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Part III: Shell Operations

Objectives

Deliverables:

Choose one of the following:

  1. Record a video of yourself performing the tasks described in part 1
  2. Create a file that lists the commands you used to perform the tasks described in part 1
  • Hint: history | tail -11

then...

  1. Complete part 2. Answer the questions in the doc, then schedule a time to meet with your mentor and discuss the commands and how they work

Part 1: Creating, adding to, reading and sorting files

  • Use echo and text redirection > to create a file and add at least one line of text
  • Use echo and text redirection >> to append lines to a second file
  • Use cat <file1> <file2> to output the contents of both files
  • Use cat and text redirection to create a third file
  • In one terminal windows use text redirection and echo to append lines to a file, in a second terminal window use tail -f to view the lines being appended. Stop the tail -f
  • Use tail -f | grep <pattern> and continue to append lines to the file. Notice that only lines that match your pattern are displayed by grep.
  • Do the above, but invert the match so only lines without the pattern are displayed.
    • grep -v
  • Use cat and sort to display the contents of all the files alphabetically.
    • cat three.txt | sort
  • Use cat and wc -l to count the number of lines in all the files.
    • cat three.txt | wc -l
  • Use head to display the first 3 items from the above step

Part 2

  • Inspect your environment with env

  • export variable on the command line export TESTVAR1=testvar1

  • Use grep to find that variable in your environment

    • env | grep $TESTVAR1

  • Set a second variable on the command line without exporting it

    • Is that variable in the environment?
      • Without export the variable is only available to the shell
      • With export, the variable is added to the environment is available to all processes(?)
      • Question: is this related to the source command ?
      • https://jvns.ca/blog/2017/03/26/bash-quirks/

  • export that variable. No need to re define it.

    • Does it show up in the env now?

  • Use which to find the path of the grep binary

    • which grep

  • What are the permissions on the grep binary? ls -l

  • Make a directory bin in your home directory

  • Create a script that displays ‘Hello World’ and the in the $HOME/bin directory.

    • Can you run it without using the bash or sh command?
      • No. I have to change the file permission from having just read and write access to having access to execute

  • What are the permissions on the files you created above?

    • read and write access

  • Make the script executable using chmod +x

    • chmod +x say_hello.sh
    • Now the permissions look like this: -rwxr-xr-x

  • Find use env and grep to find the $PATH environment variable

    • env | grep $PATH

  • Add $HOME/bin to your $PATH, be sure to export it

    • The script should now be executable from anywhere without using its full path
    • Can you find your script using which
      • which say_hello.sh

  • Open a new terminal window.

    • Can you still use which to find your script?
    • Why or why not?
    • Is $HOME/bin still in your $PATH

  • Find hidden files in your home directory using ls -a $HOME

  • Edit .bash_profile to make your change to $PATH persist

  • Three ways to set env variables:

    • foo=bar (local scope)
    • export foo=bar (global scope inside your terminal window => 1 session)
    • putting the declaration into a file and sourcing the file x

  • echo "echo $var" >> into your script

  • Set var=test in your terminal and run the script.

    • Does it output “test”

  • export var and run the script.

    • Does it output “test”
    • Why or why not?

  • Open a new window and run the script

    • Does it output “test”
    • Why or why not?

  • echo "export var2='exported in the script'" >> into your script and run it

  • source your script and see if $var2 is set this time.

    • Did $var2 get set in your current shell?
      • yes
    • Why or why not?
      • source runs the script in the current environment so the scope overlaps. If you do not source it runs in a sub-shell and cannot set new variables in the current session.

  • Create another executable script that outputs “Goodbye World” in you home directory but not in your bin directory. Create a symlink to the goodbye_world script in the bin directory so that it is accessible if your path is set correctly. https://www.youtube.com/watch?v=l_1Q3DG3uoE

  • Inspect your symlink using ls -l

    • lrwxr-xr-x
    • You should now be able to run your new script from anywhere even though it is only a symlink in the bin directory
      • When is this pattern helpful?
        • When you have something that is used in multiple projects and you don’t want to recreate the resource in multiple places

  • Use Homebrew to install python, ansible and terraform

  • Use ls -l $(which <program>) to see if the binaries are linked. If they are not use brew link and which to see where the binaries exist

    • ex: ls -l $(which ansible)

  • Inspect the path of each binary and determine if it is a symlink and if so, where the actual binary is on the file system

    • ls /usr/local/bin/terraform shows me that terraform is a symlink (on my machine, it’s pink and has an @ at the end of the name)
    • realpath /usr/local/bin/ansible => /usr/local/Cellar/ansible/2.5.4/libexec/bin/ansible
    • realpath /usr/local/bin/terraform => /usr/local/Cellar/terraform/0.11.7/bin/terraform
    • realpath /usr/local/bin/python => /usr/local/Cellar/python@2/2.7.15_1/Frameworks/Python.framework/Versions/2.7/bin/python2.7

  • Symlinks

    • To keep a directory organization and have bin files and keep in organized like brew install