SRTechOps Linux

Basic commends:

SSH Server :

🔧 1. Install SSH Server
Run this on Ubuntu (not Windows):


sudo apt update
sudo apt install openssh-server

🔄 2. Start the SSH service


sudo systemctl start ssh

3. Enable SSH to start on boot


sudo systemctl enable ssh

4. Check SSH status


sudo systemctl status ssh
Make sure it says: active (running)

Extra: Check if Port 22 is Open


sudo ufw allow ssh
sudo ufw enable

Then verify:


 sudo ufw status

ssh ranjith@192.168.0.102

🔐 whoami

→ Shows the current logged-in username
📝 Example: whoami → srtechops

📂 pwd


→ Prints the current working directory path
📝 Example: /home/srtechops

📁 mkdir – Make Directory


→ Creates folders (one or more)
📝 Example: mkdir sample → Creates a folder named "sample"

mkdir devops aws azure → Creates multiple folders

rmdir – Remove Empty Directory


→ Deletes empty folders only
📝 Example: rmdir sample

rmdir devops aws azure


📋 ls – List Files and Directories


→ Shows items inside the current directory
📝 Example: ls

ls -l → Long format

ls -a → Shows hidden files

ls -lt → Sort by modified time

ls -ltr → Time-based + reverse

ls -F → Shows file type

ls aws/ → Lists items in aws folder

📄 File Commands

📄 touch

→ Creates a blank file

📝 touch file.txt


🗑️ rm – Remove Files/Folders

→ Deletes files/folders

📝 Examples:

rm file.txt → Deletes file

rm -r aws/ → Removes folder + content

rm -rf aws/ → Force delete

rm *.txt → Deletes all .txt files

rm f* → Deletes files starting with f


📖 cat – Read & Write Files

→ Read/write files (no editing lines)

📝 Examples:

cat file.txt → Show content

cat > file.txt → Overwrite content (Ctrl + D to save)

cat >> file.txt → Append content

cat -n file.txt → Show lines numbered

tac file.txt → Show file in reverse


✍️ CLI Text Editors

📝 nano – Easy text editor

nano sample.txt

Save: Ctrl + O

Exit: Ctrl + X


🧠 vi or vim – Advanced text editor

Open: vi file.txt

Type i → Insert mode

Esc → Exit insert mode

:wq! → Save and quit

:q! → Quit without saving

📣 echo – Print in Terminal
📝 echo "welcome to devops class"


➕ To save output:

echo "welcome" >> sample.txt → Auto creates file

📜 History & Search
📜 history – Shows command history
📝 Examples:


history

history -d 55 → Delete entry 55

history -c → Clear history

history >> file.txt → Save to file


🔎 grep – Search in text

📝 Examples:

grep "git" file.txt

grep -i "g" → Case-insensitive

grep -i -c "g" → Count matches

grep -i -n "g" → Show line numbers

grep -i -v "g" → Show lines without match


🔁 Combine with history:

history | grep -i "ls"

history | grep -c "ls"


🛠️ sed – Stream editor

📝 Examples:

Replace for display only: sed "s/c/c++/" file.txt

Replace & save: sed -i "s/py/python/" file.txt

Case-insensitive: sed -i "s/py/python/I"

Delete last line: sed -i '$d' file.txt

Delete line 2: sed -i '2d' file.txt

Delete lines from 6 to end: sed -i '6,$d' file.txt


🛠 File/Folder Movement & Copy

🚚 mv – Move or Rename

📝 Examples:

mv js java → Rename

mv file folder/ → Move

mv .hidden visible → Unhide

mv file folder/newname → Move + rename


📋 cp – Copy files/folders

📝 Examples:

cp a.txt b.txt → Copy file

cp a.txt folder/ → Copy into folder

cp -r folder1/ folder2/ → Copy folder


🔼 File View Commands

🔼 head – First few lines

head file.txt → First 10 lines

head -n 5 file.txt

🔽 tail – Last few lines

tail file.txt → Last 10 lines

tail -n 2 file.txt

tail -f file.txt → Live log view


🗃️ Compression

🗜️ zip – Compress files

📝 Examples:

zip name.zip *.txt

zip -sf name.zip → Show content

zip -d name.zip file → Remove file

zip -e secure.zip *.txt → Password zip

unzip name.zip → Extract


📂 tar – Archive tool

📝 Examples:

tar -cf file.tar *.txt → No compression

tar -czvf file.tar.gz *.txt → With compression

tar -tvf file.tar → Show detailed content

tar -xvf file.tar → Extract


🌐 Network Tools

🌐 wget – Download file
📝 Example: wget https://example.com/file.zip


🔐 ssh – Remote login

📝 Example:

ssh user@ip

ssh [e-mail]


🔑 Password protected login

🔐 GPG – Encrypt/Decrypt
🔒 Encrypt: gpg -c file.tar
🔓 Decrypt: gpg -d file.tar.gpg > file.tar


