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Information, Navigation, and Management Commands

Information, Navigation, and Management Commands

Overview

What is a shell?

  • User interface for running commands

  • Interactive language

  • Scripting language, can be used to automate tasks

  • Default shell: Bash. Other shells: sh, ksh, tcsh, zsh, and fish. Check shell with

    printenv SHELL

    This returns the path to the default shell program

Shell command applications

Getting information
  • whoam: User name
  • id: User ID and group ID
  • uname: OS name
  • ps: Running processes
  • top: Resource usage
  • df: Mounted file systems
  • man: Reference manual
  • date: Today’s date
Navigating and working with files and directories

Files

  • Files
    • cp: Copy fiile
    • mv: Change file name or path
    • rm: Remove file
    • touch: Create empty file, update file timestamp
    • chmod: Change/modify file permissions
    • ws: Get count of lines, words, characters in file
    • grep: Return lines in file matching pattern
  • Directory
    • ls: List files and directories
    • find: Fined files in directory tree
    • pwd: Get present working directory
    • mkdir: Make directory
    • cd: Change directory
    • rmdir: Remove directory
Printing file and string contents
  • cat: Print file contents
  • more: Print file contents page-by-page
  • head: Print first N lines of file
  • tail: print last N lines of file
  • echo: Print string or variable value
File compression and archiving
  • tar: Archive a set of files
  • zip: Compress a set of files
  • unzip: Extract files from a compressed zip archive
Performing network operations
  • hostname: Print hostname
  • ping: Send packts to URL and print response
  • ifconfig: Display or configure system network interfaces
  • curl: Display contents of file at a URL
  • wget: Download file from URL
Monitoring performance and status of the system, its components and applications
Running batch jobs, such as ETL operations.

Informational Command

  • Display user information
  • Verify identity or identify user account
CommandUsage
whoamiReturn user name
idUser or Group ID
  • id -u returns the numerical ID of the user
  • id -u -n returns the name corresponding to the numerical user ID
    • uname (Unix name)Returns OS information
  • Identify system or diagnose issues
    • df (disk free)Show disk usage to monitor disk usage or check space
    ps (process status)Monitor or manage processes
    top (table of processes)Task manger: Monitor system performance and reosurce usage
    echoPrint string or variable value
    dateDisplay system date and time
    man <command> (manual)Show manual for any command

    is a free and open-source collaborative documentation effort, which provides documentation that is more accessible than the traditional man pages, along with practical examples.

    Example

    Screenshot of the tldr client displaying the tar command.

    ls

    File and Directory Navigation Commands

    pwd, cd

    ls

    截屏2024-08-16 17.53.20

    find: Find files in directory tress.

    • Example:

      截屏2024-08-16 17.21.33

      • iname: case insensitive

    File and Directory Management Commands

    mkdir

    rm

    If you want to remove multiple files, for example by using a wildcard to find all filenames matching a pattern, it’s best practice to confirm or deny each deletion by including the -i option, which creates a prompt to ask for confirmation before every deletion.
    Be careful when deleting files or directories! There is normally no way to restore a deleted file once it is deleted, as there is no trash folder. This is why you should always back up, or archive, your important files. You will learn more about archiving files soon.

    rmdir: Remove empty directory

    touch: Create empty file, update file date

    cp: Copy file or directory (-r) to destination

    mv: Move file or directory

    • when the source and target directories are the same, you can use mv to rename a file. Example: use mv to rename users.txt to user-info.txt by entering the following command:

      mv users.txt user-info.txt
    You should always use caution when moving a file. If the target file already exists, it will be overwritten, or replaced, by the source file.

    chmod (change mode): Change file permissions

    • Specify which permissions to change with a combination of the following characters:

      OptionDescription
      r, w, xPermissions: read, write, and execute
      u,g, oUser categories: user, group, and all others
      +, -Operations: grant and revoke

    • Example

      截屏2024-08-16 17.43.31

    Managing File Permissions and Ownership

    Three possible levels of file ownership in Linux: user, group, and other.

    • Whoever creates a file, namely the user at the time of creation, becomes the owner of that file by default.
    • A group of users can also share ownership of a file.
    • The other category essentially refers anyone in the universe with access to your Linux machine - careful when assigning ownership permission to this level!

    Only an official owner of a file is allowed to change its permissions. This means that only owners can decide who can read the file, write to it, or execute it.

    Viewing file permissions

    Example:

    $ echo "Who can read this file?" > my_new_file
$ more my_new_file
Who can read this file?
$ ls -l my_new_file
-rw-r--r-- 1 theia users 25 Dec 22 17:47 x

    Here we’ve echoed the string "Who can read this file?" into a new file called my_new_file. The next line uses the more command to print the contents of the new file.

    Finally, the ls command with the -l option displays the file’s (default) permissions: rw-r--r--.

    • The first three characters (rw-) define the user permissions. You, being the user, have the permission rw-, which means you have read and write permissions by default, but do not have execution permissions. Otherwise there would be an x in place of the last -.
    • The next three (r--) the group pemissions. The final three (r--) the other permissions.

    Looking at the entire line, rw-r--r--, you can see that anyone can read the file, nobody can execute it, and you are the only user that can write to it.

    The - at the very beginning of the line in the terminal means that the permissions are referring to a file. If you were getting the permissions to a directory, you would see a d in the front for “directory”.

    Directory permissions

    The permissions for directories are similar but distinct for files. Though directories use the same rwx format, the symbols have slightly different meanings.

    The following table illustrates the meanings of each permission for directories:

    Directory PermissionPermissible action(s)
    rList directory contents using ls command
    wAdd or remove files or directories
    xEnter directory using cd command

    Making a file private

    You can revoke read permissions from your group and all other users by using the chmod command.

    Example:

    $ chmod go-r my_new_file
$ ls -l my_new_file
-rw------- 1 theia users 24 Dec 22 18:49 my_new_file

    In the chmod command, go-r is the permission change to be applied, which in this case means removing for the group (g) and others (o) the read (r) permission.

    Executable file

    A Linux file is executable if it contains instructions that can be directly interpreted by the operating system. Basically, an exectuable file is a ready-to-run program. They’re also referred to as binaries or executables.

    Script is a particular kind of executable. shell scripting, or more specifically Bash scripting, which is writing scripts in Bash (born-again shell), a very popular shell scripting language. A shell script is a plain text file that can be interpreted by a shell.

    Formally speaking, for a text file to be considered an executable shell script for a given user, it needs to have two things:

    1. Execute permissions set for that user
    2. A directive, called a “shebang”, in its first line to declare itself to the operating system as a binary