Lab 9: Linux Admin

Comprehension questions

What are the parts of a bash command?

A bash command has several key parts:

%%{init: {'theme': 'base', 'themeVariables': {'fontFamily': 'monospace'}}}%%

graph TD
    A["command<br/><code>ls</code>, <code>cd</code>, <code>echo</code>"] --> B["options/flags<br/><code>-l</code>, <code>-a</code>, <code>-r</code>"]
    B --> C["arguments<br/><code>filenames</code>, <code>paths</code>"]
    C --> D["redirects<br/><code>&gt;</code>, <code>&lt;</code>, <code>|</code>"]

    style A fill:#d2562b,stroke:#fff,stroke-width:2px,color:#fff
    style B fill:#2a6f77,stroke:#fff,stroke-width:2px,color:#fff
    style C fill:#5b8c5a,stroke:#fff,stroke-width:2px,color:#fff
    style D fill:#e8a33d,stroke:#000,stroke-width:2px,color:#000

Bash Command Structure

Part	Description	Example
Command	The program to run	ls, cd, echo, grep
Options/Flags	Modify behavior, prefixed with (single) or (double)	- or --
Arguments	What the command operates on	files, paths, text
Redirects	Send output elsewhere	> file, < input, | pipe

Example: ls -la /home | grep .txt

Command: ls | Flags: -la | Argument: /home | Pipe: | | Chain: grep .txt

What is the difference between a relative and an absolute path?

The main difference is that absolute paths always start with / (and works from anywhere), while relative paths do not start with / and are interpreted from the current working directory (you can use .. to go up one level, . for current directory).

%%{init: {'theme': 'base', 'themeVariables': {'fontFamily': 'monospace'}}}%%

graph LR
    Path["Filesystem<br>Paths"]

    Absolute["<strong>Absolute Path</strong><br/>Starts from root<br>directory /"]
    AbEx1["<code>/home/user/documents/file.txt</code>"]
    AbEx2["<code>/etc/passwd</code>"]
    AbDescr["Always same location<br>regardless of current<br>directory"]

    Relative["<strong>Relative Path</strong><br/>Relative to current<br>directory"]
    RelEx1["<code>documents/file.txt</code>"]
    RelEx2["<code>../parent-folder/file.txt</code>"]
    RelDescr["Changes meaning based<br>on where you are<br>in filesystem"]

    Path -->|"Full from root"| Absolute
    Absolute --> AbEx1
    Absolute --> AbEx2
    Absolute --> AbDescr

    Path -->|"From current location"| Relative
    Relative --> RelEx1
    Relative --> RelEx2
    Relative --> RelDescr

    style Path fill:#e8a33d,stroke:#000,stroke-width:2px,color:#000
    style Absolute fill:#d2562b,stroke:#fff,stroke-width:2px,color:#fff
    style Relative fill:#2a6f77,stroke:#fff,stroke-width:2px,color:#fff
    

Absolute vs Relative Paths

What are some ways to direct them to run in a particular place on the filesystem?

You can run use relative/absolute paths to:

1. Change to a directory, then run the commands: cd /path/to/directory
2. Run the script with the absolute path directly: /absolute/path/to/file.sh
3. Run the script relative to current location: relative/path/file.sh
4. Run the script in the current directory (explicit): ./file.sh

%%{init: {'theme': 'base', 'themeVariables': {'fontFamily': 'monospace'}}}%%

graph LR
    Control["Ways to Control<br>Execution Location"]

    CD["Use <code>cd</code>to change<br>directory first"]
    ABS["Use the absolute path<br/>(specify full path)"]
    REL["Use the relative path<br/>(but specify from<br>current location)"]
    Dot["Run from current directory with <code>./file.sh</code>"]

    Control --> CD
    Control --> ABS
    Control --> REL
    Control --> Dot

    style Control fill:#d2562b,stroke:#fff,stroke-width:2px,color:#fff
    style CD fill:#2a6f77,stroke:#fff,stroke-width:1px,color:#fff
    style ABS fill:#2a6f77,stroke:#fff,stroke-width:1px,color:#fff
    style REL fill:#2a6f77,stroke:#fff,stroke-width:1px,color:#fff
    style Dot fill:#5b8c5a,stroke:#fff,stroke-width:1px,color:#fff

Ways to Control Execution Location

How can you copy, move, or delete a file? What about to or from a server?

