linux.md

Linux sheet

File System hierarchy

Directory	Purpose	Example Contents
/bin (/usr/bin)	Essential user binaries (programs for all users)	ls, cp, mv, bash
/sbin (/usr/sbin)	Essential system binaries (mainly for root)	shutdown, ifconfig, mkfs
/etc	System configuration files (plain text)	passwd, fstab, hosts, ssh/
/opt	Optional / third-party software	google/, skype/
/usr	User-installed software, libraries, docs	bin/, lib/, share/
/tmp	Temporary files (cleared on reboot)	Session data, installer caches
/var	Variable data that changes often	log/, mail/, cache/, spool/
/home	User home directories	/home/alice, /home/bob
/root	Root user’s home directory	Configs, scripts
/boot	Boot loader files, kernel, initrd	vmlinuz, grub/
/dev	Device files (interfaces to hardware)	sda, null, tty
/proc	Virtual FS: kernel and process info	cpuinfo, meminfo, /proc/<pid>/
/sys	Virtual FS: kernel ↔ device interaction	Driver/module info
/lib	Shared libraries for essential binaries	libc.so, kernel modules
/media	Mount point for removable devices	/media/usb, /media/cdrom
/mnt	Temporary mount point (manual use)	/mnt/mydisk
/srv	Data for services (www, ftp, etc.)	/srv/ftp, /srv/www
/run	Runtime variable data (since boot)	PID files, sockets

Linux User Management Commands

Action	Command	Notes
Create a new user	sudo adduser <username>	Interactive, sets password & home dir.
Create user (minimal)	sudo useradd <username>	Creates user, but no password/home by default.
Give user root privileges	sudo usermod -aG sudo <username>	Adds user to the sudo group.
Delete a user (keep home dir)	sudo deluser <username>	Removes user but keeps files.
Delete a user (remove home dir too)	sudo deluser --remove-home <username>	Deletes user & home directory.
Change a user’s password	sudo passwd <username>	Prompts for new password.
Show all system users	cat /etc/passwd	Displays all users (including system users).
Show only real users	cut -d: -f1 /etc/passwd	Extracts just usernames.
Check currently logged-in users	who or w	Shows active sessions.
Switch to another user	su - <username>	Starts a shell as another user.
Switch to root user	sudo -i or su -	Opens a root shell.

Linux File Navigation & Directory Commands

Action	Command	Notes
Show current working directory	pwd	Displays absolute path of where you are.
List files in directory	ls	By default, hides dotfiles.
List all files (including hidden)	ls -a	Shows files starting with .
Long listing (permissions, owner, size, date)	ls -l	Human-readable details.
Long listing + hidden files	ls -la	Most commonly used.
Human-readable file sizes	ls -lh	Size in KB/MB/GB.
Sort by modification time	ls -lt	Newest first.
Reverse order	ls -lr	Combine with others (e.g. ls -ltr).
Recursive listing	ls -R	Lists subdirectories too.
Go to another directory	cd <dir>	Use cd alone to go to home.
Go up one directory	cd ..	Moves one level up.
Go back to previous directory	cd -	Switches between last two dirs.
Home directory shortcut	cd ~	Jumps to your home dir.
Create a directory	mkdir <dir>	Add -p to create nested dirs.
Remove an empty directory	rmdir <dir>	Only works if dir is empty.
View disk usage of directory	du -sh <dir>	Human-readable size.
Show disk usage of filesystem	df -h	Mounted partitions usage.

ls Options Reference

Option	Meaning	Example
ls	List files (excluding hidden)	ls
-a	Show all (including hidden . files)	ls -a
-l	Long listing format (permissions, ownership, size, time)	ls -l
-h	Human-readable sizes (with -l)	ls -lh
-r	Reverse order of sorting	ls -lr
-t	Sort by modification time (newest first)	ls -lt
-S	Sort by file size	ls -lS
-X	Sort alphabetically by extension	ls -lX
-R	Recursive list (include subdirs)	ls -R
-d */	Show only directories	ls -d */
-1	One file per line	ls -1
-i	Show inode numbers	ls -i
-F	Append / to dirs, * to executables	ls -F
-G / --color	Enable colors for file types (if supported)	ls --color=auto

