Basic commands
cd
mkdir
cp
cp – R
cat
touch
mv
whoami
id – to find the id of the user
su
ssh username@ip
sudo /ls/root – to perform root level tasks
wget & curl url -O somfile.txt – to save the file
cat /etc/*release* – to find the release version
vi Editor
- Command Mode
- Insert Mode
By default vi opens in command mode, to switch to insert mode “i” to get back to command mode esc
Command Mode
x – to delete a character
dd – to delete a entire line
yy – to copy a line
p – to paste a line
: – to write a command
:w – to write the change
:q – Quit without saving
/ – to find
:wq – save changes and quit
n – to find next occurrence
Package managers
By default rpm (redhat package manager) is the package manager but it will no install the dependencies
rpm -q – to query a package
rpm -I – to install a package
rpm -e – to remove a package
To install a package with dependencies use YUM Package manager
yum repolist – To find the available packages
ls /etc/yum.repos.d/ – To Find the available repos
cat /etc/yum-repos.d/Centos-Base.repo – To find the avaialble repo for cent-os
yum list packagename – Find the installed package
yum remove packagename – To remove a package
yum list ansible –showduplicates – To find the duplicate repository
DNS
/etc/hosts
/etc/resolv.conf
/etc/nsswitch.conf – Order of dns order is mentioned
Preference in always the local server, if not specified on the above file
Create a entry in /etc/resolv.conf with search entry to resolve the name without domain name (within organization)
A Record – Name to IP
AAAA Record (quad A record) – name to IPV6
Cname – alias record
nslookup does not lookup local server
dig – similar to nslookup
Services
The services can be started, stopped and status can be checked from the below commands
service httpd start
(or)
systemctl start httpd
systemctl stop httpd
systemctl status httpd
systemctl enable httpd – to enable during startup
systemctl disable httpd – to disable during startup
How to create an app and start using ctl commands
- Create a file under /opt/code/my_app
- Update the contents of the file
echo myworld
Test the script by running the below command
/usr/bin/python3 /opt/code/my_app
User the curl command to check whether the service is in listening state
Navigate to “/etc/systemd/system” or “/lib/systemd/system” and create a file with the service name
myapp_service
Update the contents of the file as below
[unit]
Description of the app
[Service]
Mention the service start details and the dependency services and scripts that has to be started along with the app
[Install]
To start the service during boot, mention the service after which this service has to start
Once the file is configured then stystemctl commands can be used to start, stop and to check the application status.