enter the directory and access its contents.
Permissions are represented by a 10-character string. The first character indicates the file type (regular file, directory, symbolic link, etc.), and the next nine characters represent the permissions for the owner, group, and others in groups of three.
-rwxrwxrwx
Here's what this means:
The first character '-' denotes that it's a regular file. The next three characters 'rwx' indicate that the owner has read, write, and execute permissions. The next three characters 'r-x' indicate that the group has read and execute permissions, but not write permission. The last three characters 'r--' indicate that others have only read permission.
In the octal representation, each permission is represented by a three-bit number:
Read (r) is represented by 4.
Write (w) is represented by 2.
Execute (x) is represented by 1.
Here are some common combinations:
Read and write: 4 (read) + 2 (write) + 0 (no execute) = 6
Read and execute: 4 (read) + 0 (no write) + 1 (execute) = 5
Read, write, and execute: 4 (read) + 2 (write) + 1 (execute) = 7
Read only: 4 (read) + 0 (no write) + 0 (no execute) = 4
Write only: 0 (no read) + 2 (write) + 0 (no execute) = 2
Execute only: 0 (no read) + 0 (no write) + 1 (execute) = 1
To modify permissions, you can use the chmod command. For example:
chmod 540 filename/directoryname
is used to change the ownership of files and directories. It allows you to change both the owner user and the owner group of a file or directory. It is executable by root user only.
chown <option> <new owner>:<new group> <filename>
Here's what each part of the command means:
OPTIONS: This specifies any additional options you want to use with the chown command. Some common options include -R to recursively change ownership for directories and their contents, and -v for verbose mode, which displays a message for each file processed.
NEW_OWNER: This specifies the new owner user for the file or directory.
NEW_GROUP: This specifies the new owner group for the file or directory. If you omit :NEW_GROUP, only the owner user will be changed.
chown newowner filename
chown newowner:newgroup filename
chown -R newowner directory
chown :newgroup filename
chown -v newowner:newgroup filename
| Command | Description |
|---|---|
| history | List all commands executed by a user |
| free | Free memory of a server |
| /proc/meminfo | Display memory information |
| /proc/cpuinfo | Display CPU information |
| uname -a | Show kernel information |
| du | Show directory space usage |
| whereis | Show possible locations of an application |
| which | Show which application will be run by default |
Free command will show the memory details in KB’s. If you want in MB’s then execute free –m command.
free
| Total | Used | Free | Shared | Buff/Cache | Available | |
|---|---|---|---|---|---|---|
| Mem | 989140 | 134168 | 306204 | 680 | 548768 | 712948 |
| Swap | 0 | 0 | 0 | - | - | - |
free -m
| Total | Used | Free | Shared | Buff/Cache | Available | |
|---|---|---|---|---|---|---|
| Mem | 965 | 131 | 298 | 0 | 535 | 696 |
| Swap | 0 | 0 | 0 | - | - | - |
other command to see the memory and cpu usage
cat /proc/meminfo
cat /proc/cpuinfo
du command shows the usage of the directories in Kb’s and df command shows the file systems in the server. File system means like c-drive, d-drive…..etc and df –h will show the disk utilization.
du
df -h
| Filesystem | 1K-blocks | Used | Available | Use% | Mounted on |
|---|---|---|---|---|---|
| devtmpfs | 485296 | 0 | 485296 | 0% | /dev |
| tmpfs | 494568 | 0 | 494568 | 0% | /dev/shm |
| tmpfs | 494568 | 808 | 493760 | 1% | /run |
| tmpfs | 494568 | 0 | 494568 | 0% | /sys/fs/cgroup |
| /dev/xvda1 | 8376300 | 1595524 | 6780776 | 20% | / |
| tmpfs | 98916 | 0 | 98916 | 0% | /run/user/1000 |
| tmpfs | 98916 | 0 | 98916 | 0% | /run/user/1002 |
| tmpfs | 98916 | 0 | 98916 | 0% | /run/user/1001 |
Yum is the primary tool for getting, installing, deleting, querying and managing RedHat Enterprise Linux RPM software packages from official RedHat software repositories, as well as other third-party repositories.
yum install <package name>
yum remove <package name>
yum update <package name>
yum info <package name>
yum list available
yum list installed
| Command | Description |
|---|---|
| hostname | Lists host name of the server |
| ping | Checks availability of destination server over the network |
| wget | Downloads packages/software onto Linux system |
| ifconfig | Lists IP addresses of the server |
| telnet | Connects to remote host or checks port availability status |
| curl | Accesses the application as from a browser |
It is stored in /etc/hostname we will edit the host name as we need and restart the server. "init 6" or "sudo reboot" is the command to restart the system. Don’t change this name in real-time because some DNS name are effected.
cat /etc/hostname
ping www.google.com or google.com
It is used that the destination/target server has available or not.
ping www.google.com or google.com
64 bytes from bk-in-f99.1e100.net (142.251.111.99): icmp_seq=1 ttl=51 time=1.72 ms 64 bytes from bk-in-f99.1e100.net (142.251.111.99): icmp_seq=2 ttl=51 time=1.84 ms
wget command is used to download the packages.
wget https://dlcdn.apache.org/tomcat/tomcat-9/v9.0.56/bin/apache-tomcat- 9.0.56.tar.gz
curl is working as the browser in Linux. It is used that once deploy the application will check whether the server accessible or not.
