Linux File Permissions & User Management for DevOps & Cloud

Command	Description	Why We Use It?	Best Practice
`ls`	Lists files and directories.	To see available files in a directory.	Use `ls -lah` for human-readable sizes and hidden files.
`ls -l`	Detailed file listing (permissions, owner, size, date).	To check file details and permissions.	Combine with `grep` to filter specific files (e.g., `ls -l
`ls -ltr`	Lists files sorted by modification time (oldest first).	To find recently modified files quickly.	Use `ls -ltr
`cd`	Changes the current directory.	To navigate through directories.	Use `cd -` to go back to the previous directory.
`pwd`	Prints the current working directory.	To confirm which directory you’re in.	Use `echo $PWD` in scripts for debugging paths.
`mkdir`	Creates a new directory.	To organize files into folders.	Use `mkdir -p parent/child` to create nested directories in one command.
`rm`	Deletes files.	To remove unwanted files.	Always use `rm -i` for confirmation before deletion.
`rmdir`	Deletes an empty directory.	To remove unused directories.	Use `rmdir -p parent/child` to remove parent directories if empty.
`cat`	Displays file content.	To quickly view a file’s content.	Use `less` for large files instead of `cat` to avoid scrolling issues.
`zcat`	Displays the content of a compressed file.	To read `.gz` files without extracting them.	Combine with `grep` (`zcat file.gz
`touch`	Creates an empty file or updates a file’s timestamp.	To create new files for scripts or logs.	Use `touch -d 'yesterday' file.txt` to create files with a past timestamp.
`head`	Displays the first 10 lines of a file.	To preview the start of a file.	Use `head -n 20 file.txt` to customize the number of lines displayed.
`tail`	Displays the last 10 lines of a file.	To see recent logs or file endings.	Combine with `grep` (`tail -n 50 file.log
`tail -f`	Continuously monitors a file’s changes in real time.	To track live logs (e.g., server logs).	Use `tail -f file.log
`less, more`	Opens large files for scrolling and viewing.	To read large files efficiently.	Use `less +F file.log` to view a file in real-time mode like `tail -f`.
`cp`	Copies files or directories.	To duplicate files or backup data.	Use `cp -r` for directories and `cp -i` to avoid overwriting files.
`mv`	Moves or renames files and directories.	To organize or rename files.	Use `mv -i` to prompt before overwriting an existing file.
`wc`	Counts lines, words, and characters in a file.	To analyze file content size.	Use `wc -l` to count lines and `wc -w` to count words separately.
`vi`	Opens a file in the `vi` text editor.	To edit files directly in the terminal.	Use `vim` instead for modern features, and learn shortcuts like `:q!` to exit without saving.
`ln (hard link, soft link)`	Creates file links (hard or symbolic).	To create shortcuts or reference files.	Use `ln -s target_file link_name` for soft links to prevent file duplication.
`cut`	Extracts specific sections from a file.	To process and filter data.	Use `cut -d',' -f2 file.csv` to extract the second column of a CSV file.
`tee`	Writes command output to a file while displaying it.	To save and view results simultaneously.	Use `tee -a` to append instead of overwriting an existing file.
`sort`	Sorts file content alphabetically or numerically.	To arrange and organize data.	Use `sort -u` to remove duplicates while sorting.
`clear`	Clears the terminal screen.	To remove clutter from the console.	Use `Ctrl + L` as a shortcut to clear the terminal screen.
`diff`	Compares differences between two files.	To track changes or debug scripts.	Use `diff -u file1.txt file2.txt` for a clearer side-by-side comparison.

1. ls – List Directory Contents

The ls command is used to list files and directories.

Usage:

ls                  # List all files in the current directory
ls -l               # List with detailed information (permissions, size, owner, etc.)
ls -a               # Show hidden files (files starting with `.`)
ls -lh              # List with human-readable file sizes
ls -lt              # List files sorted by modification time
ls -ltr        # List in reverse order of modification time

Example:

mkdir test_folder   # Create a test folder
cd test_folder      # Navigate inside the folder
touch file1.txt file2.txt  # Create two files
ls -l              # Check file details

2. cd – Change Directory

The cd command allows navigation between directories.

