Shell Scripting (Part-2)

Shell scripting and command-line usage are essential skills for a Linux power user.

In this part, I’ll cover the most important shell commands and scripting essentials from scratch, assuming you're aiming to become proficient.

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Basic Linux Commands

1. File Management

• ls: Lists directory contents.

    ls             # List files in the current directory
    ls -l          # List files with detailed information
    ls -a          # Include hidden files in the listing
    ls -lh         # Human-readable sizes
  

• cd: Change directory.

    cd /path/to/directory  # Navigate to a directory
    cd ..                  # Move to the parent directory
    cd ~                   # Go to home directory
  

• mkdir: Create a new directory.

    mkdir myfolder  # Create a directory named 'myfolder'
  

• cp: Copy files.

    cp source.txt destination.txt   # Copy a file
    cp -r /source/ /destination/    # Copy directories recursively
  

• mv: Move or rename files.

    mv file1.txt file2.txt    # Rename file1 to file2
    mv /source/ /destination/ # Move files
  

• rm: Remove files or directories.

    rm file.txt               # Delete a file
    rm -r directory           # Remove directory and its contents
    rm -f file.txt            # Force delete (use with caution)
  

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File Viewing and Manipulation

• cat: Display the content of a file.

    cat file.txt       # Show file contents
  

• more and less: View large files one page at a time.

    less file.txt      # View file contents (scroll with arrows)
  

• head and tail: View the start or end of a file.

    head -n 5 file.txt   # Show the first 5 lines
    tail -n 5 file.txt   # Show the last 5 lines
  

• grep: Search for patterns in files.

    grep 'pattern' file.txt         # Search for 'pattern' in file
    grep -r 'pattern' /path/        # Recursive search in directory
  

• find: Search for files in a directory hierarchy.

    find /path/ -name '*.txt'   # Find all .txt files under /path/
  

• touch: Create an empty file or update a file’s timestamp.

    touch newfile.txt           # Create a new file
  

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File Permissions

• chmod: Change file permissions.

    chmod 755 file.sh   # Give the owner full permissions, others read-execute
  

• chown: Change file ownership.

    chown user:group file.txt    # Change the owner and group
  

File Permissions Structure:

• r: Read

• w: Write

• x: Execute

Permissions are defined using numeric codes (owner, group, others):

• 7: Read, write, execute

• 6: Read, write

• 5: Read, execute

• 4: Read only

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Process Management

• ps: Show running processes.

    ps aux              # Display all running processes with detailed info
  

• top: Display real-time processes and system resource usage.

    top                 # View real-time system processes and memory usage
  

• kill: Terminate a process.

    kill PID            # Kill process by PID (Process ID)
    kill -9 PID         # Force kill a process (use carefully)
  

• &: Run a command in the background.

    command &           # Run 'command' in the background
  

• jobs: List background jobs.

    jobs                # List all background jobs
  

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Networking Commands

• ifconfig: Display network interfaces.

    ifconfig            # Show network interface info (IP address, etc.)
  

• ping: Check network connectivity.

    ping google.com     # Test network connection to a host
  

• netstat: Display network connections and listening ports.

    netstat -tuln       # Show listening ports
  

• scp: Securely copy files between hosts.

    scp file.txt user@remote:/path/   # Copy file to a remote host
  

• ssh: Securely connect to a remote host.

    ssh user@remote_ip_address    # Connect to remote machine over SSH
  

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System Monitoring and Management

• df: Check disk space usage.

    df -h                # Show disk space usage in human-readable format
  

• du: Check directory size.

    du -sh /directory/   # Show the size of a directory
  

• uptime: Check system uptime.

    uptime               # Show how long the system has been running
  

• free: Display memory usage.

    free -h              # Show system memory usage (RAM)
  

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Text Processing Commands

• awk: Pattern scanning and text processing.

    awk '{print $1}' file.txt   # Print the first column of a file
  

• sed: Stream editor for text manipulation.

    sed 's/old/new/g' file.txt  # Replace 'old' with 'new' in file
  

• cut: Remove sections from each line of files.

    cut -d':' -f1 /etc/passwd   # Display the first field (username) in /etc/passwd
  

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Shell Scripting Basics

A shell script is simply a series of commands stored in a file.

