My Cheat Sheets: 2022

Linux Commad Quick Reference

Command	Description
cd ..	Move up one level to upper directory.
cd /root	Move /root directory.
pwd	Show working directory (your current position).
ls -lah	Show all files and directory in working dir with format as list, all (show hidden files) and human readable size (eg. with Mb). I use it as general listing.
ls -ltr	ls with order by date.
ls \| wc -l	Count files on a directory.
ls -lahS	ls with order by file size.
df -h	Check disk space.
du -hd 1 /root	Check disk usage for every directories under /root directory. Useful for tracking disk space eater. Add \| sort -h to sort the result.
wget -c -url-	Use -c option to resume download.
grep -Rnw '/path/to/search' -e 'pattern'	find in files, use --include=\.{c,h} or --exclude=\.o param to include or exclude spesific file extensions. Use --exclude-dir={dir1,dir2,*.dst} param to exclude dirs.

Enabling HTTPS on Tomcat9 (using Self Signed Certificate)

source: https://tomcat.apache.org/tomcat-9.0-doc/ssl-howto.html#Configuration

Self Signed Certificate means we create ourselves the certificate, and must tell the browser to accept it. It already serve the purpose to secure connection between browser and Tomcat but you can import certificate from a Certificate Authority later on if deemed as necessary.

1. Create a local Certificate Signing Request (CSR) using keytool which is located in Java's bin directory (keytool.exe on Windows). Change \path\to\my\keystore below as needed, in my case i change it to Tomcat's conf directory (/opt/tomcat/latest/conf/.keystore)

keytool -genkey -alias tomcat -keyalg RSA -validity 1095 -keystore \path\to\my\keystore

Before we run keytool, we make sure it is accessible using which command.

then run:

keytool -genkey -alias tomcat -keyalg RSA -validity 1095 -keystore /opt/tomcat/latest/conf/.keystore

Then provide all the information that the keytool asks and don't forget to write down the password (keypass) for later use.

2. Edit conf/server.xml file, add following lines into config right after the first Connector tag (port 8080). Don't forget to change 'changeIt' word with your certificate password.

<Connector protocol="org.apache.coyote.http11.Http11NioProtocol"
           port="8443" maxThreads="200"
           scheme="https" secure="true" SSLEnabled="true"
           keystoreFile="conf/.keystore" keystorePass="changeIt"
           clientAuth="false" sslProtocol="TLS"/>

3. Open incoming port 8443 on firewall:

firewall-cmd --permanent --zone=public --add-port=8443/tcp
firewall-cmd --reload

4. Restart Tomcat, then test by accessing with https://localhost:8443/ there will be a security warning from the browser, we should add exception for this.

Below are examples add security exception on Mozilla Firefox:


(1) Click on 'Advanced' button.

(2) Click on 'View Certificate' to verify that it has the right certificate. (3) Click on 'Accept the Risk and Continue' to make exception.

Tomcat + MariaDB on CentOS 8 Stream Setup

Here a list of commands i use to setup standard Java application server using OpenJDK11 + Tomcat9 + MariaDB10.3 on fresh instaled CentOS 8 Stream (Server without GUI Package) :

#preparation
yum update
reboot
# make sure you have space
df -h
# install mariadb
yum install mariadb-server
# enabling mariadb service at startup
systemctl enable --now mariadb
# securing, setup root's password and remove test db and anonimous user
mysql_secure_installation
# test
mysql -u root -p

# install java, check available jdk
yum info java*
yum install java-11-openjdk-devel
# test
java -version

# tomcat9
# prepare user to run tomcat service, never using root
useradd -m -U -d /opt/tomcat -s /bin/false tomcat
# download and install tomcat9, please check version availability on https://www-eu.apache.org/dist/tomcat/tomcat-9/
VERSION=9.0.68
wget https://www-eu.apache.org/dist/tomcat/tomcat-9/v${VERSION}/bin/apache-tomcat-${VERSION}.tar.gz -P /tmp
tar -xf /tmp/apache-tomcat-${VERSION}.tar.gz -C /opt/tomcat/
ln -s /opt/tomcat/apache-tomcat-${VERSION} /opt/tomcat/latest
chown -R tomcat: /opt/tomcat
sh -c 'chmod +x /opt/tomcat/latest/bin/*.sh'
# check
ls -lah /opt/tomcat/latest/
ls -lah /opt/tomcat/latest/bin/
# create service for tomcat9 and put script below on it
vim /etc/systemd/system/tomcat.service

# script
[Unit]
Description=Tomcat 9 servlet container
After=network.target

[Service]
Type=forking

User=tomcat
Group=tomcat

Environment="JAVA_HOME=/usr/lib/jvm/jre"
Environment="JAVA_OPTS=-Djava.security.egd=file:///dev/urandom"

Environment="CATALINA_BASE=/opt/tomcat/latest"
Environment="CATALINA_HOME=/opt/tomcat/latest"
Environment="CATALINA_PID=/opt/tomcat/latest/temp/tomcat.pid"
Environment="CATALINA_OPTS=-Xms512M -Xmx1024M -server -XX:+UseParallelGC"

ExecStart=/opt/tomcat/latest/bin/startup.sh
ExecStop=/opt/tomcat/latest/bin/shutdown.sh

[Install]
WantedBy=multi-user.target
# end of script

# start tomcat
systemctl daemon-reload
# enabling tomcat serice at startup
systemctl enable --now tomcat
# check
systemctl status tomcat
# open port on firewall
firewall-cmd --permanent --zone=public --add-port=8080/tcp
firewall-cmd --reload

# check and remove all unnecessary software 
cd /opt/tomcat/latest/webapps/
ls -lah
rm -rf *
ls
# check log
less +G ../logs/catalina.out
# done

MySQL Replace First Letter With Uppercase


  UPDATE `user` SET `full_name`=CONCAT(UPPER(SUBSTRING(full_name,1,1)),SUBSTRING(full_name,2));

Git SSH using multiple account (GitLab)

The key is in ~/.ssh/config, here you can give aliases for every user on same host. SSH Key on each GitLab account must be already set up.


Host user1.gitlab.com
  Hostname gitlab.com
  PreferredAuthentications publickey
  IdentityFile ~/.ssh/id_rsa_user1

Host user1.gitlab.com
  Hostname gitlab.com
  PreferredAuthentications publickey
  IdentityFile ~/.ssh/id_rsa_user2

Testing:


$ ssh -T git@user1.gitlab.com
Welcome to GitLab, @user1!


$ ssh -T git@user2.gitlab.com
Welcome to GitLab, @user2!