Usage:

cd /home/user      # Change to the specified directory
cd Documents       # Change to Documents folder (relative path)
cd ..              # Move up one directory level
cd ../..           # Move up two levels
cd ~               # Change to home directory
cd -               # Switch to the last visited directory

Example:

cd ~/Downloads    # Go to the Downloads directory
cd -              # Switch back to the last directory

3. cat – Display File Contents

The cat command is used to view, create, or concatenate files.

Usage:

cat filename.txt     # Display file contents
cat file1.txt file2.txt  # Concatenate and display multiple files
cat > newfile.txt    # Create a new file and add text (Ctrl+D to save)
cat >> existing.txt  # Append text to an existing file

Example:

echo "Hello World" > sample.txt   # Create a file with content
cat sample.txt   # Display the content

4. cp – Copy Files and Directories

The cp command is used to copy files or directories.

Usage:

cp file1.txt file2.txt  # Copy file1.txt to file2.txt
cp file.txt /home/user/  # Copy file to a specific directory
cp -r folder1 folder2    # Copy directories recursively

Example:

mkdir backup            # Create a backup directory
cp sample.txt backup/   # Copy file into backup
ls backup/              # Verify the copy

5. mv – Move/Rename Files and Directories

The mv command moves or renames files.

Usage:

mv oldname.txt newname.txt  # Rename a file
mv file.txt /home/user/     # Move file to another directory
mv folder1 folder2/         # Move folder1 inside folder2

Example:

mv sample.txt document.txt  # Rename sample.txt to document.txt
mv document.txt backup/     # Move document.txt to the backup folder
ls backup/                  # Verify the move

6. rm – Remove Files and Directories

The rm command deletes files or directories.

Usage:

rm file.txt        # Delete a file
rm -r folder/      # Delete a directory and its contents
rm -i file.txt     # Prompt before deleting
rm -rf /directory  # Force delete directory (use with caution)

Example:

rm document.txt   # Delete a file
rm -r backup/     # Delete a directory with its contents

7. chmod – Change File Permissions

The chmod command modifies file permissions.

Linux File Permissions Table

Permission Type	Symbol	Octal Value	Description
Read	`r`	`4`	Allows viewing file content or listing directory contents
Write	`w`	`2`	Allows modifying or deleting file content
Execute	`x`	`1`	Allows executing a script or accessing a directory
No Permission	`-`	`0`	No access

File Permission Representation

Owner (User)	Group	Others	Symbolic Representation	Octal Representation
Read, Write, Execute	Read, Write	Read	`rwxrw-r--`	`764`
Read, Write	Read	Read	`rw-r--r--`	`644`
Read, Execute	Read, Execute	Read	`r-xr-xr--`	`554`
Read, Write, Execute	Read, Execute	Execute	`rwxr-x--x`	`751`
Read, Execute	Read	No Permission	`r-xr-- ---`	`540`

Commands to Manage File Permissions

Command	Description	Example
`ls -l`	Lists files with permissions	`ls -l myfile.txt`
`chmod 755 filename`	Sets rwx for owner, rx for group and others	`chmod 755 script.sh`
`chmod u+x filename`	Adds execute permission for owner (user)	`chmod u+x script.sh`
`chmod g-w filename`	Removes write permission for group	`chmod g-w document.txt`
`chmod o+r filename`	Adds read permission for others	`chmod o+r report.txt`
`chmod 644 filename`	Sets rw for owner, r for group and others	`chmod 644 file.txt`
`chmod -R 700 dirname`	Sets full access for owner, no access for others recursively	`chmod -R 700 myfolder/`
`chown user:group filename`	Changes file ownership to a user and group	`chown devops:developers project.txt`
`chown -R user:group directory`	Recursively changes ownership of a directory	`chown -R cloudadmin:devops data/`