Example 1: Simple Shell Script

1. Create a script file:

touch myscript.sh

2. Open the file in a text editor (e.g., nano myscript.sh) and add the following code:

#!/bin/bash

# This is a comment
echo "Hello, World!"

3. Make the script executable and run it:

chmod +x myscript.sh
./myscript.sh

Example 2: Using Variables and Arguments

#!/bin/bash
name=$1  # First argument to the script
echo "Hello, $name"

Run it:

./myscript.sh Alice

Output:

Hello, Alice

Example 3: Control Structures (If-Else, Loops)

If-Else:

#!/bin/bash
if [ $1 -gt 10 ]; then
  echo "Number is greater than 10"
else
  echo "Number is less than or equal to 10"
fi

For Loop:

#!/bin/bash
for i in {1..5}; do
  echo "Welcome $i times"
done

While Loop:

#!/bin/bash
count=1
while [ $count -le 5 ]; do
  echo "Iteration $count"
  ((count++))
done

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Redirecting Input and Output

• >: Redirect standard output to a file (overwrite).

    echo "Hello" > file.txt     # Overwrites file.txt with "Hello"
  

• >>: Append standard output to a file.

    echo "Hello" >> file.txt    # Appends "Hello" to file.txt
  

• <: Redirect input from a file.

    wc -l < file.txt            # Count lines in file.txt
  

• Piping (|): Send the output of one command as input to another.

    ls -l | grep ".txt"         # List .txt files in the current directory
  

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Permissions and User Management

• adduser: Add a new user.

    sudo adduser newuser        # Create a new user with home directory
  

• passwd: Change a user’s password.

    sudo passwd newuser         # Set password for the new user
  

• userdel: Delete a user account.

    sudo userdel newuser
  

  • usermod: Modify a user account.

      sudo usermod -aG groupname username  # Add a user to a group
    

  • groups: Show the groups a user belongs to.

      groups username      # List groups for the specified user
    

  • who: Display who is currently logged in.

      who                  # Show logged-in users
    

  • last: Show the last logged-in users.

      last                 # Display login history
    

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System Information Commands

• uname: Display system information.

    uname -a             # Show all system information
  

• hostname: Display or set the system’s hostname.

    hostname             # Show the hostname
  

• lscpu: Display CPU architecture information.

    lscpu                # Show CPU details
  

• lsblk: List block devices.

    lsblk                # Display information about all block devices
  

• df -i: Check inode usage.

    df -i                # Show inode usage for filesystems
  

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Package Management

Depending on your Linux distribution, package management commands can vary. Here are commands for Debian/Ubuntu and Red Hat/CentOS:

Debian/Ubuntu: apt

• Update package index:

    sudo apt update       # Update package information
  

• Install a package:

    sudo apt install package_name  # Install specified package
  

• Remove a package:

    sudo apt remove package_name    # Remove specified package
  

• Upgrade installed packages:

    sudo apt upgrade       # Upgrade all installed packages
  

Red Hat/CentOS: yum or dnf

• Update package index:

    sudo yum check-update       # Check for package updates
  

• Install a package:

    sudo yum install package_name  # Install specified package
  

• Remove a package:

    sudo yum remove package_name    # Remove specified package
  

• Upgrade installed packages:

    sudo yum update       # Upgrade all installed packages
  

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Cron Jobs

Cron jobs allow you to schedule tasks to run automatically at specified intervals.

• Edit crontab:

    crontab -e            # Edit the current user's cron jobs
  

Crontab Format:

    * * * * * command_to_run
    - - - - -
    | | | | |
    | | | | +---- Day of the week (0-6, 0 is Sunday)
    | | | +------ Month (1-12)
    | | +-------- Day of the month (1-31)
    | +---------- Hour (0-23)
    +------------ Minute (0-59)

Example Cron Job:

Run a script every day at 5 AM:

    0 5 * * * /path/to/script.sh

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Advanced Shell Scripting Concepts

Functions in Shell Scripts

Functions help to organize scripts into reusable blocks of code.

    #!/bin/bash

    function greet {
        echo "Hello, $1!"   # Function that greets the user
    }

    greet "Alice"          # Call the function with 'Alice' as an argument

Arrays

Bash supports one-dimensional arrays.

    #!/bin/bash

    my_array=(apple banana cherry)   # Declare an array

    echo ${my_array[0]}              # Access the first element
    echo ${my_array[@]}               # Access all elements

Error Handling

To handle errors in scripts, you can check the exit status of commands.

    #!/bin/bash

    cp file.txt /destination/
    if [ $? -ne 0 ]; then  # Check if the previous command failed
        echo "Copy failed!"
    else
        echo "Copy successful!"
    fi

Debugging Scripts

Use the -x option to enable debugging.

    bash -x myscript.sh  # Run the script in debug mode

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Environment Variables

Environment variables store system-wide values that affect the behavior of processes.

Common Environment Variables

• $HOME: Home directory of the current user.

• $USER: The name of the current user.

• $PATH: A list of directories to search for executable files.

Setting Environment Variables

    export MY_VAR="Hello World"  # Set a new environment variable
    echo $MY_VAR                  # Display the variable

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Redirection and Pipes

• Redirecting Standard Error:

    command 2> error.log        # Redirect error messages to a file
  

• Redirecting both Output and Error:

    command &> output.log       # Redirect both stdout and stderr
  

• Using Pipes:

    command1 | command2         # Pipe output of command1 to command2
  

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Conclusion

Mastering these essential commands and shell scripting concepts will greatly enhance your productivity as a Linux user. Start practicing these commands in a Linux environment to build your proficiency. If you have any specific questions or need further clarification on any topic, feel free to ask!

Happy Linux’ing :)