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understand and use essential tool

Command Line Basics​

Login methods

  • Local text-mode console: Log in directly on the computer using a text-based interface
  • Local graphical-mode console: Log in directly on the computer using a graphical interface, like a desktop
  • Remote text-mode login: Log in to a remote computer over a network using a text-based interface (e.g., via SSH or Telnet)
  • Remote graphical-mode login: Log in to a remote computer with a graphical desktop

Old terms

  • Console: The physical or main interface for interacting with a computer
  • Virtual terminal: A software-based text interface, like a command-line window
  • Terminal emulator: A program that mimics a text-based console on your screen

SSH

  • Computer (SSH Client) → Server (SSH daemon)
ssh [email protected]
---
[email protected]'s password:

Logging in and Switching Users​

Security features

  • Access controls: Defines who access resources and what they can do
  • PAM (Pluggable Authentication Modules): Handles authentication, authorization, and session management
  • Network security: Protects systems from network-based threats
  • SSH hardening: Secures the secure shell service for remote access
  • SELinux: Enforces stricter security policies
  • Others

Account types

Account TypePurposeExamplesAttributesTypical UID Range
User AccountStandard accounts for interactive login by humansbob, michael, daveUsername, Password (stored securely), User ID (UID), Group ID (GID), Home Directory, Login ShellOften starts at 1000 (>= 1000)
Superuser AccountFull administrative access to the systemrootUsername: root, UID: always 0UID is 0
System AccountUsed by system services or daemons running in the backgroundsshd, mail, daemonUsername, User ID (UID), Group ID (GID), Home Directory, typically no login capabilitiesReserved range, often below 1000
Service AccountDedicated accounts for specific applications or services to run undernginx, postgresUsername, User ID (UID), Group ID (GID), Home Directory, service-specific permissionsOften within system account range or user range

Groups

  • A collection of users, primarily used to manage permissions efficiently. Users can belong to multiple groups

Essential commands

  • User
# Display user account database
cat /etc/passwd
---
root:*:0:0:System Administrator:/var/root:/bin/sh
daemon:*:1:1:System Services:/var/root:/usr/bin/false

# Display user and group identity
id kyphan
---
uid=1001(kyphan) gid=1001(kyphan) groups=1001(kyphan),27(sudo)

# Display currently logged-in users
who
---
kyphan   tty1         2025-04-16 09:30
kyphan   pts/0        2025-04-16 10:15 (192.168.1.10)

# Display history of last logged-in users
last
---
kyphan   pts/0        192.168.1.10     Wed Apr 16 10:15   still logged in
kyphan   tty1                          Wed Apr 16 09:30   still logged in
reboot   system boot  5.15.0-generic   Wed Apr 16 09:25   - 10:30  (01:05)

# Search for information and home directory
grep -i kyphan /etc/passwd
---
kyphan:x:1001:1001::/home/kyphan:/bin/bash
  • Group
# Display group database
cat /etc/group
---
root:x:0:
sudo:x:27:kyphan
kyphan:x:1001:

Switch users

  • su
    • Switches to another user account. If no user is specified, it defaults to root
    • Requires the target user's password with su -
    • Directly using su to become root for extended periods or sharing the root password is often discouraged for security reasons
# Switch to the root user
su -
---
Password:

# Execute a single command as root
su -c "whoami"
---
Password:
root
  • sudo
    • Allows permitted users to execute specific commands as another user (typically root) without needing the target user's password
    • It requires the user's own password for authentication
    • Provides better accountability and allows granting specific, limited administrative privileges instead of full root access
# Execute a command with root privileges
sudo apt-get update
---
[sudo] password for kyphan:

# Display the sudoers configuration file
cat /etc/sudoers
---
root    ALL=(ALL:ALL) ALL
%admin ALL=(ALL) ALL
%sudo   ALL=(ALL:ALL) ALL
  • no-login shell
    • Assigns a shell like /sbin/login, /usr/sbin/nologin, or /bin/false to an account (often system accounts or even root on hardened systems) prevents users from logging in directly using that account
    • Enhances security by forcing administrative tasks through mechanisms like sudo or requiring a switch from a standard user account
# Check the login shell for the root user
grep '^root:' /etc/passwd
---
root:x:0:0:root:/root:/bin/bash

Using System Documentation​

Commands

  • --help: Displays a brief summary of their usage and options
ls --help

# Pager
journalctl --help
  • man: Displays a comprehensive manual pages
man journalctl
man man
man 1 printf
man 3 printf
  • apropos: Searches the names and short descriptions of all manual pages for a specific keyword
apropos director
apropos -s 1,8 director

# Update the manual page index database
sudo mandb

Additional System Documentation​

  • info: Accesses documentation in the GNU Info format
info bash
  • /usr/share/doc: Contains supplementary documentation provided by installed packages
cd /usr/share/doc/bash

less INTRO

grep "command" INTRO

Working With Files and Directories​

List files and directories

  • ls: Lists the contents of a directory
ls
ls -a
ls -l /var/log
ls -lah

Filesystem tree

  • Current working directory
  • Relative path
  • Absolute path
cd /var/log
cd -
cd ..