Special File Permissions (chmod)

Special Permission	Symbol	Octal	Description
Set User ID (SUID)	`s`	`4`	Allows execution as the file owner
Set Group ID (SGID)	`s`	`2`	Newly created files inherit the group
Sticky Bit	`t`	`1`	Only the file owner can delete files

Commands for Special Permissions

Command	Description	Example
`chmod u+s filename`	Adds SUID permission	`chmod u+s script.sh`
`chmod g+s directory`	Adds SGID permission	`chmod g+s shared_folder/`
`chmod +t directory`	Adds Sticky Bit (prevents others from deleting files)	`chmod +t /var/tmp/`

Verify Permissions

To check file permissions:

ls -l filename

Example Output:

-rwxr-xr-- 1 user group 1234 Jan 1 12:34 script.sh

Owner (user): rwx (read, write, execute)
Group (group): r-x (read, execute)
Others: r-- (read)

Usage:

chmod 755 file.sh   # Give execute permission to owner, read & execute to others
chmod u+x file.sh   # Add execute permission for the owner
chmod g-w file.txt  # Remove write permission from group
chmod o+r file.txt  # Give read permission to others

Example:

touch script.sh       # Create a script file
chmod +x script.sh    # Make it executable
ls -l script.sh       # Check permissions

8. chown – Change File Ownership

The chown command changes file or directory ownership.

Usage:

chown user:group file.txt   # Change owner and group
chown user file.txt         # Change only the owner
chown -R user:group folder/ # Change ownership recursively

Example:

sudo chown username file.txt  # Change ownership to the current user
ls -l file.txt                # Verify ownership change

8. pwd – Print Working Directory

Displays the current directory path.

Usage:

pwd

Output:

/home/user/Documents

9. mkdir – Create Directories

Used to create new directories.

Usage

mkdir project    # Create a single directory
mkdir -p work/code  # Create nested directories

10. rmdir – Remove Empty Directories

Used to delete empty directories.

Usage

rmdir empty_folder

11. zcat – View Compressed Files

Used to display compressed file content without unzipping.

Usage

zcat file.gz

12. touch – Create Empty Files

Creates an empty file or updates the timestamp of an existing file.

Usage

touch newfile.txt

13. head – View First 10 Lines of a File

Displays the first lines of a file.

Usage

head file.txt
head -20 file.txt  # Display first 20 lines

14. tail – View Last 10 Lines of a File

Displays the last lines of a file.

Usage

tail file.txt
tail -n 20 file.txt  # Show last 20 lines

15. tail -f – Monitor Log Files in Real-Time

Continuously displays new lines added to a file (useful for logs).

Usage

tail -f /var/log/syslog

16. less & more – View Large Files

Used for navigating large files efficiently.

Usage

less file.txt
more file.txt

less allows backward and forward scrolling.
more only moves forward.

17. wc – Word Count in a File

Counts lines, words, and characters in a file.

Usage

wc file.txt        # Show line, word, and character count
wc -l file.txt     # Show only line count
wc -w file.txt     # Show only word count

18. vi – Text Editor

Opens a file in the vi text editor.

Usage

vi file.txt

Press i to insert text.
Press ESC, type :wq to save and exit.

19. cut – Extract Specific Columns from a File

Used to extract fields from a text file.

Usage

cut -d',' -f1,3 file.csv   # Extract 1st and 3rd columns

20. tee – Save and Display Output

Redirects output to a file and displays it.

Usage

ls -l | tee output.txt

21. sort – Sort File Contents

Sorts file contents alphabetically or numerically.

Usage

sort file.txt
sort -r file.txt  # Reverse sort

22. clear – Clear Terminal Screen

Clears the terminal window.

Usage

clear

23. diff – Compare Two Files

Compares differences between two files.

Usage

diff file1.txt file2.txt

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