Blogs

1. Introduction

• - All electronic devices such as computers and smartphones are composed of hardware and software.
  • - Hardware (HW): everything physically in an electronic device
  • - Software (SW): A collection of commands given to a computer to achieve a specific purpose
• - When using the software, the computer’s CPU, RAM, and other hardware are used to perform the operation requested by the user.
  • - At this time, the Operating System (OS) allocates hardware resources as needed for the software.
    • - The operating system manages limited hardware resources such as CPU and RAM and mediates between HW and SW.
    • ex. Windows, MAC, Android, IOS etc.
• - Linux is also an operating system
  • - An operating system based on UNIX by Linus Torvals, a Finnish SW engineer.

2. Why use Linux?

• - Open source
  • - Linux is an open source operating system
  • - Anyone, whether an individual or a corporation, can install and use Linux for free.
  • - open source movement
• - Customizing
  • - Linux strictly means the Linux kernel.
    • - The kernel is a part of the OS that performs the core functions of the OS.
  • - Only the core functions that Linux OS needs to perform are defined, and the other parts can be customized and used by the user according to his/her own use.
• - Stable operation
  • - Since it is open source, various users can verify it in real time.

3. Linux(UNIX) Required Commands

3.1. CLI(Command Line Interface) commands

• - Made for i18n (internationalization)
• - UTF-8 is used as the default character set
• - It is recommended to use the en_US locale as it is affected by the setting of the LANG environment variable

3.1.1. File

• - Path
  • - command
    • pwd: print working directory
    • cd: change directory
  • /: root directory
  • ~: home directory
  • -: previous directory
  • - Path type
    • - Absolute path (abs-path): path starting from the root directory
    • - Relative path: path starting from the current directory (.)
• - Check
  • - command
    • - ls, file, stat, which, find
  • - file mode bit: 3+9 bit system representing UNIX file permissions
    • - how to write
      • - Symbolic mode:
        • How to mark with rwx symbol
        • Consists of owner, group, and others parts, each with 3 spaces
        • -r: readable, -w: writable, -e: executable
      • - Octal mode: A method of expressing bits in octal notation
  • - stat: outputs status of file, meta data of file
    • - meta data: Modifying information, not content (file name, creation time, permissions)

  • - touch: update meta data of file, create an empty file if file does not exist

  • - find: find directory
    • -name filename: search for files with the same name as filename
    • -size n: search for files of size n
    • -mtime n: search for files with modified time n
    • -inum n: Search for files with inode number n
    • -max(min)depth level: Search for files with a maximum (minimum) depth of level in the subdirectory of the location to be searched
    • $ex1.$ find . -name '*k.data' -a -size 1M (-a: AND, -o: OR)
    • $ex2.$ find -name "*.tmp" -exec rm {}\;
      • - *.tmpfiles are put in {}
      • - The meaning of \ is that the command is executed while searching one by one.
      • - \+ finds everything and executes the command at once
    • practice 1. Find general files whose contents have been changed within the last 24 hours under the current directory and save the list as mtime_b24.txt file
      • find ./ -mtime -1 -type f > mtime_b24.txt
    • practice 2. If it goes beyond the 3rd level under the current directory, it is not searched, and all files that satisfy the condition are copied to the ~/backup directory.
      • find ./ -maxdepth 3 ... -exec cp {} ~/backup \;

• - stdio (standard input/output)
  • File channel: channel for input/output to file
    • - A kind of virtualization layer that allows standardized input/output methods to be used to input/output channels to files without being directly transferred to hardware.
    • - file channel: object with meta information for input/output to a file
    • - Enables simplicity of I/O interface in C language
  • File descriptor (often used as file descriptor, fd)
    • - Unique idenfifier attached to file channels, named numerically
    • - Starting from positive number 0 and increasing
    • - Reserved file descriptors: 0 (stdin), 1 (stdout), 2 (stderr)
    • - The fd value is given within the process. Can’t I open the same file twice?
      • - Even if the same process opens the same file, a new pd value is assigned. It’s possible, but there’s a possibility of overlapping updates.

