## System Information ### `uname` - Display system information - `-a`, `--all`: Display all information, including kernel name, version, hardware architecture, etc. - `-r`, `--kernel-release`: Display the kernel release. - `-m`, `--machine`: Display the machine's hardware architecture (e.g., x86_64). - `-s`, `--kernel-name`: Display the kernel name (e.g., Linux). ### `hostname` - View or set the system's host name - `-i`, `--ip-address`: Display the host's IP address. - `-f`, `--fqdn`: Display the Fully Qualified Domain Name. ### `lscpu` - Display CPU architecture information This command is usually used without parameters to output detailed CPU information. ### `free` - Display memory usage - `-h`, `--human`: Display in a human-readable format (e.g., KB, MB, GB). - `-m`, `--mebi`: Display in MB. - `-g`, `--gibi`: Display in GB. - `-t`, `--total`: Display the sum of physical memory and swap. - `-s `: Display continuously at a specified interval. ### `df` - Display file system disk space usage - `-h`, `--human-readable`: Display in a human-readable format. - `-i`, `--inodes`: Display inode information instead of block usage. - `-T`, `--print-type`: Display the file system type. ### `lsblk` - List block devices - `-f`, `--fs`: Display filesystem information (UUID and mountpoint). - `-m`, `--perms`: Display permission information. - `-p`, `--paths`: Display full device paths. ### `uptime` - Display system uptime and load This command is usually used without parameters to output the system uptime, number of logged-in users, and average load. ## Process Management ### `ps` - Report a snapshot of current processes - `aux`: The most common combination, shows all processes for all users (including those without a terminal) with detailed information. - `-ef`: A common System V style combination, shows a full-format listing of all processes. - `-eo `: Customize output format, e.g., `ps -eo pid,ppid,cmd,%mem,%cpu --sort=-%mem`. - `--sort`: Sort by the specified field. ### `top` - Display real-time process information - `top` is an interactive command. Inside, you can use: - `P`: Sort by CPU usage (default). - `M`: Sort by memory usage. - `k`: Kill a process (will prompt for PID). - `q`: Quit. ### `htop` - Interactive process viewer `htop` is an enhanced version of `top` with a friendlier interface and more features (requires separate installation). ### `pstree` - Display processes as a tree - `-p`, `--show-pids`: Show PIDs. - `-a`, `--arguments`: Show command line arguments. - `-u`, `--uid-changes`: Show user identity changes. ### `kill` - Send a signal to a process - `kill `: Sends the default termination signal (TERM, 15). - `kill -9 ` or `kill -KILL `: Sends the force kill signal, which cannot be ignored. - `kill -1 ` or `kill -HUP `: Sends the hangup signal, often used to make services reload their configuration. ### `pkill` / `killall` - Kill processes by name - `pkill `: Kill processes by name. - `killall `: Similar effect, but matches the process name more strictly. - `-f` (pkill only): Match against the full command line. - `-u `: Only kill processes belonging to the specified user. ## Network Management ### `ip` - Show and manipulate routing, devices, policy routing and tunnels - `ip addr show` or `ip a`: Show addresses for all network interfaces. - `ip link show`: Show the state of all network interfaces. - `ip route show`: Show the kernel routing table. - `ip neigh`: Show the neighbor (ARP) cache. ### `ping` - Test network connectivity - `-c `: Specify the number of packets to send. - `-i `: Specify the interval in seconds between packets. ### `ss` - Socket statistics - `-t`: Display TCP sockets. - `-u`: Display UDP sockets. - `-l`: Display listening sockets. - `-n`: Do not resolve service names (show port numbers directly). - `-p`: Show the process using the socket. - **Common combination**: `ss -tulnp` ### `netstat` - Show network connections (legacy) - **Common combination**: `netstat -tulnp` (similar to `ss -tulnp`, but `ss` is more efficient). ### `dig` / `nslookup` - DNS query tools - `dig example.com`: Query the A record for a domain. - `dig example.com MX`: Query the Mail Exchange record. - `nslookup example.com`: Similar functionality, but `dig` provides more detailed output. ### `tcpdump` - Packet Capturing - `-i `: Listen on a specific network interface, e.g., `eth0` or `any` (listen on all interfaces). - `-c `: Stop after capturing a specified number of packets. - `-w `: Write the raw packets to a file instead of parsing and printing them to the screen. - `-r `: Read packet data from a file for analysis. - `-n`: Don't convert IP addresses to hostnames. - `-nn`: Don't convert protocol and port numbers to their corresponding service names. - `-s0`: Capture full packets instead of truncating them. - `host `: Filter expression, only capture traffic with the specified