File Search in Linux

Action	Command	Notes
Find by filename	find /path -name "file.txt"	Case-sensitive search.
Find by filename (case-insensitive)	find /path -iname "file.txt"	Ignores case.
Find by extension	find /path -name "*.log"	Matches all .log files.
Find by multiple patterns	find /path -name "*.jpg" -o -name "*.png"	OR condition.
Find only directories	find /path -type d -name "config"	Use -type d.
Find only files	find /path -type f -name "*.sh"	Use -type f.
Find by size	find /path -size +100M	Files larger than 100MB.
Find by modified time (last 7 days)	find /path -mtime -7	-mtime +7 = older than 7 days.
Find by access time (last 1 day)	find /path -atime -1	Time since last access.
Find empty files/dirs	find /path -empty	Useful for cleanup.
Execute command on results	find /path -name "*.tmp" -exec rm {} \;	Deletes all .tmp files.
Limit depth of search	find /path -maxdepth 1 -type f	Only in top-level dir.

locate Command (fast, database-based)

Action	Command	Notes
Find file quickly	locate file.txt	Uses a prebuilt database.
Case-insensitive search	locate -i file.txt	Ignores case.
Show count of matches	locate -c file.txt	Only count results.
Limit output to first 10 results	locate -n 10 file.txt	Prevents long output.
Refresh database (needed after new files created)	sudo updatedb	Updates /var/lib/mlocate/mlocate.db.
Search for partial name	locate docu	Matches document.txt, docutils/, etc.

• Use find when you need precision and real-time results.
• Use locate when you just need a fast filename search.

Wildcard Characters in Linux

Wildcards are special characters used to match multiple files or patterns in commands like ls, cp, mv, rm, find, etc.

🔹 Basic Wildcards

Wildcard	Meaning	Example	Matches
*	Matches zero or more characters	ls *.txt	a.txt, notes.txt, report1.txt
?	Matches exactly one character	ls file?.txt	file1.txt, fileA.txt (but not file12.txt)
[ ]	Matches any one character inside brackets	ls file[123].txt	file1.txt, file2.txt, file3.txt
[! ] or [^ ]	Matches any one character NOT inside brackets	ls file[!0-9].txt	fileA.txt, fileX.txt (but not file1.txt)

🔹 Character Ranges

Wildcard	Meaning	Example	Matches
[a-z]	Any lowercase letter	ls [a-z]*	apple, banana, zebra
[A-Z]	Any uppercase letter	ls [A-Z]*	File, Test, Zoo
[0-9]	Any digit	ls file[0-9].txt	file1.txt, file9.txt
[a-zA-Z]	Any letter (upper/lower)	ls [a-zA-Z]*	dog, Cat, Zoo
[0-9a-f]	Hex characters	ls *[0-9a-f]	data1, filea, logf

🔹 Advanced Patterns

Wildcard	Meaning	Example	Matches
{ }	Brace expansion → generate multiple patterns	ls {file1,file2}.txt	file1.txt, file2.txt
{start..end}	Brace expansion with sequence	echo {1..5}	1 2 3 4 5
**	Matches files/directories recursively (if enabled with shopt -s globstar in Bash ≥ 4.0)	ls **/*.txt	Finds all .txt under current dir and subdirs

File Creation with Wildcards (via brace expansion)

Command	What it does	Example Result
touch file{1..5}.txt	Create numbered files	file1.txt file2.txt file3.txt file4.txt file5.txt
touch {a,b,c}.log	Create multiple named files	a.log b.log c.log
mkdir project_{A..C}	Create multiple directories	project_A project_B project_C
cp file.txt{,.bak}	Create a backup copy	file.txt and file.txt.bak

File Deletion with Wildcards

Command	What it does	Example
rm *.tmp	Delete all .tmp files in current dir	Removes a.tmp, b.tmp
rm file?.txt	Delete files with exactly 1 character after "file"	Removes file1.txt, fileA.txt (but not file12.txt)
rm [0-9]*.log	Delete .log files starting with a digit	Removes 1.log, 2025.log
rm -r test*	Delete all dirs/files starting with test	Removes test1/, testcase.txt

Be careful with rm + wildcards!

