Linux Flashcards

Question

Can you breakdown this command? chmod u+x [filename]

Answer 1

Breaking Down chmod u+x [filename] chmod: This is the command used to change the permissions of a file or directory. u+x: u: Stands for "user," but in this context, it specifically refers to the file or directory's owner. The owner is typically the creator of the file or directory, unless ownership has been changed. +: This indicates that you're adding a permission. x: Stands for "execute," which allows the file to be run as a program or the directory to be entered and executed in. [filename]: This should be replaced with the actual name of the file or directory you want to modify. What Does u+x Do? When you run chmod u+x [filename], you're instructing the system to add execute (x) permissions to the user/owner (u) of the file specified by [filename]. Here's what happens: If [filename] is a regular file, adding execute permissions means that the user who owns the file can now run it as a program. This is common for script files or compiled programs that need to be executed directly from the command line. If [filename] is a directory, adding execute permissions allows the owner to enter (cd into) the directory and access files and subdirectories within it. Note that for directories, execute permission is necessary to access the contents of the directory. Practical Example Suppose you have a script file named script.sh that you've just created or downloaded. By default, it might not have execute permissions, which means you can't run it directly from the shell. To enable execution, you would use: chmod u+x script.sh After this command, as the owner, you can run the script by typing ./script.sh in the terminal. Security Considerations Adding execute permissions should be done cautiously. Executable files can perform any operations that the user has permission to perform. Hence, you should only add execute permissions to files that you trust and that require execution for their intended function. In summary, chmod u+x [filename] is a way to ensure that the owner of the file has the necessary permissions to execute it, enhancing functionality while maintaining control over who can execute the file.

Answer 2

chown This command allows you to change the owner and/or the group owner of a file or directory. chown [owner][:[group]] [file] Parameters: [owner]: Specifies the new owner's user ID or name. [group]: Optionally, specifies the new group ID or name. If only a colon is used with no group name, the group of the file is changed to the login group of the new owner. [file]: The file or directory whose ownership you want to change. Example 1: Changing the Owner To change the owner of a file named example.txt to a user named john: chown john example.txt Example 2: Changing the Owner and Group To change both the owner and the group of a file named example.txt to john and admin respectively: chown john:admin example.txt Example 3: Changing the Group Only If you want to change only the group of a file and leave the owner unchanged, you can use: chown :admin example.txt Recursive Change If you need to change the ownership of a directory and all of its contents (files and subdirectories), you can use the -R (recursive) option: chown -R john:admin /path/to/directory This recursively changes the owner and group of all files and subdirectories within /path/to/directory to john and admin.

Answer 3

kill Steps to Kill a Process: Find the PID: Before you can kill a process, you need to know its PID. You can find the PID by using commands like ps, top, or pgrep. For example: ps aux | grep process_name Use the kill Command: Once you have the PID, you can use the kill command to terminate the process. kill 1234 Specifying a Signal: You can also specify a different signal with the kill command if needed. For example, if the process does not terminate with the default TERM signal, you might need to send the KILL signal, which forcefully terminates the process and cannot be ignored: kill -9 1234 Here, -9 represents the KILL signal, which forcefully stops the process. Common Signals: SIGTERM (-15): Gracefully terminates the process. The process can clean up before it terminates. SIGKILL (-9): Forcefully terminates the process. The process cannot ignore this signal and does not perform any cleanup. If you prefer to terminate processes by name instead of PID, you can use killall or pkill: killall firefox pkill -f firefox

Answer 4

ps To list all the processes running on a Linux system, the ps command is commonly used. This command displays information about active processes. By itself, ps shows only the processes associated with the terminal session you are using, but it can be combined with various options to extend or change the display. Commonly Used ps Options: ps aux: This is a widely used option combination to list all the running processes on the system, including those started by other users. a: Show processes for all users u: Display the user/owner of the process x: Include processes without a controlling terminal ps aux This command will output several columns of information, such as: USER: The username of the process owner. PID: Process ID. %CPU: CPU usage of the process. %MEM: Memory usage of the process. VSZ: Virtual memory size of the process. RSS: Resident set size, the non-swapped physical memory the process is using. TTY: Terminal type associated with the process. STAT: Process state. START: Starting time or date of the process. TIME: Total CPU time used. COMMAND: Command with all its arguments as a string. ps aux -o pid,user,comm top

Answer 5

ssh ssh john@192.168.1.100 With -p option ssh -p 2222 john@192.168.1.100 To avoid typing a password each time you connect, you can use SSH keys. First, generate a key pair on your client machine (if you haven't already) using: ssh-keygen Then, copy your public key to the remote server using: ssh-copy-id john@192.168.1.100 After this, you can connect to the server without a password, as long as your private key is accessible and not protected by its own passphrase (or you have the passphrase saved in a key manager). Running a command remotely: ssh john@192.168.1.100 "ls -l /home/john"

Answer 6

In Linux, a service (often referred to as a daemon) is a background process that is designed to run independently of user sessions. Services often start at system boot (or when manually started) and run continuously until the system shuts down. They do not have a direct interface with the user, but they work in the background to handle system tasks, user requests, network communications, or other processes. Characteristics of Linux Services: Background Execution: Services run in the background and do not require user interaction. Long-running: These processes are typically initiated at system startup and continue running until the system is shut down. System or Application Tasks: Services can perform a variety of tasks, from system-related jobs like logging, system monitoring, and managing network connections, to application-specific tasks such as web serving, database management, or file sharing. Managed by Init Systems: Services are usually managed by an initialization system. The traditional system used was SysVinit. However, most modern Linux distributions have adopted systemd as their init system, which provides more features and a unified standard for managing services. systemd is the most common init system and service manager on current Linux distributions. It uses units to manage different resources. The type of unit used for services is called a "service unit". Basic Commands to Manage Services with systemd: Start a service: sudo systemctl start [service-name] Stop a service: sudo systemctl stop [service-name] Enable the service to start at boot: sudo systemctl enable [service-name] Check the status: sudo systemctl status [service-name] sudo systemctl restart [service-name]

Answer 7

When you launch or print $?, Bash outputs the exit status of the most recently completed command.

Answer 8

When the code in the file is loaded in the memory and executed by the processor, it becomes a process. A thread is a unit of execution within a process. A process has at least one thread. It is called the main thread.

Answer 9

It is an executable file. It contains the code or a set of processor instructions, that is stored as a file on disk. When the code in the file is loaded in the memory and executed by the processor, it becomes a process.

Answer 10

When the code in the file is loaded in the memory and executed by the processor, it becomes a process. An active process also includes the resources the program needs to run. They are managed by the operating system. Some examples of these resources are - processor registers - program counters - stack pointers - memory pages assigned to the process for its heap and stack Each process has its own address space and they can not corrupt each others address space. Chrome tabs example

Answer 11

A thread is a unit of execution within a process. A process has at least one thread. It is called the main thread. A process may have many threads. Threads within a process share a memory address space and they can corrupt each other when they malfunction. And threads can communicate with each other using the shared memory space. One misbehaving thread can take down the entire process.

Answer 12

Scheduler - context switching. This is expensive. https://www.youtube.com/watch?v=4rLW7zg21gI&ab_channel=ByteByteGo

Linux Flashcards

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