Linux Tutorials and Command Line Guides

List users in Linux with getent, /etc/passwd, who, and id. Filter normal and system accounts, see logged-in sessions, and check whether a user exists on Ubuntu and RHEL.

Configure OpenSSH authentication in sshd_config: password, public key, keyboard-interactive, host-based, GSSAPI, AuthenticationMethods, and Match blocks on Linux.

Use OpenSSL to view PEM, CRT, CER, chain, bundle, PFX, and P7B certificates. Check expiry, SAN, fingerprint, remote TLS certs, and verify chains.

List groups in Linux with getent, groups, id, and /etc/group. Check user group membership, list group members, and compare primary vs supplementary groups.

Learn how to configure http_proxy, https_proxy and no_proxy environment variables in Linux temporarily, permanently, system-wide, and for tools like curl, wget, apt, git and Docker.

Install Node.js on Ubuntu with apt nodejs, NodeSource setup scripts, NVM for multiple versions, or the official nodejs.org linux-x64 tar.xz under /opt, verify with node --version and npm --version, and uninstall per …

Back up a PKCS12 truststore, check the keytool alias and SHA-256 fingerprint, import a CA only when missing, and exit cleanly for Ansible, systemd, and CI pipelines.

Complete keytool tutorial for Java keystores and truststores - generate PKCS12 keys, import CA-signed chains, automate truststore imports with Bash, sign and verify JAR files with jarsigner, build mTLS keystores and …

Create a code-signing keypair with keytool, sign a sample JAR with jarsigner, verify the signature, and read the PKIX chain warnings jarsigner prints for a self-signed certificate.

Create Java mTLS keystore and truststore files with keytool and OpenSSL, then run a mutual TLS HttpsServer lab that fails without a client cert and succeeds.

Set javax.net.ssl.trustStore and keyStore JVM properties after building a PKCS12 file with keytool: property names, -D flags, JAVA_TOOL_OPTIONS, and a lab that fails without trustStore then succeeds.

Fix PKIX after importing into the wrong cacerts. Find the Java truststore your app actually uses, including bundled JRE cacerts, and import with keytool.

Renew an expired TLS certificate in an existing Java PKCS12 keystore by checking expiry with keytool -list -v, exporting a CSR from the same alias, and importing the CA-signed reply without deleting the private key.

Fix keytool Invalid keystore format by matching JKS or PKCS12 storetype to the file, avoiding PEM certificates as keystores, and repairing empty or corrupt keystore paths.

Build a custom PKCS12 Java truststore with keytool for Docker and Kubernetes: RUN importcert in the image, set JAVA_TOOL_OPTIONS with trustStoreType=PKCS12, or use an initContainer with a mounted CA Secret and emptyDir …

Find which Java cacerts file your application actually uses: compare JAVA_HOME, java.home, bundled JRE paths, and javax.net.ssl.trustStore overrides before importing a CA certificate.

Fix Java No subject alternative names present by adding DNS and IP SAN entries with keytool -ext SAN so the certificate matches the hostname or IP clients use.

Fix keytool Failed to establish chain from reply by importing missing intermediate and root CA certificates before the server reply, using the correct keystore alias, or importing a complete PKCS7 chain bundle.

Fix keytool Certificate not imported, alias already exists by listing aliases, deleting or renaming the old entry, and re-importing to the correct keystore with a unique alias.

Fix keytool Input not an X.509 certificate by importing valid PEM, DER, or supported certificate-chain input instead of HTML, private keys, PFX files, or malformed PEM.

Fix java.security.UnrecoverableKeyException: Cannot recover key by aligning keystore and private key passwords in Tomcat, Spring Boot, and KeyManagerFactory.

Fix keytool Keystore was tampered with, or password was incorrect by checking storepass, file path, JKS vs PKCS12, cacerts, import passwords, and corruption.

Enable Spring Boot HTTPS on port 8443 with a PKCS12 keystore from keytool, set server.ssl.key-store in application.properties, add an optional truststore for private CAs, and verify with Java HttpClient.

Enable Tomcat 10 HTTPS on port 8443 with a PKCS12 keystore from keytool: import the CA chain before the server certificate, set certificateKeystoreFile in server.xml, and verify the TLS handshake.

Fix javax.net.ssl.SSLHandshakeException PKIX path building failed by importing your private CA chain into a PKCS12 truststore with keytool, then point the JVM at it with javax.net.ssl.trustStore — without editing …

Generate a private key with keytool -genkeypair, build a CSR with -certreq, sign it with a local OpenSSL CA, and replace the self-signed entry using -importcert on the same alias.

Fix keytool error Public keys in reply and keystore do not match by importing the signed certificate into the same alias that generated the CSR. Verify public-key hashes and distinguish this error from missing CA chain …

Generate a PKCS12 keystore with keytool -genkeypair, add DNS and IP Subject Alternative Names for localhost and 127.0.0.1, verify with keytool -list -v, and see why CN alone fails modern HTTPS hostname checks.

Import a CA or TLS certificate into Java cacerts on Ubuntu, Debian, RHEL, and Rocky Linux with keytool -importcert: locate cacerts, use a copy for safe testing, default password changeit, and distro-specific system trust …

Import a TLS chain into a Java PKCS12 keystore with keytool: root CA and intermediate as trustedCertEntry, server reply on the PrivateKeyEntry alias, correct order, -trustcacerts, and chain verification with keytool …