Servers linux

Schedule recurring commands with cron: edit crontab -e, use the five fields (minute hour day month weekday), and log output to a file or to journal. Use systemd timers as an alternative for services. Verify the job runs and check logs when it fails.

Learn the standard Linux directory layout: /etc for config, /var for variable data, /home for users, /tmp for temporary files. Find where config, logs, and binaries live so you can back up, restore, and troubleshoot without guessing.

Create full or incremental backups with tar (archives) and rsync (mirror or incremental), store them off the host, and verify with listing or checksum. Use this for config and data backup before changes or for scheduled retention.

Find what is using disk space with du and df, locate large files and dirs, and free space by removing logs, caches, and old packages. Use this when the disk is full or you need to prevent full before it happens.

Create and delete user accounts with useradd and userdel (or adduser on Debian), set passwords and SSH keys, and assign users to groups. Use this to grant shell or SFTP access without sharing root and to clean up when someone leaves.

List and filter processes with ps, pgrep, and top; find what is using CPU or memory; send signals (kill, killall) and interpret state (D, Z, R). Use this to debug high load, stuck processes, or what is bound to a port.

Install a single package on Debian/Ubuntu (apt install) or RHEL/Fedora (dnf install), resolve dependencies, and verify the binary and config location. Use this when you need a specific tool or service and the distro provides it.

Start, stop, restart, and enable systemd units; check status and read logs with journalctl. Use this to bring services up after boot, restart after config changes, and diagnose why a unit failed.

Send SIGTERM or SIGKILL to a process with kill or killall; use signals to stop, reload, or force-stop. Understand what each signal does so you can stop runaway or stuck processes safely.

Install, upgrade, and remove packages on Debian/Ubuntu (apt) and RHEL/Fedora (dnf/yum). Run updates safely: check changelogs, take backups, and verify services after reboot. Use this to keep servers patched without breaking running workloads.

Diagnose and fix permission denied on files, directories, and SSH. Check owner, group, and bits with ls; fix with chmod and chown; for SSH check authorized_keys and server-side permissions. Never use chmod 777 as a fix.

Before going live: verify hostname, time, network, firewall, SSH, updates, backups, monitoring, and app config. Use this so nothing is left in default or broken state when the server is put in production.

Configure and test DNS resolution: set nameservers in resolv.conf or netplan, use getent and dig to verify; add static entries in /etc/hosts when you need a fixed name without DNS. Use this when the server cannot resolve hostnames or you need to override a name.

Convergence checklist for the servers-linux topic: ensure you have covered fundamentals, operations, failure recovery, and hardening. Use this to confirm you have the right guides for your role and to find the next step when you are stuck.

Use the Linux shell to navigate, inspect, and change the system: cd, ls, cat, grep, find, ps, systemctl, and redirection. Verify commands with exit codes and output so you can operate and debug servers without a GUI.

Grant and revoke sudo access: add users to the sudo or wheel group, or add rules in /etc/sudoers and /etc/sudoers.d. Use visudo to avoid syntax errors. Restrict commands or require a password so only authorized users run as root.

Define a Linux server as a headless machine or VM that runs services and accepts network requests. Learn when to choose a server over a workstation, and what access (SSH, console) and skills you need before provisioning or installing Linux for services.

Understand how Linux file and process permissions work: owner, group, and others; read, write, execute; numeric modes and chmod/chown. Use this to fix permission denied errors and to grant least privilege to services and users without using chmod 777.

Diagnose SSH connection failures: check network (ping, port open), sshd running, firewall allows port 22, and key or password accepted. Use this when SSH connection is refused, times out, or permission denied so you can fix or use console access.

Add a third-party or vendor repository to APT (sources.list or .list in sources.list.d) or DNF (.repo in /etc/yum.repos.d) so you can install packages from it. Use this when you need software not in the default distro repos, and to keep repo config auditable and reversible.

Set environment variables for a user in ~/.profile, ~/.bashrc, or ~/.bash_profile, and for a systemd service in the unit file or EnvironmentFile. Use this when an app or script needs VAR=value across logins or at service start.

Use iostat, iotop, and /proc/diskstats to see disk throughput and which processes are doing the most I/O. Use this when the system is slow and CPU and memory look fine, or when tuning storage or diagnosing latency.

Use free, /proc/meminfo, and top or smem to see total RAM, used, free, buffers, cache, and swap. Identify processes by RSS or VSZ. Use this when diagnosing high memory use, before adding swap, or when tuning application memory limits.

Check and raise the open file limit (nofile) for the shell with ulimit, and for a systemd service or system-wide with limits.conf or the unit file. Use this when you see 'too many open files' or when running high-connection services.

Use du, find, and ncdu to locate the largest files and directories so you can free disk space or move data. Use this when a filesystem is full or you need to identify what is consuming space before cleanup or resizing.

Create a swap file or partition, enable it with swapon, and add it to fstab so it survives reboot. Size swap based on workload: often equal to RAM for small servers, or less when you have plenty of RAM. Use this when you see OOM or need to reduce memory pressure.

Set the default systemd target (multi-user vs graphical) with systemctl set-default so the system boots to the desired run level. Use this when converting a desktop install to headless, or to force graphical or console-only boot.