Local operations include cp, mv and rm:

Command	Description
cp /source/destination	copy a file
cp -r source_dir destination_dir	copy a directory recursively
mv source destination	move or rename a file
rm filename	delete a file (permanent, no recovery)
rm -r directory	delete a directory and it’s contents

Server operations (via SSH) include scp, rsync, and sftp

Command	Description
scp local_file user@server:/remote/path	copy to server
scp user@server:/remote/file local_path	copy from server
scp -r local_dir user@server:/remote/path	copy directory to server
rsync -av local_dir user@server:/remote/path	sync files (better for large operations)
sftp user@server	interactive file transfer session

Key note: rm is permanent! There is no trash can in Linux. Use the -i flag to prompt before deletion: rm -i filename

Mind map of: operating system, Windows, MacOS, Unix, Linux, Distro, Ubuntu

%%{init: {'theme': 'base', 'themeVariables': {'fontFamily': 'monospace'}}}%%

mindmap
  root((Operating System))
    Commercial
      Windows
        GUI-focused
        Proprietary<br>kernel
      MacOS
        Unix-based
        Proprietary<br>kernel
        Desktop &<br>laptop
    Unix family
      Unix
        Original<br>design
        POSIX<br>standard
        BSD, Solaris
      Linux
        Free & open<br>source
        POSIX<br>compatible
        Kernel +<br>utilities
        Distros
          Ubuntu
            Debian-based
            User-friendly
            Server &<br>desktop
          CentOS
            RedHat-based
            Enterprise
          Fedora
            Cutting<br>edge
            RedHat-based
        Distro
          Linux kernel +<br>tools
          Different package<br>managers
          Different<br>defaults
          Community<br>focused

Operating System Family Tree

Key relationships between operating systems:

The Operating System is the software that manages hardware and runs programs. Windows and MacOS are commercial proprietary operating systems. Unix is the original multiuser system, developed at AT&T Bell Labs in the 1970s; it began as proprietary software, and its design was later standardized as POSIX.

Linux is a free, open-source Unix-like kernel created by Linus Torvalds, and the distro (Distribution) is the Linux kernel packaged with utilities, package manager, defaults. Ubuntu is a popular Linux distro based on Debian.

What are the 3x3 options for Linux file permissions? How are they indicated in ls -l?

The 3×3 permission matrix:

Owner (User)	Group	Other
Read (r) = 4	Read (r) = 4	Read (r) = 4
Write (w) = 2	Write (w) = 2	Write (w) = 2
Execute (x) = 1	Execute (x) = 1	Execute (x) = 1

How they appear in ls -l:

-rw-r--r-- 1 user group 1234 Jan 15 10:30 file.txt
^         ^^^^^^ ^^^^^^ ^^^^^^
|         |      |      |
|         |      |      +-- Other: read only
|         |      +--------- Group: read only
|         +-------------- User: read & write
+----------------------- File type (- = regular file, d = directory)

Breaking down permissions:

• Position 1: File type (- file, d directory, l link)
• Positions 2-4: Owner permissions (rwx)
• Positions 5-7: Group permissions (rwx)
• Positions 8-10: Other permissions (rwx)

Octal notation (chmod):

• chmod 755 file: Owner: rwx (7), Group: rx (5), Other: rx (5)
• chmod 644 file: Owner: rw (6), Group: r (4), Other: r (4)
• chmod 700 file: Owner: rwx (7), Group: (0), Other: (0)
• chmod 600 file: Owner: rw (6), Group: (0), Other: (0): typical for SSH keys