Needful CLI Commands:

Computers understand only binary language so to interact with computer machines we need some bridge which fills this communication gap and this is done by shell.
Our operating system has a special user program known as shell which can read human language(commands) and can interact with the kernel to perform various tasks.

Here are some important cli commands:

Commands related to files and folders:

• Create a file/folder (touch,cat,mkdir,)

touch file.txt #create an empty file if it does not exist
cat > file.txt #if file does not exist; it can create file and let us write the content
mkdir foldername   #create only single folder
mkdir -p a/b/c     #create nested folders , work (parent exist or not)

• Navigating folders

cd foldername      #move into a folder
cd ..              #Go one level back
cd or cd ~         #go to home directory
cd -               #go to previous working directory
cd /               #go to root directory
pwd                #print the current directory

• Write/Overwrite content (echo,cat)

echo "hello world!" > file.txt
cat > file.txt #it can also create file and let you overwrite content, ctrl + d to save

• Append content to file (echo,cat)

echo "Append line!" >> file.txt
cat >> file.txt   # type and press ctrl + d to save

• Move/Rename a file/folder to another directory (mv)

mv file.txt /path/to/another/directory/
mv oldname.txt newname.txt
mv oldname newname            #rename a folder
mv foldername /path/to/dir/   #move folder to a different directory

• Delete a File/Folder (rm,rmdir)

rm file.txt #No recycle bin-it's gone!
rmdir foldername   #remove an empty folder
rm -r foldername   #remove folder and its contents(recursive)
rm -rf foldername  #force remove folder without prompt

• Empty a File (>,:)

> file.txt
: > file.txt

• View File Content (cat)

cat file.txt

• Other useful viewers (less,more)

less file.txt
more file.txt

• Copying folders/files

cp -r folder1 folder2    #copy folder and its contents
#If folder2 is exist before copying folder1 then folder2 will have whole folder1
#If folder2 is not exist then there will be folder2 same as folder1
cp file1.txt file2.txt  #copying file1 to file2

• Search inside a File (grep)

grep "keyword" file.txt #show that line

• Edit a file using editor (nano , vim)

nano file.txt  #simple
vim file.txt   #advanced

• Hidden file/folders (using .)

touch .hiddenfile.txt #create hidden file
cat .hiddenfile.txt  #To open hidden file
ls -a  #List all hidden files
mkdir .hiddenfolder      #create a hidden folder(starts with a dot)
ls -a                    #show hidden folders

• Listing contents

ls       #list files/folders
ls -l    #long listing with details
ls -a    #show hidden files and folders
ls -R    #list recursively
ls -dl file.txt  #long listing for specific files

• Search files (find)

#Let say we want to search all file named "hello.txt"
find . -name "hello.txt" -type f

#Let say we want to delete all files  having extension ".txt"
find . -name "*.txt" -type f -delete

find . -type d                     #find all folders in current directory
find /path -name foldername        #search for a folder by name
find . -name foldername -type d    #search for a folder by name

#let say we want to search a folder name hello
find . -name hello -type d
find ./ -name "*hello*" -type d | xargs rm -r 

#let say we want to delete all folder in nested ,named hello
find . -name hello -type d -delete

• Giving file permission (chmod)

  Using Symbolic mode

#Make it readable, writable, and executable for the owner:
chmod u+rwx hello.txt
#Make it readable and executable for group:
chmod g+rx hello.txt
#Remove write permission for others:
chmod o-w hello.txt

Using Numeric mode
Each permission has a number:
r = 4 , w = 2 , x = 1
Read + Write = 4 + 2 = 6
Read + Execute = 4 + 1 = 5
Write + Execute = 2 + 1 = 3
Read + Write + Execute = 4 + 2 + 1 = 7

#Owner ➔ 7 (rwx)
#Group ➔ 5 (r-x)
#Others ➔ 5 (r-x)

chmod 755 hello.txt 

chmod 755 foldername     #change folder permissions

• Changing the owner (chown)

#Let's say you want to give ownership of hello.txt to a user named vishal.
#You need sudo because changing ownership usually needs admin rights.
sudo chown vishal hello.txt

#Change owner and group together
sudo chown vishal:developers hello.txt #owner->vishal , group->developers

• grep commands (grep)

  Key Notes:

  • -i makes search case-insensitive

  • -r enables recursive directory searching

  • -o with wc -l is the proper way to count total word occurrences

  • Quotes around search patterns ("") are good practice for multi-word searches

  • -v is useful for filtering out unwanted lines

  • -n helps locate matches by line numbers in files

 # 1. Basic search - finds lines containing "harry" (case-sensitive)
grep "harry" tempFile.txt

# 2. Inverted search - shows lines NOT containing "harry"
grep -v "harry" tempFile.txt

# 3. Numbered results - shows lines with "harry" including line numbers
grep -n "harry" tempFile.txt

# 4. Count matching lines - shows count of lines containing "Harry" (not total occurrences)
grep -c "Harry" tempFile.txt

# 5. Show only matches - prints each "Harry" match on separate line
grep -o "Harry" tempFile.txt

# 6. Count characters - counts total characters in all "Harry" matches (not recommended)
grep -o "Harry" tempFile.txt | wc -m

# 7. Count words - counts words in output (misleading for this use)
grep -o "Harry" tempFile.txt | wc -w

# 8. Count occurrences - correct way to count total "Harry" words in file
grep -o "Harry" tempFile.txt | wc -l

# 9. Recursive search - searches for "harry" in current directory and subdirectories
grep -r harry ./

# 10. Recursive case-insensitive search in home directory - looks for "aur" in ~/
grep -ri aur ~/

# 11. Same as 11 but with exact word matching (better practice)
grep -ri "aur" ./

Commands related to Process/Ports

ps -ef | grep -i 'chrome' | grep -v grep | awk '{print $2, $3}'     # Show Chrome's process ID and its parent process ID
open -a "safari"                                                    # Open safari on mac
chrome $                                                            # Open chrome on linux 
pkill -i safari                                                     # Force stop Safari browser
ps -A -o pid,ppid,comm,%cpu,%mem | sort -k4 -nr | head -n 3         # Top 3 CPU consuming processes
ps -A -o pid,ppid,comm,%cpu,%mem | sort -k5 -nr | head -n 3         # Top 3 memory consuming processes
python3 -m http.server 8000                                        # Start a simple HTTP server on port 8000
kill -9 $(lsof -ti :8000)                                          # Kill the process running on port 8000
python3 -m http.server 90                                          # Start a server on port 90 
netstat -a                                                         # Show all active network connections and ports
lsof -i :5432                                                      # Show process using port 5432
pgrep Telegram                                                     # Show the pid of Telegram
pgrep Telegram | xargs kill                                         # Kill Telegram by getting pid of telegram

Commands related to Pipes

curl -L -o Harry_Potter.txt "url"  # Download the Harry Potter book text file
cat Harry_Potter.txt | head -3                                     # Show first 3 lines of the book
cat Harry_Potter.txt | tail -10                                    # Show last 10 lines of the book
cat Harry_Potter.txt | grep "Harry" -o | wc -w                     # Count occurrences of the word "Harry"
cat Harry_Potter.txt | grep "Ron" | wc -w                          # Count number of lines containing "Ron"
cat Harry_Potter.txt | grep "Hermione" | wc -w                     # Count number of lines containing "Hermione"
cat Harry_Potter.txt | grep "Dumbledore" | wc -w                   # Count number of lines containing "Dumbledore"
cat Harry_Potter.txt | head -200 | tail -100                       # Show lines from 101 to 200
cat Harry_Potter.txt | grep -oE '\w+' | sort -u | wc -w                # Count number of unique words in tempFile.txt

Commands related to Managing Software

brew install htop                                                  # Install 'htop' 
brew install vim                                                   # Install 'vim' text editor
brew install nginx                                                 # Install Nginx web server
brew uninstall nginx                                               # Uninstall Nginx from system

Some Miscellaneous commands

ifconfig                                                           # Display network interface configuration
nslookup google.com                                                # DNS lookup for google.com
ping google.com                                                    # Test internet connectivity
which node                                                         # Show path to Node.js binary
which code                                                         # Show path to VS Code binary
curl ifconfig.me                                                   # Show public IP on mac
ipconfig getifaddr en0                                             # Show private IP on mac

# All in one local + private ip address
echo "Local IP: $(ipconfig getifaddr en0 2>/dev/null || hostname -I | awk '{print $1}')" && echo "Public IP: $(curl -s ifconfig.me)"