Use journalctl to view systemd journal logs by unit, time, priority, or boot. Follow logs in real time, filter by service name, and export for debugging. Use this when diagnosing service failures, boot issues, or security events.

Schedule a reboot during a maintenance window; notify users, stop or drain services if needed, run reboot or shutdown -r, and verify the system and services after boot. Use this when applying kernel or critical updates, or recovering from a hung state.

Check whether SELinux is enabled and in enforcing or permissive mode with getenforce and sestatus; read denials in the audit log or ausearch. Use this when access is denied and permissions look correct, or when hardening or debugging a RHEL/CentOS/Fedora system.

Set the system hostname persistently with hostnamectl or by editing /etc/hostname and /etc/hosts. Use this when provisioning a server, cloning a VM, or fixing duplicate hostnames so the machine has a unique, predictable name for logs and SSH.

Set the system timezone with timedatectl or a symlink in /usr/share/zoneinfo, and enable NTP (systemd-timesyncd or chrony) so the clock stays correct. Use this when deploying a server, fixing certificate or log ordering issues, or after restoring a snapshot.

Configure SSH client with ~/.ssh/config for hosts, keys, and options; use ssh-agent to hold keys so you do not type passphrases repeatedly. Use this to simplify SSH and SCP to servers and to avoid exposing keys to every command.

Recover when the root or critical partition is full: free space quickly by clearing logs and caches, find and remove or move large files, then fix rotation and monitoring so it does not recur. Use this when the system is read-only or services fail with no space.

Fix broken apt or dnf state: repair dpkg with apt --fix-broken or dpkg --configure -a; clear lock files; reinstall the package or restore the package list from backup. Use this when apt or dnf fails with dependency or status errors.

Enable and configure a host firewall with ufw (Ubuntu/Debian) or nftables (RHEL/modern): allow SSH first, then HTTP/HTTPS or app ports; deny by default. Verify with ufw status or nft list ruleset so the server is protected and still reachable.

Set a static IP or use DHCP on Linux using netplan (Ubuntu), nmcli (NetworkManager), or /etc/network/interfaces. Verify with ip addr and ping; ensure DNS and default route work so the server is reachable and can reach the internet.

Find which process is using CPU with top, ps, or pidstat; identify the thread or code path; fix the cause (loop, leak, or load) or throttle and scale. Use this when the system is slow or load average is high and you need to pinpoint the consumer.

When something is wrong: preserve logs and state, identify scope (one host or many), restore service or isolate, then root-cause and fix. Use this so incidents are handled consistently and evidence is kept for post-mortem.

Prevent logs from filling the disk: configure logrotate for app logs and limit journald size (SystemMaxUse, RuntimeMaxUse). Verify rotation runs and disk usage stays bounded so the system does not crash from a full disk.

Mount a disk or partition manually with mount, add an entry to /etc/fstab for boot mount, and verify with mount and df. Use this when adding data disks or moving data to a new volume so the system mounts it correctly after reboot.

When the server cannot reach the network or others cannot reach it: check interface, IP, route, DNS, and firewall. Use ping, ip, ss, and traceroute to isolate whether the problem is local, routing, or remote.

Create physical volumes, volume groups, and logical volumes with pvcreate, vgcreate, lvcreate; extend an LV when you add disk or need more space. Use this when you want flexible storage that can grow or span disks.

Diagnose OOM: check dmesg and journalctl for oom-killer, identify the process killed and what was using memory; fix by adding RAM, limiting process memory, or fixing leaks. Use this when the system kills processes or becomes unresponsive and logs show out-of-memory.

Create a partition table and partitions with fdisk or parted, format with mkfs.ext4 or mkfs.xfs, then mount and add to fstab. Use this when adding a new data disk so the system can use it safely after reboot.

When a filesystem is mounted read-only due to errors or full disk: free space if full, run fsck to repair errors, then remount read-write. Use this when writes fail with 'read-only file system' or after an unclean shutdown.

Restore from a tar archive or rsync backup to the original or a new path; verify permissions and service config; bring services back up and confirm the app works. Use this after data loss or to roll back a bad change.

Harden sshd: disable password auth and root login, allow only key-based auth, use a non-default port if desired, and restrict users with AllowUsers. Verify with a new session before closing the current one so you do not lock yourself out.

Checklist for securing a new or existing Linux server: SSH key-only and no root login, firewall default deny, updates, non-root service user, minimal packages, and logging. Use this before putting a server in production or during a security review.

Diagnose why a systemd unit fails to start: read journalctl -u UNIT, check config and paths, fix permissions and dependencies (sockets, mounts), then restart. Use this when systemctl start fails or the unit is in failed state.

When the server is slow: check load average, CPU, memory, disk I/O, and network; use top, iostat, and vmstat to find the bottleneck. Use this to decide whether to scale, optimize, or fix a bug.

When apt or dnf reports dependency conflicts or broken packages: identify the conflicting packages, choose to remove or replace, use --fix-broken or manual dependency resolution. Use this when install or upgrade fails due to dependency loops or version conflicts.

When Linux fails to boot, use rescue or recovery mode from the installer or a live image to mount the root filesystem, fix fstab or initramfs, and repair broken config. Use this when the system hangs at boot, drops to emergency mode, or fails to mount root.