Create files

  • touch: Creates an empty file if it doesn't exist, or updates the access and modification timestamps of an existing file
touch /home/kyphan/text.txt

Create directories

  • mkdir
mkdir -p /home/kyphan/folder

Copy files and directories

  • cp: Copies files or entire directory structures
cp hello.text folder/
cp hello.txt folder/hello_copy.txt

# If hello does not exist, it copies only the contents, if not, it copies the entire folder/ to be hello/folder/
cp -r folder/ hello/

Move files

  • mv: Moves files/directories to a different location or renames them
mv abc.txt hello/
mv abc.txt abcd.txt
mv folder/ new_folder/

Delete files and directories

  • mv: Removes files and directories
rm abc.txt

rm -r folder

Inodes

  • An inode (index node) is a data structure on Unix-like filesystems
  • Each file and directory on the filesystem has a unique inode
  • It stores metadata about the file or directory, excluding the filename and the actual file content
  • Key metadata includes file type, permissions, owner, file size, timestamps, link count, pointers, etc.
# Create the file and write text into it
echo "Picture of Milo the dog" > Pictures/family_dog.jpg

# Display detailed status information
stat Pictures/family_dog.jpg
---
File: Pictures/family_dog.jpg
Size: 49 Blocks: 8 IO Block: 4096 regular file
Device: fd00h/64768d Inode: 52946177 Links: 1
Access: (0640/-rw-r-----) Uid: ( 1000/ aaron) Gid: ( 1005/ family)
...
  • How it works
    • When accessing a path like Pictures/family_dog.jpg, the system locates the Pictures directory
    • It searches the directory's contents for the entry family_dog.jpg
    • This entry contains the corresponding inode number
    • The system uses the inode number to find the inode structure itself
    • From the inode, it reads metadata (like permissions) and gets the pointers to the data blocks to retrieve the file's content
  • Relationship
    • A directory entry links a filename to an inode number
    • The inode stores metadata and points to the data blocks

img

Hard links

  • A hard link is simply another directory entry (another filename, potentially in a different directory) that points to the exact same inode as the original file
  • Benefit
    • Allows multiple filenames to refer to the same file data without duplicating the data itself, thus saving storage space
    • Instead of having two separate copies of a large file, you have two names pointing to one set of data blocks via the shared inode
# ln path_to_target_file path_to_link_file

# Recursively copy the contents of Aaron's Pictures directory to Jane's
cp –r /home/aaron/Pictures/ /home/jane/Pictures/

# Create a hard link: Jane's path now points to the same inode as Aaron's path
ln /home/aaron/Pictures/family_dog.jpg /home/jane/Pictures/family_dog.jpg

# Display detailed status information
stat Pictures/family_dog.jpg
---
File: Pictures/family_dog.jpg
Size: 49 Blocks: 8 IO Block: 4096 regular file
Device: fd00h/64768d Inode: 52946177 Links: 2
Access: (0640/-rw-r-----) Uid: ( 1000/ aaron) Gid: ( 1005/ family)
...

# The link count for inode 52946177 decreases from 2 to 1
rm /home/aaron/Pictures/family_dog.jpg

# The link count for inode 52946177 decreases from 1 to 0
rm /home/jane/Pictures/family_dog.jpg
  • Data deletion
    • The actual file data (disk blocks) associated with an inode is only marked for deletion (and eventually overwritten) when its link count reaches zero
    • Removing a hard link (rm) just deletes that specific directory entry and decrements the inode's link count

Limitiations and considerations

  • Only hardlink to files, not folders
  • Only hardlink to files on the same filesystem
# Add existing user 'aaron' to the supplementary group 'family'
useradd -a -G family aaron

# Add existing user 'jane' to the supplementary group 'family'
useradd -a -G family jane

# Change file permissions: owner=read/write, group=read/write, others=none
chmod 660 /home/aaron/Pictures/family_dog.jpg

Soft (symbolic) links

  • Unlike a hard link which points directly to an inode, a symbolic link's content is simply the pathname (text string) of another file or directory (the target)
  • It is used to create convenient access points, redirect paths, or link across different filesystems or partitions

img

# ln -s path_to_target_file path_to_link_file

# Create a symbolic link named 'family_dog_shortcut.jpg' and points to the absolute path '/home/aaron/Pictures/family_dog.jpg'
ln –s /home/aaron/Pictures/family_dog.jpg family_dog_shortcut.jpg

ls -l
---
lrwxrwxrwx. 1 aaron aaron family_dog_shortcut.jpg -> /home/aaron/Pictures..

readlink family_dog_shortcut.jpg
---
/home/aaron/Pictures/family_dog.jpg

echo "Test" >> fstab_shortcut
---
bash: fstab_shortcut: Permission denied

ls -l
---
lrwxrwxrwx. 1 aaron aaron family_dog_shortcut.jpg -> /home/aaron/Pictures..

ln –s Pictures/family_dog.jpg relative_picture_shortcut
  • If the target file or directory is deleted, moved, or renamed, the symbolic link is not automatically updated or removed
  • It continues to point to the original path, which now leads nowhere. Such links are called "broken" or "dangling" links - "No such file or directory" error

Limitiations and considerations

  • Softlink to files and folders
  • Softlink to files on different filesystem as well

List, Set, and Change Standard Ugo/RWX Permissions -- Continue​

Lab​

Logging in and System Documentation​

man ssh | grep version
ssh -V

apropos hostname
hostnamectl

mandb

man ssh | grep verbose
ssh -v alex@localhost

ls -la /home/bob/data

ssh bob@dev-host01
touch myfile

apropos ssh
sudo mandb

apropos "NFS mounts"
touch nfsmount.conf
mkdir -p /home/bob/lfcs

touch /home/bob/lfcs/lfcs.txt

cp -r /tmp/Invoice/ /home/bob/

cp -a /home/bob/myfile.txt /home/bob/data/

cp -r /home/bob/lfcs /home/bob/old-data

rm /home/bob/lfcs/lfcs.txt

mv /home/bob/lfcs/ /home/bob/new-data/

rm -rf /home/bob/lfcs

ln -s /tmp /home/bob/link_to_tmp

ln /opt/hlink /home/bob/hlink

mv /home/bob/new_file /home/bob/old_file

mkdir -p /tmp/1/2/3/4/5/6/7/8/9

ls --full-time
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