Linux: Viewing and controlling processes

• A process can be started from the GUI or the command line.
• A process can start another process.
• A process can only be created by another process.

Commands for Viewing and Controlling Processes

• ps reports a snapshot of current processes.
• top displays tasks.
• jobs lists active jobs.
• bg places a job in the background.
• fg places a job in the foreground.

Viewing Processes

• The ps command shows: PID (Process ID), TTY (Teletypewriter, controlling terminal), TIME (cumulative CPU time), and CMD (command name).
• Process states include: Running (R), Sleeping (S), Waiting for an event (S), Uninterruptible sleep (D), Waiting for I/O (D), Traced or stopped (T), and Defunct or zombie (Z).
• A zombie process is a terminated child process not cleaned up by its parent.
• Characters used with process states: < is high priority, N is low priority, L has pages locked into memory, S is for real-time and custom I/O, l is a session leader, + is in the foreground process group, and | is multi-threaded.
• The ps command with the aux option shows: USER (owner's user ID), %CPU (CPU usage percentage), %MEM (memory usage percentage), and VSZ (Virtual Memory Size).
• RSS (Resident Set Size) is the physical memory amount (in kilobytes) a process uses.
• START is the process start time; a date is used for values over 24 hours.

The top Program

• The top program displays real-time system information, including uptime, user count, load averages, task summaries, and CPU/memory usage.
• Load average indicates processes waiting to run, with values for the last 1, 5, and 15 minutes.
• The field descriptions from the top program include:
• %Cpu(s): CPU usage statistics.
• us: CPU time used by user processes.
• sy: CPU time used by kernel processes.
• ni: CPU time used by "nice" (low priority) processes.
• id: CPU time idle.
• wa: CPU time waiting for I/O.
• hi: CPU time servicing hardware interrupts.
• si: CPU time servicing software interrupts.
• st: CPU time stolen by the hypervisor.
• KiB Mem: RAM used in kilobytes.
• KiB Swap: Swap space used in kilobytes.
• PID: Process ID.
• USER: Username of the process owner.
• PR: Priority of the process.
• NI: Niceness value.
• VIRT: Total virtual memory used by the process.
• RES: Resident size (non-swapped physical memory) in kilobytes.
• SHR: Shared memory size in kilobytes.
• S: Process status.
• %CPU: CPU time percentage used since the last update.
• %MEM: Memory percentage used since the last update.
• TIME+: Cumulative CPU time used by the process and its children.
• COMMAND: Process name or command used to start the process.
• Interrupt a process with Ctrl + C.
• Putting a process in the background: type top &.
• Use the jobs command to see stopped jobs.
• Return a process to the foreground: type fg %1.
• Move a process to the background: type bg %1.

Networking

• A computer network facilitates communication between resources through connected computers and computing resources.
• To establish network access: have a network interface, run network services, and have access to a network-broadcast device.
• A network interface connects to the network.
• Network services include firewall, interface device, network authentication, and network file systems.
• Network-broadcast devices:
• A Hub broadcasts messages to all subnet devices.
• A Switch broadcasts messages to a single subnetwork device using MAC addresses.
• A Router directs messages to the appropriate network or subnetwork based on IP addresses.
• A Gateway connects different local area networks.
• TCP/IP has four layers: Network Interface, Internet, Transport, and Application.
• The Network Interface Layer places/receives TCP/IP packets on/off the network medium.
• The Internet Layer handles addressing, packaging, and routing with core protocols like IP, ARP, ICMP, and IGMP.
• IP (Internet Protocol) does IP addressing, routing, fragmentation, and packet reassembly.
• ARP (Address Resolution Protocol) resolves Internet layer addresses to the Network Interface layer (hardware) address.
• ICMP (Internet Control Message Protocol) provides diagnostic functions and reports IP packet delivery errors.
• IGMP (Internet Group Management Protocol) manages IP multicast groups.
• The Transport Layer provides session and datagram communication services via TCP and UDP.
• TCP (Transmission Control Protocol) establishes TCP connections, sequences packets, acknowledges sent packets, and recovers lost packets.
• UDP (User Datagram Protocol) is used for small data transfers, avoiding TCP overhead, or when upper layers ensure reliable delivery.
• The Application Layer allows applications to access services and defines protocols for data exchange.
• Common Application Layer protocols: HTTP, FTP, SMTP, and Telnet.
• HTTP (Hypertext Transfer Protocol) transfers web page files.
• FTP (File Transfer Protocol) is for interactive file transfer.
• SMTP (Simple Mail Transfer Protocol) transfers email messages and attachments.
• Telnet is a terminal emulation protocol for remote host login.
• Protocols facilitating TCP/IP network use and management include: DNS, RIP, and SNMP.
• DNS (Domain Name System) resolves hostnames to IP addresses.
• RIP (Routing Information Protocol) exchanges routing information between routers on IP internetworks.
• SNMP (Simple Network Management Protocol) collects and exchanges network management information between a console and devices.

Networking Commands

• ping sends ICMP ECHO_REQUEST packets to network hosts.
• traceroute/tracepath prints the route taken by packets to a network host.
• ip shows/manipulates routing, devices, policy routing, and tunnels.
• netstat prints network connections, routing tables, interface statistics, masquerade connections, and multicast memberships.
• ftp is an Internet file transfer program.
• wget is a non-interactive network downloader.
• ssh is an OpenSSH SSH client (remote login program).