Understanding the Linux Command-Line Interface (CLI)

Dongguozi(東郭子) asked Zhuangz(莊子) : What is the Tao(道)? Where is it located?
Zhuangzi replied: It is everywhere.
Dongguozi asked: Can you give an example?
Zhuangzi answered: It is in the body of a small ant.
Dongguozi questioned: The Tao is in such a lowly place?
Zhuangzi responded: It is also in the weeds.
Dongguozi inquired: Why does it become even lower?
Zhuangzi explained: It is in the shards of a broken pot.
Dongguozi asked: Why does it keep deteriorating?
Zhuangzi replied: It is in dung and urine!!!
Dongguozi exclaimed: What the..FxxK. ~!@87#$%^&*..e04!..

Shell is a textual language and it is an interpreter language used for Kernel. The general programs on a computer, for the sake of execution speed and confidentiality, are developed using high-level programming languages. These high-level languages are then compiled into machine code, which the computer can execute, using a compiler.

On the other hand, an interpreter language, like Shell, does not translate the entire program at once. Instead, it interprets and executes one line of instructions at a time, communicating with the relevant application software or the core of the operating system. So, when a user enters commands in a text interface, they are translated by the Shell interpreter and executed by the Linux kernel, one line at a time.

Scripting languages, are a collection of commands in a text interface that work together to accomplish a specific action. Some common interpreter languages for PCs include Shell, Python, JavaScript, and HTML.

Interpreted languages have their advantages and disadvantages. Here are some of them:

• Advantages:
  • Easy to read and learn:
    Software written in high-level languages needs to be compiled into machine code that the computer can execute in order to be validated. If there are bugs, the original source code needs to be modified and then recompiled into machine code for validation. On the other hand, interpreted languages can be executed without going through a compiler, making them relatively easier to learn and read.
  • Good interactivity:
    Because only one line is executed at a time, it is easier to interact with commands.
  • Easy cross-platform:
    As long as the operating system or application can interpret the interpreted language, it can be cross-platform. For example, if a web page is written in HTML, as long as the application can interpret HTML language, it can be accessed across different platforms. Whether the user is using a mobile phone, PC, or iPad, and regardless of whether they are using Edge/Firefox/Chrome or other browsers, they will get similar results when browsing the same web page. Additionally, UNIX/Linux follows an important design philosophy of "Choose portability over efficiency." Therefore, it strongly recommends using Shell Script as much as possible to complete programs.
    
• Disadvantages :
  • Slower execution speed::
    Interpreted languages generally have slower execution speeds compared to compiled languages since the code is translated and executed line by line.
  • Code protection:
    Interpreted languages make it easier for others to read and modify the source code, as it is typically distributed in its original form. This can be a concern for proprietary or sensitive applications.

• Superuser (root)
  Because Linux is a multi-tasking operating system, each file or command has permissions to regulate. For example, others cannot peek at my files without authorization, and I cannot delete or modify other people's files or operate certain commands without authorization. The only exception is to log in with the "root" account; the "root" account is also called "superuser" or "system administrator" or "super user".
  
  Logging in as the "root" account in Linux grants unrestricted access, almost without any restrictions. However, with great power comes great responsibility, and it is a double-edged sword. Improper operations can potentially damage the entire system. Therefore, unless necessary, such as for maintenance or administration purposes, it is advisable not to log in as the "root" account. It is recommended to perform routine operations using a regular user account. The more rules and regulations followed, the more secure the system tends to be..
  
  In Fedora, the default text-based login prompt is displayed as "[username@hostname+current_directory]$". If you log in with the "root" account, the symbol at the end, which is usually "$", will change to "#" to indicate to the operator that they are logged in as the root user.

• "\" :Remove alias
  
  Entering the escape character "\" before the command means "removing aliases"; for example, most Linux distributions make the files with different attributes display different colors in order to make it easy for users to understand at a glance when issuing the ls command. In fact, it is to execute ls --color=tty means that ls --color=tty is the alias of ls , but if you think it is unnecessary, enter "\" (such as \ls ) before the command to remove the alias. example:
  
  Example:
  

  $ ls ↵ Enter ←The Fedora 8 command "ls" actually executes "ls --color=tty", so the output files with different attributes will display different colors bin   dev   home   lost+found misc  net   proc  sbin        srv   tmp   var boot  etc   lib   media       mnt   otp   root  selinux     sys   usr $ \ls ↵ Enter  ← input "\" before the command means "remove alias", the output will not be colorful bin   dev   home   lost+found misc  net   proc  sbin        srv   tmp   var boot  etc   lib   media       mnt   otp   root  selinux     sys   usr

  
  
• "COMMAND" :Commands and instructions .
  
