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Lecture #4: Managing your environment, searching your files, and bash scripts

Learning objectives

Unix/Linux Command-line

  1. read and change environment variables and aliases
  2. configure the environment by chaning the .bashrc file
  3. Read and change permissions on files to share with others
  4. Search within files with grep
  5. Search among files with find
  6. Construct loops with bash
  7. Invoke conditionals with bash
  8. Write a bash script for repetitive tasks

Textbook Reference

Chapter 1, Pages 29-38

Activities

  1. open a terminal

  2. managing your environment

    • important environment variables

      • echo to see the values of each one: $USERNAME, $PWD, $PATH, $GROUP, $HOME, $PS1
      • env to see them all

    • set an environment variable with export bestClass=EP476 - note: no $

      • use echco to test that you set it correctly

    • set the prompt with export PS1="Have a great day <name>: [\w]"

    • make an alias to ls -ltr with alias lT='ls -ltr'

    • save your environment by editing .bashrc

      • new prompt
      • alias ls to be ls --color=auto

  3. permissions

    • find a partner and log in to each other's computer using ssh <username>@tux-??

    • change to the /tmp directory

    • make a directory there with your username and copy your sports.txt file to there

    • can you access each other's directories?

    • examine the permissions of those directories

    • change the permissions to allow cd but not ls

      • chmod o+x . or chmod a+x . or chmod o+x /tmp/<username>

    • make a subdirectory there with the other person's username

    • who can see that new directory?

    • change the permissions to allow cd and ls of the new directory

    • place a copy of sport.txt into the new directory

    • change the permissions of sport.txt to allow the other person to read it

    • copy the other person's sport.txt file to your own directory in /tmp

    • who can read/see which files?

  4. history

    • review the history of your commands using history
    • save the last 5 lines of your history to a file using history | tail -n 5 > recent_history.txt

  5. Searching in files: grep

    • go to the data/elements directory in the sandbox data
    • find the oxidization states of all elements with grep oxidization *
    • count the number of times that melting occurs in each file with grep -c melting *
    • find the files that do not have melting points with grep -c melting * | grep :0

  6. Searching for files: find

    • find all the files in the data directory that:

      • have ane in their name: find . -name "*ane*"
      • have a size larger than 1000 bytes: find . -size +1000c
      • have a size between 1000 and 10000 bytes: find . -size +1000c -size -10000c
      • have a size between 1000 and 10000 bytes and have ane in their name: find . -size +1000c -size -10000c -name "*ane*"

  7. Loops - consider python

    • basic syntax/construct: for <loop variable> in <list of values>; do .... ;done

    • go to data/elements directory

    • make a subdirectory tmp

    • copy all the elements to the tmp directory with new. prepended to its name

      • for element in *.xml; do cp $element tmp/new.$element; done

    • there are many ways to create a list of values, best to think of a string converted to items separated by whitespace

      • wildcards are interpretted as lists of files in the local directory, e.g. *.xml for all the files in this directory that match that pattern

      • surrounding a command with backticks (```) makes the output of that command useable as a string, e.g. find . -name "*.xml" for all the files in this directory or any subdirectory that match that string

  8. Conditionals

    • basic syntax/construct: if <test>; then ....; fi

    • many ways to format tests<http://tldp.org/LDP/Bash-Beginners-Guide/html/sect_07_01.html>_

    • some common examples for in scripts:

      • if [ "$#" -ne 1 ]; then will test whether there was 1 and only 1 argument
      • if [ -a "$1" ]; then will test whether the file name specified in the first argument exists

  9. Making scripts

    • like any language:

      • set variables
      • execute commands
      • conditionals
      • loops

    • many ways to execute

      • change file permissions to be executable and run like any command
      • source <filename>

  10. Getting help

    • It is very common for expert software engineers to rely on Google for assistance.
    • One of the best resources is Stack Overflow and Q&A posted there often comes up near the top of a Google search

Command-line Cheat Sheet