    • used as communication between PIPE processes

    • - A type of IPC (inter-process communication)
      • Anonymous pipe
        • serial connection of processes (A|B|C)
        • Temporarily created and destroyed pipes
        • Created by using | (vertical bar) in the shell.
        • $ex$. find ~ | wc -l
          • find ~: find all files under the home directory
          • | Since there is a pipe, since it is wc (word count), it is per line, so I want to know how many files are under the home directory.
          • The output of the find command (stdout) is concatenated with the input (stdin) of the wc command.
          • Has the same meaning as find ~ > tmp.txt; wc -l < temp.txt; rm tmp.txt.
          • fd: 1 is connected to fd: 0 via a pipe
          • When using the wc -l option, the number of lines is counted.
      • Named pipes
        • In Unix, the implementation of a named pipe is called a FIFO pipe.
        • It is structured like a file, so there is a path+filename.
        • Expresses that having a path is named.
        • mkfifo or POSIX C API
  • Redirection
    • Link the direction of a channel to another place
    • A > B : connect (save) A’s stdout to file B
      • ex. ls -a > ls.txt
    • A < B : link A’s stdin to file B
    • A >> B: direction is the same as “>”, append mode
      • ex. strace ls 2> strace.txt
        • 2> means a command that connects file descriptor number 2 to a file.
        • Save the output of stderr, which is fd 2, to a file.
  • cat: default filter to freely link stdout to files
    • Used to output the contents of a file to stdout
    • Used to redirect input from stdin and output it to a file
    • $ex$. cat ~/.bashrc
    • $ex$. cat > hello.txt Then, if you write Hello world and do ^D, Hello world is entered into hello.txt and exited.
• - Change data
  • - command
    • - cp, mv, rm, mkdir/rmdir, ln
    • - mkdir: make directory
    • - rmdir: remove directory (In many cases, files and directories are deleted together with rm -rf instead of rmdir.)
    • - cp: copy
    • - mv: move, rename
    • - rm: remove
• - Meta change
  • - command
    • - chmod: change mode
    • - chown, chgrp: change owner/group
• - Archive
  • - tar
    • An archive is a grouping of multiple files.
    • tar -ctxv
      • -c (create): create an archive
      • -t(test) : test the archive
      • -x (extract) : Extracts a file from an archive
      • -v (verbose): output detailed information (not used in practice)
      • f archive-file: Archive file name to input/output --exclude file: Exclude the file from the target
    • $ex$. tar c *.c > arc_c.tar == tar cf arc_c.tar *.c (just give the f option. *.c files go into arc_c.tar)
• - Compress
  • - gzip, zstd
    • The compression rate is xz > bzip2 > zstd > gzip > lz4
    • xz: Compression rate is good, but slow.
    • zstd: used a lot these days $ex$. The classic way to use tar and gzip together
    • compression: tar c /etc/*.conf | gzip -c > etc.tar.gz
    • release: gzip -cd etc.tar.gz | tar x

3.1.2. Text

• - Editor
  • - vim(vi)
• - Filter
  • - cat(tac), head, tail, less/more, sort
• - Regex
  • - grep, sed, awk

3.1.3. Job control

• - jobs, fg, bg

3.1.4. Process control

• - kill, pkill, pgrep, strace(tracing)

3.1.5. Networking

• - nc (net cat), curl, wget

3.1.6. Disk

• - df

3.1.7. System

• - free, top, ps, pidstat, lshw

3.2. Admin commands

3.2.1. Package

• - Redhat: rpm, yum
• - Debian: dpkg, apt

3.2.2. Network

• - status: ss, netstat(old fashion)
• - config: nmcli, ip
• - ssh
• - packet: tcpdump, wireshark, tshark

3.2.3. Files and kernel

• - lsof
• - sysctl

3.2.4. Disks

• - fdisk, parted, mkfs, mount, lsblk, blkid, grubby, udisksctl

3.2.5. User

• - useradd, groupadd, usermod
• - passwd, chpasswd