  The command is a necessary part, and the difference from the DOS command is that the upper and lower case have different meanings, and the Linux command can be built in the shell (such as cd, echo, etc.) or can be an executable file (such as /bin/ ls , /bin/mv, etc.) or an alias , if it is an executable file, its location is in its environment variable "PATH" and you don't need to enter the path of the command (you can enter echo $PATH to query the path).
  
  The order of executing the command is to find the alias first, then find the shell built-in if it cannot be found, and then find the executable file if it cannot be found again.
  
  Example:
  

  $ type -a pwd ← Check that the command "pwd" is a shell built-in or executable file or alias pwd is a shell builtin ← There are two commands "pwd", one is the shell built-in, and the other executes File "/bin/pwd pwd is /bin/pwd $ cd /var/mail↵ Enter ← Enter directory "/var/mail" $ pwd↵ Enter ←Execute the built-in "pwd" command in the shell (built-in takes precedence) /var/mail← Built-in command "pwd" displays the link file $ /bin/pwd↵ Enter ← Change the executable file "/bin/pwd "See if the output is different from the built-in /var/spool/mail ← Executive file command "/bin/pwd" shows the real path

  
  
• "-" :The symbol of the option.
  There is at least one space between the command and the option symbol "-", and the option character is directly connected to the "-" without a space, and multiple options "-" can be combined.
  For example:
  • ls -a: There is at least one space or more between the command ls and the symbol "-" of the option, so ls    -a and ls -a are the same and ls-a is wrong syntax (no spaces), and ls - a is also Incorrect syntax (because of a space after "-").
  • Multiple options "-" can be merged into ls -alR , of course, as long as you are happy, you can also partially merge them such as ls -al -R .
  • ls -L≠ ls -l because the case sensitivity of the command and option characters has different meanings.
    
    
  Spaces can also be used for the options of a few commands, such as ps aux , but there are only a few commands in this process.
  
  
• "--" :Sub-options
  Some commands also have sub-options "--", such as mv --help or ls --color=no .
  
  
• "PARAMETER" :Parameter or also known as argument.
  Parameters can be files or variables or paths or combinations thereof or other symbols such as stat -c%F%i /etc/fstab or more + 30 /etc/prelink.conf or shutdown -r +3 .
  
  
• 〝\〞↵ Enter : Escape Character
  If a command is too long or for other requirements, the escape character "\" + ↵ Enter can be used. Then, on the next line, you can continue entering the unfinished part. In a CLI the "\ ↵ Enter" sequence will be treated as invisible or ignored.
  
  Example:
  

  $ cat /etc/fst\↵ Enter ←Enter the blue text + <Enter> > ab↵ Enter ←When entering the blue text, the ">" symbol is automatically generated and does not need to be entered. LABEL=/                 /                       ext3    defaults        1 1 LABEL=/boot             /boot                   ext3    defaults        1 2 tmpfs                   /dev/shm                tmpfs   defaults        0 0 devpts                  /dev/pts                devpts  gid=5,mode=620  0 0 sysfs                   /sys                    sysfs   defaults        0 0 proc                    /proc                   proc    defaults        0 0 LABEL=SWAP-sda3         swap                    swap    defaults        0 0 $ cat /etc/fstab↵ Enter ←This example is the same as the previous one, but the previous example used "\ <Enter>" to input it in two lines. LABEL=/                 /                       ext3    defaults        1 1 LABEL=/boot             /boot                   ext3    defaults        1 2 (The rest is omitted.)

  
  
• ";" :command terminator
  The semicolon can be used to execute multiple commands on a single line. For example:
  
  Example:
  

  $ clear;cd /;ls↵ Enter ←When there are multiple commands, you can use the ";" to separate one command from another bin   dev  home  lib         media  mnt  opt   root  selinux  sys  usr boot  etc  lib   lost+found  misc   net  proc  sbin  srv      tmp  var

  
  
• ↵ Enter :Command input is complete
  The command will be accepted only after pressing ↵ Enter command.
  

• <Ctrl+Alt+F(1~7)> Switching terminals
  Most of the Linux distributions, you can press Ctrl+Alt+F1 ~ Ctrl+Alt+F7 to switch between different terminals tty#, Fedora 8 defaults tty1~tty6 as Text operation interface and tty7 is a graphics operation interface (different versions may be different).
  
