Linux commands

Directories

List items.

List using ~$ ls command. Flags -a -l -r -t

-t are used to sort files by time.
-a Is used to display all files, including hidden files and folders.
-r reverses the list of files.
-l lists details of all files.

Change directory

~$ cd <path/to/folder>
Go back ~$ cd ..

To check the present working directory, use ~$ pwd.

Create directory

~$ mkdir <foldername>

Delete

Delete file: ~$ rm <filename>
Delete empty folder: ~$ rm <dir>
Delete non-empty folder: ~$ rm -r <dir>

Print

Use ~$ cat <filename>. Example: ~$ cat index.py.

Print some lines: ~$ head -n 5 index.py (prints 5 lines from the head). For the last lines, use ~$ tail -n 5 index.py.

Copy or Move

~$ cp [OPTIONS] SOURCE DESTINATION
Flags: -r, -v.

~$ mv [OPTIONS] SOURCE DESTINATION

Flags:

-v Verbose: Displays detailed information about the move operation.
-n Prevents overwriting files in the destination.

Rename

~$ mv oldname.txt newname.txt

Soft and Hard link

In Linux, a hard link refers to a physical copy of a file, while a soft link (or symbolic link) refers to a reference or pointer to the original file.

Soft copy:~$ ln -s path/orignal.sh copy.sh
Hard copy: ~$ ln path/orignal.sh copy.sh

Difference

Check the difference between two files:

~$ diff first.sh second.sh

System Level Commands

~$ uname: Displays system information.
- -a: Shows all information.
- -r: Displays the kernel version.
- -n: Displays the system's network hostname.
- -m: Shows the machine's hardware architecture (e.g., x86_64).
~$ uptime: Displays how long the system has been running.
- -p: Displays in a human-readable way.
~$ date: Displays the current date and time.
~$ who and ~$ whoami:
- ~$ who: Displays information about all users currently logged into the system.
- ~$ whoami: Displays the current logged-in user.
~$ id: Displays information about a user’s identity, including user ID (UID), group ID (GID), and group memberships.
~$ shutdown and ~$ reboot: Used to shut down or reboot the system.

User and Group Management

User

~$ sudo useradd -m <newuser>: Using -m will create a home directory for the user.
~$ sudo passwd <newuser>: Set the password for the user.
~$ sudo userdel <username>: Delete the user.

Group

~$ sudo groupadd <newgroup>: Creates a new group.
~$ sudo gpasswd -a <user> <newgroup>: Adds a user to the group.
~$ sudo groupdel <groupname>: Deletes a group.

File Permissions

Inspect Permissions of a File

~$ ls -l: Use this command to inspect the permissions of files and folders.

Symbolic Representation	Numeric Representation	Owner (User)	Group	Others	Description
`rwxrwxrwx`	`777`	read, write, execute	read, write, execute	read, write, execute	Full permissions for everyone
`rw-rw-r--`	`664`	read, write	read, write	read	Owner and group have read/write access; others have read-only access
`rwxr-xr-x`	`755`	read, write, execute	read, execute	read, execute	Owner has full permissions; group and others have read/execute access
`rw-r--r--`	`644`	read, write	read	read	Owner has read/write access; group and others have read-only access
`rwx------`	`700`	read, write, execute	none	none	Only the owner has full access
`r-xr-xr-x`	`555`	read, execute	read, execute	read, execute	Owner, group, and others have read/execute access
`r--r--r--`	`444`	read	read	read	Read-only access for everyone
`---x--x--x`	`111`	execute	execute	execute	Only execute access for everyone

~$ chmod 400 <file>: Change file permissions.
~$ chown <user> <file>: Change the owner of the file.
~$ chgrp <groupname> <file>: Change the group of the file or folder.

Networking

~$ netstat Displays active network connections, routing tables, and network interface statistics
~$ ping <domain or ip>
~$ ifconfig Displays or configures network interfaces
~$ traceroute vs ~$ tracepath Tracks the route packets take to reach a destination
~$ mtr <domain or ip> Combines the functionality of ping and traceroute for real-time network diagnostics
~$ nslookup <domain or ip> Queries DNS servers to obtain domain name
~$ hostname Displays or sets the hostname of the system
~$ iwconfig Displays wireless network interfaces
~$ arp table for mapping IP addresses to MAC addresses
~$ dig Provides detailed DNS query information
~$ whois Retrieves domain registration details
~$ nmap <domain or ip> scans networks to discover hosts, services, and open ports
~$ wget download files from the web
~$ watch runs a command repeatedly at specified intervals
~$ curl test API

awk, sed, grep

awk

~$ awk '{print}' index.py print the file
~$ awk '{print $1, $2}' index.py $<no> no means column no
~$ awk '/word/ {print}' index.py /<filterword>/ filter rows concerning filter word
~$ awk '/word/ {count++} END {print count}' index.py made a var count and incremented it every time a filter word was found in a row.

sed

~$ sed -n '/INFOp/' index.log print rows containing INFO word
~$ sed -i 's/INFO/DATA/g' index.log replace INFO with DATA -i case sensitive g globally s means substring
~$ sed -n -e '/INFO=/' -e '/INFOp/' index.log print count of rows having INFO substring and print rows.

grep

~$ grep INFO index.log print rows having INFO word.
~$ grep -i Info index.log not case sensitive.
~$ grep -i -c INFO index.log print rows including counts.

Example:

~$ ps aux | grep ubuntu

Linux Commands

Essential Linux commands includes.

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