Intro to linux basics

My study resource is Introduction to Linux from Devops Daily

shell basics is possible using bash terminal on macos but for the other sections i used multipass ubuntu

Shell Basics

Bash Script are commonly used but zsh are also popular and the defualt for macOS

command [opt] [args]

most linux commands have this structure, options (flags) like what used - or -- and its modify how the run, args thats like the text you need to print or the file path, etc

Navigating the File System

pwd to show your current location in the file system

pwd

output:

/Users/mas

ls to list the file and dircetorities

ls

there’re common options like -l that gave you a long format with details, -a show hidden files, -h appers the file sizes with SI prefixes (KB, MB, GB)

ls -lah

you can combine the options flags

cd to change directory

cd /path/to/directory

there’re useful shortcuts like cd ~ to go to home dir, cd - go to previous dir, cd .. Go up one directory, cd goes to home dir

File Management

Create a directory with mkdir

mkdir my_directory

you can add -p flag and create a parent/child directory

Create an empty file/files with touch

touch file1.txt file2.txt

Copy with cp

cp source.txt destination.txt       # Copy a file
cp -r source_dir destination_dir    # Copy a directory recursively

mv used for moving and renaming file/dir

mv old_name.txt new_name.txt       # Rename a file
mv file.txt /path/to/directory/    # Move a file
mv dir1 dir2                       # rename(move) a directory

Remove files with rm

rm file.txt        # Remove a file
rm -r directory    # Remove a directory and its contents
rm -i file.txt     # Ask for confirmation before removing
rm -rf file.txt    # Remove without asking for confirmation

if the dir is empty you can use rmdir to remove it

Viewing File Content

Show file/files contents with cat

cat file1.txt file2.txt 

use less for large files which allows scrolling

less large_file.txt

Navigation in less:

space For Next Page,

b For Previous page,

q to quit,

/[keyword] For search,

n For next search result

i think that nano a lot easier

show first 10 lines from the file using head

head file.txt
head -n 20 file.txt    # Show first 20 lines

Show the last 10 lines with tail:

tail file.txt
tail -n 20 file.txt    # Show last 20 lines
tail -f log_file.txt    # Follow the file (show updates in real-time)

The -f option is extremely useful for monitoring log files

Finding Files and Content

find used to search for files/directories in the directories

find /path/to -name "filename"

add -type d flag to search for directories insted of files

grep to search for patterns inside files

grep "search_term" file.txt

add . insted of file name to search inside all files on the current directory

Working with Text Files

Edit text files with nano

nano file.txt

Save command output to a file with > (overwrites) or >> (appends)

echo "Add this line" >> notes.txt

Connect commands with | to use one command’s output as another’s input:

ls -la | grep "Dec"    # List files modified in December
cat file.txt | sort    # Sort the contents of file.txt
cat file.txt | grep "pattern" | wc -l    # Count lines containing "pattern"

Command Execution Control

running multiple commands sequentially using ;

mkdir new_dir; cd new_dir; touch file.txt

or run second command when the first one succeds using &&

mkdir new_dir && cd new_dir

or run second command when the first one fails using ||

ping -c 1 google.com || echo "Internet connection failed"

run command in background using &

command &

View previously used commands with history

history
history | grep "find"    # Search for previous find commands

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The Filesystem Hierarchy Standard (FHS)

the photo visualize the root directory and the other directories with understandable explaination and style (Generated Using Grok)

Important subdirectories and files

/                          root of the whole tree
├── bin/                   essential user binaries (bash, ls, cp, mv, rm)
├── sbin/                  system admin binaries (usually root)
├── lib/                   libraries those /bin and /sbin programs need
├── etc/                   system-wide config
│   ├── passwd             user accounts
│   ├── group              groups
│   ├── fstab              mounts
│   ├── hosts              name → IP
│   ├── ssh/               SSH config
│   └── sudoers            sudo rules
├── home/                  one dir per normal user  (~ = yours)
├── root/                  root user's home (not under /home)
├── usr/                   most programs, libs, docs  ("Unix System Resources")
│   ├── bin/               user commands
│   ├── sbin/              admin commands
│   ├── lib/               libraries
│   ├── share/             arch-independent data
│   ├── local/             software you install locally
│   └── include/           C headers
├── opt/                   add-on / commercial packages
├── var/                   data that changes while the system runs
│   ├── log/               logs
│   ├── spool/             print and mail queues
│   ├── cache/             caches
│   └── lib/               variable state
├── tmp/                   world-writable temp files (often wiped on reboot)
├── dev/                   device files (sda, tty, null, zero, random)
├── proc/                  virtual: processes and kernel status (cpuinfo, meminfo)
├── sys/                   virtual: devices, drivers, kernel objects
├── media/                 removable media (USB, CD)
└── mnt/                   temporary mounts

File Permissions and Ownership

when you list files and their permissions using ls -l you see some weird things liks this:

-rwxrw-r-- 1 user group 4096 Oct 1 12:34 file.txt

The permissions are represented by the first 10 characters

Position 1: File type

Positions 2-4: Owner permissions

Positions 5-7: Group permissions

Positions 8-10: everyone else permissions

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Permisions are represented numerically like the example : chmod 755 file

• Owner: 7 (rwx)
• Group: 5 (r-x)
• Others: 5 (r-x)

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Permisions characters and numbers:

read -> r (4)
write -> w (2)
execute -> x (1)
read only -> (4)

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Changing Permissions

Use chmod to change file permissions

chmod 755 file.txt    # Set permissions using numeric mode
chmod u+x file.txt    # Add execute permission for user
chmod g-w file.txt    # Remove write permission from group
chmod o=r file.txt    # Set others permission to read-only
chmod a+x file.txt    # Add execute permission for all (user, group, others)

Changing Ownership

Use chown to change file ownership:

chown user file.txt    # Change the owner
chown user:group file.txt    # Change both owner and group
chown :group file.txt    # Change only the group

Use chgrp to change just the group:

chgrp group file.txt

For recursive changes (affecting subdirectories and their contents) add -R option

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Special Permissions

The ten characters in ls -l are type, then owner, group, other

The special bit overwrites that class’s x:

type  owner     group     other
 -    r w s     r - x     r - x     setuid   -rwsr-xr-x
 -    r w x     r - s     r - x     setgid   -rwxr-sr-x
 d    r w x     r w x     r w t     sticky   drwxrwxrwt

File Types in Linux

the types indicated by the first character in the permissions string:

-: Regular file → contains data, text or program code

d: Directory → Special files contais list of other files

c: Character device

b: Block device

p: Named pipe

s: Socket

l: Symbolic link (symlinks):

shortcuts, pointing to another file or directory

example:

        Target File                         Link Name
ln -s /etc/nginx/sites-available/mysite /etc/nginx/sites-enabled/mysite

it looks like this when you see the permision of it:

lrwxrwxrwx 1 user group 8 Jan 1 12:34 link -> target

Hard links reate additional directory entries for the same file and share the same inode as the original file, You can’t create hard links to directories or across file systems.

Working with Hidden Files

any file starts with . consider hidden file

ls -a # show all file even hidden ones

Common hidden files and directories:

.bashrc: Bash shell configuration

.profile: Login shell configuration

.ssh/: SSH keys and configuration

.config/: Application configurations

.cache/: Cached data

Finding Files and Information

Understanding Inodes

Disk Usage and Management

File System Types

Mounting File Systems

Special File Operations