• Always run ls first to confirm matches:

ls *.tmp

• Use -i option for interactive deletion:

rm -i *.tmp

File Types Overview

Symbol (ls -l)	Type	Description	Example
-	Regular file	Contains data (text, binary, images, executables).	/etc/passwd, /bin/ls
d	Directory	A folder containing files and other directories.	/home/, /etc/
l	Symbolic (soft) link	Pointer to another file/directory.	/lib -> /usr/lib
c	Character device file	Device that transfers data character by character.	/dev/tty, /dev/random
b	Block device file	Device that transfers data in blocks (storage).	/dev/sda, /dev/loop0
p	Named pipe (FIFO)	Enables inter-process communication (FIFO queue).	/tmp/mypipe
s	Socket	Used for inter-process or network communication.	/run/docker.sock

🔹 Commands to Identify File Types

Command	Purpose	Example
ls -l	Shows file type in the first character of permissions	-rw-r--r-- file.txt → - = regular file
file <filename>	Displays the file’s actual type (content-based)	file a.out → ELF 64-bit executable
stat <filename>	Detailed file info including type, size, inode	stat /etc/passwd

🔹 Example ls -l

$ ls -l
-rw-r--r--  1 alice alice   1200 Sep 25 10:30 notes.txt      # Regular file
drwxr-xr-x  2 alice alice   4096 Sep 25 10:31 projects       # Directory
lrwxrwxrwx  1 alice alice     11 Sep 25 10:32 lib -> usr/lib # Symbolic link
crw-rw-rw-  1 root  tty     5, 0 Sep 25 10:33 /dev/tty       # Character device
brw-rw----  1 root  disk    8, 0 Sep 25 10:34 /dev/sda       # Block device
prw-r--r--  1 alice alice      0 Sep 25 10:35 mypipe         # Named pipe
srwxr-xr-x  1 root  root       0 Sep 25 10:36 docker.sock    # Socket

Character Device vs Block Device

Character Device (c)

• Sends/receives data one character (byte) at a time.
• Slower (byte-by-byte)
• For devices like keyboard, mouse, serial ports
• Example: /dev/tty, /dev/random

Block Device (b)

• Sends/receives data in blocks (chunks)
• Faster (block access, buffering)
• For devices like hard drives, SSDs, USBs
• Example: /dev/sda, /dev/nvme0n1

Named Pipe vs Socket

Named Pipe

• One-way (half-duplex)
• Exists as a special file in filesystem (/tmp/mypipe)
• Works between processes on the same machine

Block Device (b)

• Two-way (full-duplex)
• Exists as a file, but mainly used for process-to-process or network communication
• Works between processes on the same machine or across network

Soft and hard links

Hard Link

• A hard link is another name for the same file.
• Both the original file and the hard link point to the same inode (same data on disk).
• Deleting the original file does not delete the data as long as one hard link remains.
• Restrictions:
  • Can’t link directories.
  • Can’t cross filesystems/partitions.

ln sourcefile.txt hardlink.txt ## both the file has to be on same partition of disk

Soft Link

• A soft link is like a shortcut (pointer) to another file’s path.
• It has its own inode, which just stores the target filename
• If the original file is deleted, the soft link becomes broken (dangling).
• Can point to:
  • Files or directories
  • Across filesystems/partitions

ln -s sourcefile.txt softlink.txt

graph TD
    subgraph Hard_Link
        F1[file.txt]
        HL[hardlink.txt]
        IN1[inode#1001 → Data Blocks]
        F1 --> IN1
        HL --> IN1
    end

    subgraph Soft_Link
        F2[file.txt]
        SL[softlink.txt]
        IN2[inode#2002 → Data Blocks]
        F2 --> IN2
        SL --> F2
    end