source or destination IP. - `port `: Filter expression, only capture traffic with the specified source or destination port. - `src host ` / `dst host `: More precisely specify source/destination IP. - **Common Combination**: `tcpdump -i eth0 -nn -s0 -w output.pcap 'port 80 and host 192.168.1.1'` ## Service Management (Systemd) ### `systemctl` - Control the systemd system and service manager - `status `: View the detailed status of a service. - `start `: Start a service. - `stop `: Stop a service. - `restart `: Restart a service. - `reload `: Reload configuration without interrupting the service. - `enable `: Enable service to start on boot. - `disable `: Disable service from starting on boot. - `is-active `: Check if a service is currently running. - `is-enabled `: Check if a service is enabled on boot. ### `journalctl` - Query the systemd journal - `-u `: Show logs for a specific service only. - `-f`, `--follow`: Follow new logs in real-time. - `-n `: Show the last N lines of logs. - `--since "YYYY-MM-DD HH:MM:SS"`: Show logs since a specific time. - `--until "YYYY-MM-DD HH:MM:SS"`: Show logs until a specific time. ## File and Directory Operations ### `ls` - List directory contents - `-l`: Use a long listing format. - `-a`, `--all`: Show all files, including hidden files starting with `.`. - `-h`, `--human-readable`: Use with `-l` to show file sizes in a human-readable format. - `-t`: Sort by modification time, newest first. - `-R`: Recursively list the contents of subdirectories. ### `cd` - Change directory - `cd ~`: Change to the current user's home directory. - `cd -`: Change to the previous working directory. - `cd ..`: Change to the parent directory. ### `grep` - Search text - `-i`, `--ignore-case`: Ignore case distinctions. - `-r`, `--recursive`: Recursively search all files under a directory. - `-v`, `--invert-match`: Show non-matching lines. - `-n`, `--line-number`: Show the line number of matching lines. - `-C `: Show matching lines and their context (N lines before and after). ### `find` - Search for files in a directory hierarchy - `-name ""`: Search by file name (e.g., `*.log`). - `-type `: Search by file type (`f`: file, `d`: directory, `l`: link). - `-size `: Search by file size (e.g., `+10M` for greater than 10MB). - `-mtime `: Search by modification time (e.g., `-7` for within the last 7 days). - `-exec {} \;`: Execute a command on each found file. - **Common Combinations**: - `find . -name "*.log" -mtime -7`: Find all `.log` files modified in the last 7 days in the current directory. - `find / -type f -size +1G -exec ls -lh {} \;`: Find all files larger than 1GB under the root directory and display their details. ### `cp` / `mv` - Copy/Move files - `-r`, `--recursive`: Operate on directories recursively. - `-i`, `--interactive`: Prompt for confirmation before overwriting. - `-v`, `--verbose`: Show detailed process. ### `rm` - Remove files or directories - `-r`, `--recursive`: Recursively remove directories and their contents. - `-f`, `--force`: Force removal without prompting. - `-i`, `--interactive`: Prompt for confirmation before each removal. - **Note**: `rm -rf` is a very dangerous command, use with caution. ### `chmod` - Change file permissions - `u` (user), `g` (group), `o` (other), `a` (all). - `+` (add permission), `-` (remove permission), `=` (set permission). - `r` (read, 4), `w` (write, 2), `x` (execute, 1). - **Example**: `chmod 755 script.sh` or `chmod u+x script.sh`. ### `chown` - Change file owner - `chown : `: Change both user and group. - `-R`, `--recursive`: Operate on files and directories recursively. ## Compression and Decompression ### `tar` - File archiving tool - `-c`, `--create`: Create a new archive. - `-x`, `--extract`: Extract files from an archive. - `-f`, `--file`: Specify the archive filename. - `-v`, `--verbose`: Show detailed process. - `-z`: Compress/decompress with gzip (.tar.gz). - `-j`: Compress/decompress with bzip2 (.tar.bz2). - **Common combinations**: `tar -czvf` (compress), `tar -xzvf` (decompress). ### `zip` / `unzip` - .zip format compression tool - `zip -r archive.zip directory`: Recursively compress a directory. - `unzip archive.zip`: Extract to the current directory. - `unzip archive.zip -d /path/to/destination`: Extract to a specified directory. ## Performance Monitoring ### `iostat` - Report CPU and I/O statistics - `-x`: Display extended statistics. - `-d`: Display device utilization report. - `iostat 1 10`: Display once per second, 10 times total. ### `vmstat` - Report virtual memory statistics - `vmstat 1 10`: Display once per second, 10 times total. ### `du` - Estimate file space usage - `-h`, `--human-readable`: Display in a human-readable format. - `-s`, `--summarize`: Display only a total for each argument. - `-d `, `--max-depth=`: Specify the directory level depth to display. - **Common combination**: `du -sh `. ### `lsof` - List open files - `-i`: List all network connections. - `-i :`: List processes using a specific port. - `-p `: List files opened by a specific PID. - `-u `: List files opened by a specific user. - **Common Combination**: `lsof -i :8080`: Find out which process is using port 8080. ## Firewall Management ### `firewall-cmd` (firewalld) - `--state`: Check the firewall running state. - `--list-all`: List all configurations in the current zone (including open ports and services). - `--add-port=/ --permanent`: Open a port permanently. Requires `--reload` to take effect. - `--remove-port=/ --permanent`: Remove a port permanently. - `--add-service= --permanent`: Allow a service permanently (e.g., http). - `--reload`: Reload firewall rules for permanent rules to take effect. ### `ufw` (Uncomplicated Firewall) - `status`: Check ufw status. `ufw status numbered` shows rule numbers. - `allow `: Allow traffic on a specific port or service. - `deny `: Deny traffic on a specific port or service. - `delete `: Delete a rule by its number. - `enable` / `disable`: Enable or disable the firewall. ### `iptables` - Low-level Packet Filtering Tool - **Basic Concepts**: `iptables` uses chains to manage rules. The most common built-in chains are `INPUT` (for incoming traffic), `OUTPUT` (for outgoing traffic), and `FORWARD` (for forwarding traffic). - `-L, --list [chain]`: List all rules in the specified chain. - `-n, --numeric`: Display IP addresses and port numbers in numeric format. - `-v, --verbose`: Show detailed information, including packet and byte counters. - `--line-numbers`: Show rule numbers, which is useful for deleting or inserting rules. - `-A, --append `: Append a new rule to the end of a chain. - `-I, --insert [rule_number] `: Insert a new rule at a specific position in a chain (defaults to the beginning). - `-D, --delete `: Delete a rule. - `-p, --protocol `: Specify the protocol (e.g., `tcp`, `udp`, `icmp`). - `--dport `: Specify the destination port. - `-s, --source

`: Specify the source IP address or subnet. - `-d, --destination

`: Specify the destination IP address or subnet. - `-j, --jump `: Specify what to do with a matching packet (`ACCEPT`: allow, `DROP`: discard, `REJECT`: reject). - `-F, --flush [chain]`: Flush all rules in the specified chain. - **Common Combinations**: - `iptables -A INPUT -p tcp --dport 22 -s 192.168.1.0/24 -j ACCEPT`: Allow SSH connections from a specific IP range. - `iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT`: Allow traffic for all established and related connections. - `iptables -P INPUT DROP`: After setting all `ACCEPT` rules, set the default INPUT policy to drop for enhanced security. - **Saving Rules**: `iptables` rules are lost on reboot and must be saved with a specific command: - `iptables-save > /etc/sysconfig/iptables` (CentOS/RHEL) - `iptables-save > /etc/iptables/rules.v4` (Debian/Ubuntu, requires `iptables-persistent` package) ## User and Permission Management ### User and Group Management - `useradd `: Create a new user. - `-m`: Create the user's home directory at the same time. - `-s `: Specify the user's login shell. - `passwd `: Set or change a user's password. - `userdel `: Delete a user. - `-r`: Remove the user's home directory at the same time. - `usermod `: Modify user attributes. - `-aG `: Add a user to a supplementary group, must be used with `-G`. - `-l `: Change the username. ### File Permissions - `sudo `: Execute a command with superuser (root) privileges. - `chown : `: Change the owner and group. - `-R`: Operate recursively. - `chmod `: Change permissions. - `-R`: Operate recursively. ## Common Docker Commands ### `docker ps` - List containers - `-a`, `--all`: List all containers, including stopped ones. - `-q`, `--quiet`: Only display container IDs. ### `docker images` - List images - `-a`, `--all`: List all images, including intermediate layer images. ### `docker run` - Create and run a new container - `-d`: Run in detached mode (background). - `-p :`: Port mapping. - `-v :`: Mount a volume. - `--name `: Assign a name to the container. - `--rm`: Automatically remove the container when it exits. ### `docker logs` - View container logs - `-f`, `--follow`: Follow log output in real-time. - `--tail `: Show the specified number of lines from the end of the logs. ### Other Core Commands - `docker pull :`: Pull an image from a repository. - `docker stop `: Stop a running container. - `docker start `: Start a stopped container. - `docker rm `: Remove one or more containers. - `docker rmi `: Remove one or more images. ### `docker-compose` - Multi-container application orchestration - `up`: Create and start containers. - `-d`: Run in the background. - `down`: Stop and remove containers, networks. - `-v`: Also remove named volumes defined in `volumes`.

Linux Server Operations Command Guide