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The Shell
Mary Piper, Radhika Khetani, Meeta Mistry, Jihe Liu
October 26, 2020

Learning Objectives

  • Review shell commands and concepts

Setting up

This workshop assumes that you have either a) taken our Introduction to command-line interface workshop or b) been working on the command-line and are already fluent with shell/bash. We ask that you complete the exercises below, to refresh some basic commands that you will be using over the course of the workshop. For each section we have relevant materials linked as a helpful reference.

Opening up a terminal window

NOTE: This mandatory pre-work does not require you to login to the O2 cluster.

On your local laptop, you will need to open up your terminal window. This will be different depending on what kind of operating system (OS) you are working on.

With Mac OS

Macs have a utility application called "Terminal" for performing tasks on the command line (shell), both locally and on remote machines.

Please find and open the Terminal utility on your computers using the Spotlight Search at the top right hand corner of your screen.

With Windows OS

By default, there is no built-in Terminal that uses the bash shell on the Windows OS. So, we will be using a downloaded program called "Git BASH" which is part of the Git for Windows tool set. Git BASH is a shell/bash emulator. What this means is that it shows you a very similar interface to, and provides you the functionality of, the Terminal utility found on the Mac and Linux Operating systems.

Please find and open Git BASH.

Tip - Windows users can use another program called Putty instead of a bash emulator to log in to remote machines, but it is a little more involved and has different capabilities. We encourage you to take a look at it, but we will not be covering it in this workshop.

Downloading the example data folder

We will be exploring the capabilities of the shell by working with some RNA-Seq data. We need to download the data to our current folder using the link below. To do so, follow the step-by-step instructions below.

1. Find out what folder we are currently inside. To do this, we can use the 'print working directory' command:

$ pwd

On a Mac your current folder should be something starting with /Users/, like /Users/marypiper/.

On a Windows machine your current folder should be something starting with /c/Users/marypiper. To find this in your File explorer try clicking on PC and navigating to that path.

Once you have identified which folder you are in, this is where we will be downloading your data.

2. Click on the link below then go to file > download to download the data". This will automatically download the folder to your downloads folder. If you downloaded the data previously as a part of the Basic Shell workshop, you do not need to download it again unless you have deleted it.

3. Once you have downloaded the file to the correct location, go back to your terminal window and type the 'list' command:

$ ls

ls stands for 'list' and it lists the contents of a directory.

You should see unix_lesson.zip as part of the output to the screen.

4. Finally, to decompress the folder:

  • Double click on unix_lesson.zip on a mac. This will automatically inflate the folder.
  • If you are on windows, press and hold (or right-click) the folder, select Extract All..., and then follow the instructions.

5. Now when you run the ls command again you should see a folder called unix_lesson, which means you are all set with the data download!

$ ls

6. Go into the folder for the lesson

on mac type:

$ cd unix_lesson

on windows type:

$ cd unix_lesson/unix_lesson

Reviewing shell commands

Shell basics

We are going to start this review with some basic commands pertaining to navigating around the filesystem. Helpful reference materials are listed below:

  1. Change directory into the unix_lesson/ directory.
  2. Take a quick look at the Mov10_oe_1.subset.fq file (located in raw_fastq directory) using less from unix_lesson/, without changing directories.
  3. Use a shortcut to move out of the directory to the parent of unix_lesson/.
  4. Change directories into the raw_fastq/ folder with a single command.
  5. What does the ~ in the command prompt mean?
  6. What is the full path to the unix_lesson directory?
  7. List all the files in the raw_fastq directory.
  8. Modify the above command using the * wildcard to only list those files that have "oe" in their names.
  9. How many and which commands have you run so far?

Searching and redirection

Next, we will search our files for specific patterns and redirect the results to file. Helpful reference materials are listed below:

  1. Create a new directory called shell_review/ within the unix_lesson/ directory.
  2. Search the file unix_lesson/reference_data/chr1-hg19_genes.gtf for lines containing the string "MOV10". Save the output in the shell_review/ directory with a new name - "Mov10_hg19.gtf".
  3. Use vim to open the newly created file unix_lesson/shell_review/Mov10_hg19.gtf and add a comment at the top specifying how this file was created and the source of the content. Save the modified file and quit vim.
  4. In the new file "Mov10_hg19.gtf", how many lines contain the word "exon"?

Loops and shell scripts

  1. Use the for loop to iterate over each FASTQ file in raw_fastq and do the following:
    • Print the name of the current file
    • Generate a prefix to use for naming our output files, and store it inside a variable called sample.
    • Dump out the first 40 lines into a new file that will be saved in shell_review
  2. Place the above for loop into a shell script using vim and run it.

Permissions

There is a folder in the HBC training shared space on the O2 cluster called intro_rnaseq_hpc. Below we have displayed a long listing of its contents.

total 714
drwxrwsr-x  3 mm573 hbctraining 1111 Aug 22  2017 bam_STAR
drwxrwsr-x  8 mp298 hbctraining 1914 May 21  2018 bam_STAR38
drwxrwsr-x  2 mm573 hbctraining  522 Oct  6  2015 bam_tophat
drwxrwsr-x  2 mm573 hbctraining  240 Oct 19  2015 counts
drwxrwsr-x  2 mm573 hbctraining  260 Oct 19  2015 counts_STAR
-rw-rw-r--  1 mm573 hbctraining 2416 Aug 22  2017 DE_script.R
-rw-rw-r--  1 mm573 hbctraining 2064 Mar 28  2018 DESeq2_script.R
drwxrwsr-x  2 mm573 hbctraining  705 Oct  6  2015 fastqc
drwxrwsr-x  2 mm573 hbctraining  272 Jan 31  2018 full_dataset
-rw-rw-r--  1 mm573 hbctraining  216 Nov 10  2015 install_libraries.R
-rw-rw-r--  1 mm573 hbctraining  117 Oct 19  2015 install_libraries.sh
drwxrwsr-x 78 mm573 hbctraining 1969 Aug 22  2017 R-3.3.1
drwxrwsr-x  3 mp298 hbctraining  234 Feb 27  2019 reference_data_ensembl38
drwxrwsr-x  2 mm573 hbctraining  555 Oct  5  2015 reference_STAR
drwxrwsr-x  2 rsk27 hbctraining  260 Aug 22  2017 salmon.ensembl37.idx
drwxrwsr-x  2 mm573 hbctraining  306 Oct  6  2015 trimmed_fastq
  1. How many owners have files in this folder?
  2. How many groups?
  3. Are there any executable files in this folder?
  4. What kind of access does the user mm573 have to the full_dataset/ directory?
  5. You are considered as "other" or everyone else on this system (i.e you are not part of the group hbctraining. What command would allow the user mm573 do to take away your ability to look inside the full_dataset/ directory?

Environment variables

  1. Display the contents of the $HOME variable on your computer.
  2. Use the which command to check where the executable file for the pwd command lives in the directory structure.
  3. How does shell know where to find the executable file for the pwd command?
  4. Display the contents of the variable that stores the various paths to folders containing executable command files.

Review your answers

This lesson has been developed by members of the teaching team at the Harvard Chan Bioinformatics Core (HBC). These are open access materials distributed under the terms of the Creative Commons Attribution license (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.