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linux command syntax

commands typically have the syntax:

command option(s) arguments(s)

options

• modify the way command works.
• usually consist of hyphen or dash followed by a single letter.
• some commands accept multiple options which can usually be grouped together after a single hyphen.

arguments

• most commands are used together with one or more arguments.
• some commands assume a default argument if none is supplied.
• arguments are optional for some commands and required by others.

Example cp (copy files)

cp -r Documents Backup 
# cp → command 
# -r → option (recursive, copy directories)
# Documents Backup → arguments (source and destination)

File Permissions

Linux is a multi-user system, so file permissions control who can read, write, or execute files and directories.

1. Permission Types

Each file/directory has three basic permissions:

• r (read) → view contents of a file, list files in a directory
• w (write) → modify a file, create/delete files in a directory
• x (execute) → run a file as a program/script, enter a directory

2. Permission Groups

Permissions are assigned to three categories:

• User (u) → the file’s owner
• Group (g) → members of the file’s group
• Others (o) → everyone else on the system

3. Viewing Permissions

$ ls -l
-rwxr-xr-- 1 alice developers 120 Sep 26 10:00 script.sh

Breakdown:

• → file type (- = file, d = directory, l = link, etc.)
• rwx → user (owner) permissions
• r-x → group permissions
• r-- → others permissions
• alice → file owner
• developers → group

So:

• Owner (alice) → read, write, execute
• Group (developers) → read, execute
• Others → read only

4. Changing Permissions

Using chmod (change mode)

• Add/Remove permissions

chmod u+x script.sh    # add execute for user
chmod g-w script.sh    # remove write for group
chmod o+r script.sh    # add read for others    
chmod a-x script.sh    # remove execute for all

• Set exact permissions with numbers (octal notation)

chmod 755 script.sh

# Breakdown:
# 7 = rwx (4+2+1) (for user)
# 5 = r-x (4+0+1) (for group)
# 5 = r-x (4+0+1) (for others)

Numeric File Permissions (Octal Notation)

Number	Binary	Permission	Meaning
0	000	---	No permission
1	001	--x	Execute only
2	010	-w-	Write only
3	011	-wx	Write + Execute
4	100	r--	Read only
5	101	r-x	Read + Execute
6	110	rw-	Read + Write
7	111	rwx	Read + Write + Execute

To cd to a directory we must have executable right to dir

5. Changing Ownership

• Change the owner:

chown bob file.txt

• Change the group:

chgrp developers file.txt

• Change both:

chown bob:developers file.txt

Access Control list (ACL)

ACLs let you give more fine-grained permissions than the standard Linux permissions (owner, group, others).

• Linux files have permissions like rwx for owner, group, and others.
• Sometimes, you want to allow specific users or groups different permissions than these default ones.
• ACL lets you do that.

---

View ACL of a file

getfacl filename

# Example output

# file: test.txt
# owner: user1
# group: staff
user::rw-
user:user2:r--
group::r--
mask::r--
other::r--

Set ACL for a user

# -m → modify ACL
setfacl -m u:username:permissions filename

# Example
setfacl -m u:john:rw test.txt

Set ACL for a group

setfacl -m g:groupname:permissions filename

# Example
setfacl -m g:editors:rwx test.txt

Remove ACL entry

setfacl -x u:john filename

Default ACLs for directories

# You can give ACLs to new files created inside a directory.
setfacl -d -m u:john:rw /mydir

Check ACL of a directory

getfacl /mydir

To remove all entries

setfacl -b path/to/file # for all users

Note:

• As you assign the ACL permission to a file/directory it adds + sign at the end of the permission.
• Setting w permission with ACL does not allow to remove a file.