  
• <↑>, <↓> History search
  After the input string + ↵ Enter, most Linux distributions will record this string, and you can use ↑、↓ keys to query the previously input string.。[Note]
  • Reviewing command history with the "history" command
    As mentioned above↑,↓are used to call out the string (command) that has been input, but how do we know what command will appear when we press↑ or ↓ ? Just enter history to know.
    
    Example:
    

    $ history↵ Enter The following are the strings (commands) that have been entered after entering the "history" command.   1 cat proc/cpuinfo   2 ls   3 rm ex07-7.cpp

    In the above example, the leftmost number is the "command number", and the right number is the command. If there are too many history records, sometimes the command you want to find will be lost. You can use history -c to clear it.
    
    
  • <!> Specifying command number
    If the command number is far from the command I want, won't I have to press ↑ or ↓ endlessly? In this case, using "!" for referencing and searching is very convenient. The usage of ! is as follows.
    • !  !:
      Enter two consecutive〝!!〞o repeat the last instruction.
    • !  #:
      (# is a number) the command to specify the command number; for example, input!80 to specify the command with the command number = 80.
    • !  -#:
      (# is a number) specifies the command counting from the last command, counting down to the # command; for example !-3 。
      
      
• <Ctrl+C> Stopping current operation
  If the operation process is too long or other reasons to give up the current operation, you can use the job control Ctrl + C to stop the current operation.
  
  Example:
  

  $ seq 1 10000000↵ Enter 1 2 3 4 8Ctrol+C ←press <Ctrl+C> to give up the current operation

  
  
• <Ctrl+D> Logging out or ending a file
  Ctrl+D in the text interface has two functions:
  • Log out the current user:
    In the login prompt of the text interface, when the cursor is at the leftmost position, press Ctrl + D to log out the current user. (Press Ctrl + Alt + Backspace in the GUI to log out the user.)
    
    Example:

    $           _Ctrl+D ← Press <Ctrl+D> at this time, it is invalid because the cursor is not at the leftmost position $ ls_Ctrl+D ← Press <Ctrl+D> at this time Invalid, the reason is the same as above $ _Ctrl+D ← Press <Ctrl+D> at this time to establish, because the cursor is at the leftmost position, the current user will be logged out

    
    
  • End input:
    In some CLI, press Ctrl + D to represent End of File (EOF).
    
    Example:

    $ cat > EOF-test↵ Enter Hello, this is a EOF test↵ Enter Ctrl+D ← press <Ctrl+D> at this time to end input [aaa@localhost ~]$ cat EOF-test ←verify Look at the newly created file Hello, this is a EOF test

    
    
• <Tab> Autocompletion
  In the text interface, I personally find the "Tab completion" feature using the Tab key to be the most useful. Tab completion significantly reduces the chance of typing errors and saves typing time. It applies to both "Command-Line Completion" and "Filename Completion."
  
  • Command-Line Completion:
    When you press Tab in the text interface, it examines the characters already entered to determine the possible commands the user might enter next. If there are multiple possibilities, pressing Tab again will display a list of all the matching commands
    
    Example:

    $ lsTab ⇌  Tab ⇌  ←Input "ls" and press <Tab> If there is no response, it means that there are many possibilities, press <Tab> again and all matching commands will be listed

    
    
  • Filename Completion:
    After the command input is completed, space more than one space , enter some characters and press Tab , at this time, it will help the user to enter the matching file name according to the space between the characters that have been entered. If there are multiple files that match, click again  Tab lists all matching file names. .
    
    Example:

    $ ls /etc/hosts.aTab ⇌llow _ ←Enter the blue text,automatically completes filenames when you press <Tab>

    
    
• <Ctrl+R> History search + autocompletion
  Ctrl+R CLI Ctrl+R is a combination of "history search" and "auto-completion." It only auto-completes commands that have been previously entered. Explaining it in detail would take some time, so I encourage you to experience it yourself.
  
  
• Input control
  When inputting commands, it is inevitable to make typos. Some input control keys can be used properly to reduce the time for correction:
  
  • <Ctrl+U> Deleting the entire line
    
    Example:

    $ LS /ETC Ctrl+U ←At any time, pressing <Ctrl+u> before entering will delete the entire line

    
    
  • <Ctrl+W> Deleting input before the cursor
    
    Example:

    $ 0123456789 Ctrl+W ← Input before the cursor will be deleted

  • <Ctrl+K> Deleting input after the cursorr
    
    Example:

    $ 0123456789 Ctrl+K ← Input after the cursor will be deleted

    
    
  • <Ctrl+L> Clearing the screen above the cursor
    Pressing Ctrl+L in text mode will clear any content above the cursor.
    