👤 USER MANAGEMENT

$ - Normal user - access file and directories

- Admin root User or Root user - access to All

(install, uninstall apps and update)

su root
password: root123
su - root

adduser :

useradd ranjith

automatically create user account group and home

set password:
passwd ranjith123

cat /etc/passwd - show all users

cat /etc/group - show all groups

su - ranjith - swith home directory

user rename :

usermod -l ranjith ranjithkumar - l new log

group rename :

groupmod -n ranjithkumar ranjith

renane home dir :
mv ranjith ranjithkumar

Delete user :

userdel ranjithkumar

group automatically deleted

Delete home dir :

rm -rf ranjithkumar

userdel ranjithkumar -- remove

👤 USER MANAGEMENT

💡 User Types


$ → Normal user (limited access)

# → Root user (full access: install, delete, update)

🔐 Switch to Root

su root
# OR
su - root

➕ Create User


useradd ranjith      # Add user only (no home)

adduser ranjith      # Recommended – creates home + group + prompts password

🔑 Set Password


passwd ranjith

📜 View All Users and Groups


cat /etc/passwd     # List all users

cat /etc/group      # List all groups

🔄 Switch User


su - ranjith         # Switch to user and home

✏️ RENAME USER / GROUP / HOME
🧑 Rename User

usermod -l newname oldname

# Example:

usermod -l ranjith ranjithkumar

👥 Rename Group

groupmod -n newgroup oldgroup

🏠 Rename Home Directory


mv /home/oldname /home/newname

usermod -d /home/newname -m newname

🗑️ DELETE USER / HOME / GROUP

❌ Delete User Only


userdel ranjith

❌ Delete Home Directory


rm -rf /home/ranjith

🔥 Delete User with Home Directory


userdel --remove ranjith

💡 Group is NOT deleted automatically unless it's a private user group and unused.

🔐 File Permissions

Permission types:

r = read (4)

w = write (2)

x = execute (1)


🛠️ Change with chmod:

Examples:


chmod u+x file → Add execute

chmod g-r file → Remove read

chmod o-rwx file → Remove all from others

chmod 777 file → Full access

chmod 600 file → Owner only

👥 Change Ownership:

Owner: chown harini file.txt

Group: chown :dev file.txt

Both: chown harini:dev file.txt

Directory: chown -R user:group folder/


🔍 File Info & Search
🔍 Find OS: cat /etc/os-release
🔍 Find binary: whereis git
🔍 Search: find -name f1.txt
🔍 Find empty files: find -type f -empty
🗑️ Delete empty dirs: find -type d -empty -delete

🔗 File Linking

🔗 Hard Link: ln file.txt hard.txt

🔗 Soft Link: ln -s file.txt soft.txt


 Kill:

📊 Process Management

📊 Top Processes: top

Normal: kill 2693

Force: kill -9 2693


🌟 What is crontab?

crontab stands for "cron table". It's a file where you define tasks to run automatically at a set time or date.

🔁 Cron = scheduler
📄 Crontab = list of scheduled tasks


🔧 Basic Crontab Command


 crontab -e Edit your crontab file
crontab -l  List current cron jobs
crontab -r  Remove all cron jobs
crontab -u username -l  View cron for another user (root only)

Crontab Syntax


*  *  *  *  *  command_to_run
|  |  |  |  |
|  |  |  |  └──── Day of the week (0 - 6) [Sun=0]
|  |  |  └─────── Month (1 - 12)
|  |  └────────── Day of the month (1 - 31)
|  └───────────── Hour (0 - 23)
└──────────────── Minute (0 - 59)

📅 Common Examples


Time    Crontab Syntax  Description
Every minute    * * * * *   Runs every minute
Every hour  0 * * * *   Runs at minute 0 of every hour
Daily at 5am    0 5 * * *   Runs at 5:00 AM every day
Every Monday at 3pm 0 15 * * 1  Runs at 3:00 PM on Mondays
On Jan 1st at midnight  0 0 1 1 *   New Year execution

📂 Run a Script with Crontab

Suppose you have a script at /home/user/backup.sh
Make it executable:

chmod +x /home/user/backup.sh
Now schedule it (e.g., every day at 2 AM):

0 2 * * * /home/user/backup.sh

✅ Tips


Always give full paths (e.g., /usr/bin/python3, /home/user/script.sh)

Use >> /path/to/logfile.log 2>&1 to log output and errors

Edit crontab using crontab -e

🧪 Test Crontab

Try a quick job:


* * * * * echo "Hello from cron!" >> /home/user/cronlog.txt

This appends "Hello from cron!" to the file every minute.

✅ 1. Shutdown Immediately

sudo poweroff

sudo shutdown now


✅ 2. Shutdown After 1 Minute

sudo shutdown

(Default is 1-minute delay)


✅ 3. Shutdown at a Specific Time

sudo shutdown 22:00

(This shuts down at 10:00 PM)


✅ 4. Restart Instead of Shutdown

sudo reboot


✅ Cancel Scheduled Shutdown

sudo shutdown -c