Lab 3: Bash Scripting

Bash Scripting: redirection, pipes, basic bash scripting, executing scripts, conditional statements, repetitive statements, and environment variables.

Setup

• This lab assumes that you’re using Bash Shell, the default shell on most systems.
• You need to have a text editor (e.g., nano, vi).

Notes

• Redirection
  • > is the output (stdout) redirection operator. » appends output to an existing file
  • < is the input (stdin) redirection operator
  • 2> is the error (stderr) redirection operator
• Pipes use the symbol | and are used to connect the stdout of one command into the stdin of another command.
• Below is a list of commands you will be using in these lab exercises. Make sure you read the manual of each one using the command man.
  • grep
  • wc
  • curl
  • du
  • history
• If your date or time is incorrect, then you need to sync it with a tool like ntpd. See how to install and run it here.

Exercises

1. Redirection

• Use the du tool to find the disk usage of each file at /var/log. Use the -h option to show the size information in human-readable format (Kilobyte, Megabyte, Gigabyte, etc.): du -h /var/log
  • Re-run the command and redirect the output into a file named log-size.txt
  • Re-run the command and redirect the errors into a file named error.log.
  • Re-run the command to redirect the errors into a file named error.log and the output into a file named log-size.txt.
• Save the history of commands you previously executed since you logged in into a file named mycommands.txt. Hint: use the history command and redirect its output.

2. Pipes

• Count how many programs/commands in /bin. Hint: You need to use the ls and wc commands.
• Run curl -h. Find the command line option for including headers in the output. Hint: pipe the output of curl -h into grep.

3. Basic Bash Scripting

• Simple script
  • Write a shell script that creates a unique file.
    • Hint: You may add the timestamp to the end of the file name, which can be obtained using date +%s.
  • Change permissions on your script so that you can run it.
  • Run the script in normal mode. It should run without errors.
• Running a remote script:
  • Download the following script using curl and run it remotely:

    • Option 1: Run the script directly by piping its content from curl into bash.

      • curl https://gitlab.com/cpit490/examples/raw/master/hello.sh | bash -
        • Note: This is obviously considered a bad idea for security reasons, so you should check always the content of the script before executing it.

    • Option 2: Download the file locally, change its permission, and execute it:

      curl -O https://gitlab.com/cpit490/examples/raw/master/hello.sh
      chmod u+x hello.sh
      ./hello.sh
      

• Conditional execution
  • Use the logical operator && to control the sequence of command execution. This is useful when you want to run the second command only if the first command is successful.
    • ls ~/bin && cd ~/bin

4. Conditional statements in Bash

• Download this shell script using curl -O https://gitlab.com/cpit490/examples/raw/master/if-statements.sh. This script checks for IPv4 connectivity using ping and displays an error message if the connection is down.

  • Run the script.

  • Edit the script, so it asks the user if he wants to re-run ping and save the output into a file. Example:

    ./if-statements.sh
    

    This script checks for IPv4 connectivity
    ping: sendto: No route to host
    Network is down
    Would you like to check again and save the output into a file? [y/n]:y
    ping: sendto: No route to host
    

5. Repetitive statements in Bash

• Fix the following script that should display the size of the home directory for each bash user in the system:

  # Loop through the list of directories in /home/*
  for ...
    du -h -d 0 $userDir
  done
  

6. Environment Variables

• Rename the script in step # 4 from if-statements.sh into check-network.sh and add it to a directory called bin under your home directory (~/bin/).
  • Open the file ~/.bash_profile in your text editor (e.g., nano ~/.bash_profile).
  • Append the path to ~/bin to the environment variable $PATH. This means you will go to the end of the file and add this line PATH=$PATH:/home/<your-user-name>/bin.
  • Save it and reload it by running: source ~/.bash_profile.
  • Move into a different directory (e.g., cd ~/) and run the script by its name only: check-network.sh

Submission

Submit your answers with screenshots showing the commands you executed as a PDF file by the due date.

Lab submissions are due one hour before the next week’s lab begins.