DevOps Journey Week 2 : Shell Scripting

I’ve been diving into the essentials of Linux and Shell Scripting as part of my DevOps journey. Here’s a summary of what I’ve learned so far.

Operating Systems (OS)

• Role: The OS is like the bridge between the software and hardware. It manages:

  • Device Management: Connects hardware components with software.

  • Memory Management: Keeps track of memory allocation and usage.

  • Process Management: Handles running processes and multitasking.

  • System Handling: Ensures the overall system stability and efficiency.

Linux

• Why Linux? It’s a powerful, open-source OS widely used in production.

• Kernel: The core of Linux that manages devices, memory, processes, and system resources.

• Popular Distros:

  • Ubuntu: User-friendly and widely used in both dev and production environments.

  • CentOS: Known for its stability and enterprise features.

  • Others: Fedora, Redhat, Debian, Kali Linux.

Shell Scripting

Purpose: Automates tasks and makes system management easier.

Key Commands:

File and Directory Management:

• pwd - Shows the current working directory.

    pwd
    # Output: /home/user
  

• ls - Lists files and directories.

    ls
    # Output: file1.txt  file2.txt  directory1
  

• ls -ltr - Lists files with permissions and timestamps.

    ls -ltr
    # Output:
    # -rw-r--r-- 1 user group 1024 Aug 24 10:00 file1.txt
    # drwxr-xr-x 2 user group 4096 Aug 24 09:45 directory1
  

• mkdir - Creates a new directory.

    mkdir new_directory
  

• touch - Creates a new file.

    touch newfile.txt
  

File Editing and Viewing:

• vim - Opens a file for editing.

    vim filename.txt
  

  • Commands:

    • esc + i (insert mode)

    • esc + :wq! (save and exit)

    • esc + :q! (exit without saving)

• cat - Displays file content.

    cat filename.txt
  

Permissions and Removal:

• chmod - Changes file permissions.

    chmod 666 file1.txt
    chmod 444 file1.txt
    chmod 777 file1.txt
  

• rm - Deletes files or directories.

    rm file1.txt
    rm -rf directory1
  

System Monitoring:

• nproc - Shows the number of CPUs.

    nproc
    # Output: 4
  

• free - Displays RAM usage.

    free -h
    # Output: 
    #              total        used        free      shared  buff/cache   available
    # Mem:          7.7G        2.4G        3.2G        310M        2.0G        4.7G
  

• top - Lists running processes.

    top
    # Displays a dynamic view of running processes
  

• ps -ef - Shows detailed process information.

    ps -ef
    # Output:
    # UID        PID  PPID  C STIME TTY          TIME CMD
    # root         1     0  0 Aug23 ?        00:00:02 /sbin/init
  

Data Handling and Networking:

• grep - Filters output by keywords.

    ps -ef | grep ssh
    # Output: Lists processes related to SSH
  

• curl - Fetches data from the web.

    curl http://example.com
  

• wget - Downloads files from the web.

    wget http://example.com/file.zip
  

• find - Locates files or directories.

    find /home/user -name filename.txt
  

• su - Switches user accounts.

    su - username
  

• awk - Processes and filters text.

    ps -ef | awk '{print $1, $2}'
    # Output: Displays the user and process ID of each running process
  

Scripting Practices

Debugging and Error Handling:

• set -x - Turns on debug mode to show command execution.

    set -x
  

• set -e - Exits the script if any command fails.

    set -e
  

• set -o pipefail - Exits the script if any command in a pipeline fails.

    set -o pipefail
  

Control Structures:

• Conditionals: Use if-else statements for decision-making.

    if [ $a -gt $b ]
    then
        echo "a is greater than b"
    else
        echo "b is greater than a"
    fi
  

• Loops: Use for loops for iteration.

    for i in {1..5}; do
        echo $i
    done
  

Pipes and Redirection:

• Use | to chain commands, passing output from one as input to another.

    ps -ef | grep mysql | awk '{print $2}'
  

#!/bin/bash vs #!/bin/sh:

• #!/bin/bash is used for Bash scripts. Bash offers more features and is commonly used for scripting.

• #!/bin/sh is used for POSIX-compliant scripts. It may be linked to different shells like Dash, which has different syntax and capabilities.

Executing Scripts:

• Run scripts using bash script_name.sh or ./script_name.sh (make sure the script has execute permissions with chmod +x script_name.sh).

    bash script.sh
    ./script.sh
  

Permissions:

• Use chmod to set permissions for files and directories:

    chmod 777 file
    chmod 644 file
  

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Refer to my github repo for shell scripts https://github.com/manjot-baj/DevOps/tree/main/Shell%20Scripting.