curl https://tomcat.apache.org/download-90.cgi
| Port Number | Service |
|---|---|
| 21 | FTP |
| 22 | SSH |
| 23 | TELNET |
| 25 | SMTP |
| 53 | DNS |
| 80 | HTTP |
| 443 | HTTPS |
If you want to know which ports are used or which ports are running then use the command
netstat -tulpn
| Proto | Recv-Q | Send-Q | Local Address | Foreign Address | State | PID/Program name |
|---|---|---|---|---|---|---|
| tcp | 0 | 0 | 0.0.0.0:111 | 0.0.0.0:* | LISTEN | 2558/rpcbind |
| tcp | 0 | 0 | 0.0.0.0:22 | 0.0.0.0:* | LISTEN | 3217/sshd |
| tcp | 0 | 0 | 127.0.0.1:25 | 0.0.0.0:* | LISTEN | 2989/master |
| tcp6 | 0 | 0 | :::111 | :::* | LISTEN | 2558/rpcbind |
| tcp6 | 0 | 0 | :::22 | :::* | LISTEN | 3217/sshd |
telnet is used that the port numbers are used or not for localhost or the target system/server.
telnet localhost 8080
Trying 127.0.0.1... telnet: connect to address 127.0.0.1: Connection refused telnet localhost 80 Trying 127.0.0.1... Connected to localhost.
• This controls the starting and stopping of services. • Service status • Service start • Service stop • Service reload • Service restart
This controls which services are set to start after reboot the server. It is mainly used while creating the AMI in AWS.
chkconfig --list >> To check the availability of services
chkconfig <service> on >> to make the services available after restart.
chkconfig <service> off >> to make the services unavailable after restart.
chkconfig httpd on
When you start a program or running an application in Linux, it actually run as a process. A Linux process (a daemon), running in foreground or in the background, uses memory and CPU resources.
| Command | Description |
|---|---|
| ps -ef | List the processes which are running in the system |
| kill / kill -9 | Terminate a process or service |
| fg | Run the program in the foreground |
| bg | Run the service in the background |
| top | List top 20 processes which are consuming more CPU |
ps –ef is like taskbar in windows.
kill command has kill the process
kill <processed> kill 11256
kill -9 is sure kill the process. In the worst cases will use the kill command.
kill -9 17564
bg command run the programs in the backend.
Sleep 500
Curser waiting for 500 seconds Ctrl+Z >> stop the service
Jobs
It is the command that what are the jobs are executing.
bg %<number>
fg command will run the bg programs in the front-end.
fg %<number>
| Command | Description |
|---|---|
| gzip | Create a compressed file |
| gunzip | Unzip a file |
| tar | Extract tar file |
gzip menas reduce the size of the file. tar command will works on directories. If you want to zip the directory then use the command
tar -cvf <destination file name> <source filename>
c = compress
v = verbose
f = file
tar -cvf 789.tar 789
Then it is created a compressed verbose file and still we have the source directory also. The cvf file will be converted as a zip file because it is a file.
If you want to unzip the directory then use the command
tar –xvf 789.tar
x = extract
v = verbose
f = file
If you want to unzip and also extract the file then we will use the command
tar -xvzf apache-tomcat-9.0.56.tar.gz
It is used for scheduling a job.
crontab -l
crontab -e
Execute a job at 8:30 on everyday morning 30 8 * * * <command>
Execute a job at 2:00 PM on every Saturday 00 14 * * 6 <command>
Execute a job at 12:00 AM on 1st July 00 00 01 06 * <command>
Execute a job at 3:30 PM on Every month 25th 30 15 25 * * <command>
| Min | Hours | Date | Month | Day of the week |
|---|
There are two types of link files, one is Soft link and the other one is Hard link.
| Soft Link (Symbolic Link) | Hard Link |
|---|---|
| Shortcut File | Backup File |
| Size of link file is equal to the number of characters in the name of the original file | Size of both files is the same |
| If the original file is deleted, the link is broken and data is lost | If the original file is deleted, the link still contains data |
Command: ln -s <sourcefile> <destinationfile> |
Command: ln <sourcefile> <destinationfile> |
If we want a softlink file then use the command
ln -s file1 file1.link
Then it creates a file, reduced the size and mapping to the source file. If we delete the source file then the linked file also deleted.
If we want hardlink file then use the command
ln file1 file1.link.
Then it creates a link file, with same size. If we delete the source file then the linked file also deleted.
Tools: Mobaxterm or winscp >>drag and drop the file
SCP(secure copy) is a command-line utility that allows you to securely copy files and directories between two systems.
scp <source file name> <username@destination host>:<destination folder>
scp file1 root@10.1.1.100:/tmp --->local to remote
scp root@10.1.1.100:/tmp /home/ec2-user/ --->remote to local
If you want to copy the directory then use the command
scp -r dir1 nani@10.1.1.100:/home/nani/
SSH keys are a pair of cryptographic keys used to secure communication between a client and a server over a network. These keys consist of a private key and a public key. The private key is kept secret and should never be shared, while the public key can be freely distributed.
Here's a breakdown of the process:
You can generate SSH keys using the ssh-keygen tool, which is typically available on Unix-based systems like Linux and macOS. To generate a new SSH key pair, you can use the following command:
ssh-keygen or ssh-keygen -t rsa -b 4096
Once you have generated the SSH key pair with name id_rsa and id_rsa.pub in the $HOME/.ssh directory
Once you have generated the SSH key pair, you'll need to add the public key to the server you want to connect to. T you will add the id_rsa.pub in the remote server $HOME/.ssh/authorized_keys file on the server.
It's crucial to set the correct permissions on the ~/.ssh/ directory and the authorized_keys file to ensure security. Here are the recommended permissions:
Here are the commands to set the correct permissions:
sudo chmod 700 ~/.ssh
sudo chmod 600 ~/.ssh/authorized_keys
By following these steps, you can generate SSH keys, add the public key to the server's authorized_keys file, and ensure proper file permissions for secure SSH communication. This setup allows you to authenticate with the server using your SSH key pair rather than a password, which is more secure and convenient, especially for automated processes or scripts.
"# basic-linux-commands"