    Example:
    

    $ ls / bin   dev   home   lost+found misc  net   proc  sbin        srv   tmp   var boot  etc   lib   media       mnt   otp   root  selinux     sys   usr $ history Ctrl+L ← the screen above the cursor will be cleared

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• hh:mm: Unambiguous time notation.
  For example, shutdown -h 10:08 means that it will automatically shut down at 10:08.
• +m: Notation for a few minutes later, 'm' is a number.
  For example, shutdown -r +5 'The system will reboot after 5 minutes' means reboot after 5 minutes', and send "The system will reboot after 5 minutes" to every logged-in user, and the sign "+" can be omitted , such as shutdown -h 5 means that it will automatically shut down after 5 minutes.
  
  
• now: means immediately.
  It is equal to +m, but m=0, such as shutdown -h now = shutdown +h +0 means that it will shut down immediately.
  

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In this example, "N" represents the previous runlevel, and "5" represents the current runlevel. This information can be useful for understanding the current state of the system and determining what services or processes are expected to be running.

It's important to note that with the introduction of systemd, which is a modern initialization and service management system used by several Linux distributions, the concept of runlevels has been replaced by targets. In systemd, you can use the systemctl command to get information about the current system state and manage services.

In fact, the related commands for shutting down, such as poweroff, halt, or shutdown, directly invoke (system call) the init command to execute the shutdown process.

Furthermore, the runlevel at startup is recorded in "/etc/inittab." In most mainstream Linux distributions, the default runlevel at startup is often set to the graphical interface (runlevel = 5). If you wish to change it to a text-based interface during startup, you can locate the following line in "/etc/inittab": "/id:5:initdefault:". The number "5" in this line represents the runlevel. By changing it to "3," the system will boot into the Command Line Interface text-based interface. To enter the GUI later, you can enter the startx command.

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• Commands that have been used but have forgotten their names, history can be used to list the commands that have been issued, which should be able to wake up the lost memory.
• If you can't find it in the long history , but you may still remember the first few words of the command name, you can use the Tab Command-line completion function to find it.
• In the Linux Filesystem Hierarchy Standard, it is mentioned that essential utility software may be stored in /bin or /usr/bin, while system administration-related software may be stored in /sbin or /usr/sbin, and so on. Therefore, you can check these directories to see if the desired commands are available.
• If you forget the built-in commands of the shell, you can enter help to list all the built-in commands of the shell, which may bring back some memories.

• Q or q : Quit the man page。
• H or h : Displays a brief help message of man
• Page Up / Page Down : Turn pages up and down.
• ← / → :  Move 8 lines left and right.
• ↑ or k :To scroll up.
• ↓ or j : To scroll down.
• Home / End : Move to the first/last page.
• / STRING: Search down for a string.
• ? STRING : Search upwards for a string.
• ESC after U : Cancel the highlighting of the search string (press the ESC key first, then press U when "ESC" appears at the bottom left of the man page ).
• n : with / or ?allows you to navigate to the next occurrence of the search term.
• N : with / or ? Allows you to navigate to the previous occurrence of the search term
• m after CHAR: To mark the bookmark of the current page,
  press the m key, and "mark" will appear at the bottom left of the man page. At this time, you can press any letter to represent the bookmark name of this page
• ' fter character: Go to the bookmarked page
  Press the ' key, "goto mark" will appear at the bottom left of the man page to the bookmarked position, and you can press the letter marked with m to return to the page just marked).
  

For more detailed usage of man, please enter man yourself.

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• q : To quit and exit the info command.
• ? : To display a summary of available key commands.
• h : Displays a brief help message of info page
• Tab⇌ : Choose between different topics (nodes).
• ↵ Enter : When the cursor is on the topic, it will enter the description of the topic.
• s : search string.。
• i : Search for topics in the current info page.
• Page Up / Page Down : Turn pages up and down.。
• ← / → / ↑ / ↓ : Tto navigate within the info page and move between different sections.
• b : Move to the very beginning topic.
• e : Move to the last topic.
• n : Move to the next topic.
• p : Move to the previous topic.

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