Writing Scripts and Working with Data
OverviewTeaching: 20 min
Exercises: 20 minQuestions
How can we automate a commonly used set of commands?Objectives
nanotext editor to modify text files.
Write a basic shell script.
bashcommand to execute a shell script.
chmodto make a script an executable program.
We’ve been able to do a lot of work with files that already exist, but what if we want to write our own files? We’re not going to type in an actual dataset, but we’ll see as we go through other tutorials, there are a lot of reasons we’ll want to write a file, or edit an existing file.
To add text to files, we’re going to use a text editor called Nano. We’re going to create a file to take notes about what we’ve been doing with the data files in
This is good practice when doing data wrangling and analysis. We can create a file called
README.txt that describes the data files in the directory or documents how the files in that directory were generated. As the name suggests, it’s a file that we or others should read to understand the information in that directory.
Let’s change our working directory to
nano to create a file called
$ cd ~/Downloads/shell-economics/data/worldbank $ nano README.txt
You should see something like this:
The text at the bottom of the screen shows the keyboard shortcuts for performing various tasks in
nano. We will talk more about how to interpret this information soon.
When we say, “
nanois a text editor,” we really do mean “text”: it can only work with plain character data, not tables, images, or any other human-friendly media. We use it in examples because it is one of the least complex text editors. However, because of this trait, it may not be powerful enough or flexible enough for the work you need to do after this workshop. On Unix systems (such as Linux and Mac OS X), many programmers use Emacs or Vim (both of which require more time to learn), or a graphical editor such as Gedit. On Windows, you may wish to use Notepad++. Windows also has a built-in editor called
notepadthat can be run from the command line in the same way as
nanofor the purposes of this lesson.
No matter what editor you use, you will need to know where it searches for and saves files. If you start it from the shell, it will (probably) use your current working directory as its default location. If you use your computer’s start menu, it may want to save files in your desktop or documents directory instead. You can change this by navigating to another directory the first time you “Save As…”
Let’s type in a few lines of text. Describe what the files in this
directory are or what you’ve been doing with them.
Once we’re happy with our text, we can press Ctrl-O (press the Ctrl or Control key and, while
holding it down, press the O key) to write our data to disk. You’ll be asked what file we want to save this to:
press Return to accept the suggested default of
Once our file is saved, we can use Ctrl-X to quit the editor and return to the shell.
Control, Ctrl, or ^ Key
The Control key is also called the “Ctrl” key. There are various ways in which using the Control key may be described. For example, you may see an instruction to press the Ctrl key and, while holding it down, press the X key, described as any of:
nano, along the bottom of the screen you’ll see
^G Get Help ^O WriteOut. This means that you can use Ctrl-G to get help and Ctrl-O to save your file.
Now you’ve written a file. You can take a look at it with
cat, or open it up again and edit it with
README.txtand add the date to the top of the file and save the file.
nano README.txtto open the file.
Add today’s date and then use Ctrl-X followed by
yand Enter to save.
A really powerful thing about the command line is that you can write scripts. Scripts let you save commands to run them and also lets you put multiple commands together. Though writing scripts may require an additional time investment initially, this can save you time as you run them repeatedly. Scripts can also address the challenge of reproducibility: if you need to repeat an analysis, you retain a record of your command history within the script.
FIXME: change to econ relevant example
One thing we will commonly want to do with sequencing results is pull out bad reads and write them to a file to see if we can figure out what’s going on with them. We’re going to look for reads with long sequences of N’s like we did before, but now we’re going to write a script, so we can run it each time we get new sequences, rather than type the code in by hand each time.
We’re going to create a new file to put this command in. We’ll call it
sh isn’t required, but using that extension tells us that it’s a shell script.
$ nano bad-reads-script.sh
Bad reads have a lot of N’s, so we’re going to look for
grep. We want the whole FASTQ record, so we’re also going to get the one line above the sequence and the two lines below. We also want to look in all the files that end with
.fastq, so we’re going to use the
grep -B1 -A2 NNNNNNNNNN *.fastq > scripted_gdp_per_capita.csv
grep command into the file and save it as before. Be careful that you did not add the
$ at the beginning of the line.
Now comes the neat part. We can run this script. Type:
$ bash bad-reads-script.sh
It will look like nothing happened, but now if you look at
scripted_gdp_per_capita.csv, you can see that there are now reads in the file.
We want the script to tell us when it’s done.
bad-reads-script.shand add the line
echo "Script finished!"after the
grepcommand and save the file.
- Run the updated script.
$ bash bad-reads-script.sh Script finished!
Making the script into a program
We had to type
bash because we needed to tell the computer what program to use to run this script. Instead, we can turn this script into its own program. We need to tell it that it’s a program by making it executable. We can do this by changing the file permissions. We talked about permissions in an earlier episode.
First, let’s look at the current permissions.
$ ls -l bad-reads-script.sh
-rw-rw-r-- 1 dcuser dcuser 0 Oct 25 21:46 bad-reads-script.sh
We see that it says
-rw-r--r--. This shows that the file can be read by any user and written to by the file owner (you). We want to change these permissions so that the file can be executed as a program. We use the command
chmod like we did earlier when we removed write permissions. Here we are adding (
+) executable permissions (
$ chmod +x bad-reads-script.sh
Now let’s look at the permissions again.
$ ls -l bad-reads-script.sh
-rwxrwxr-x 1 dcuser dcuser 0 Oct 25 21:46 bad-reads-script.sh
Now we see that it says
x’s that are there now tell us we can run it as a program. So, let’s try it! We’ll need to put
./ at the beginning so the computer knows to look here in this directory for the program.
The script should run the same way as before, but now we’ve created our very own computer program!
You will learn more about writing scripts in a later lesson.
Moving and Downloading Data
So far, we’ve worked with data that is pre-loaded on the instance in the cloud. Usually, however, most analyses begin with moving data onto the instance. Below we’ll show you some commands to download data onto your instance, or to move data between your computer and the cloud.
Getting data from the cloud
There are two programs that will download data from a remote server to your local
(or remote) machine:
curl. They were designed to do slightly different
tasks by default, so you’ll need to give the programs somewhat different options to get
the same behaviour, but they are mostly interchangeable.
wgetis short for “world wide web get”, and it’s basic function is to download web pages or data at a web address.
cURLis a pun, it is supposed to be read as “see URL”, so its basic function is to display webpages or data at a web address.
Which one you need to use mostly depends on your operating system, as most computers will only have one or the other installed by default.
Let’s say you want to download some data from Ensembl. We’re going to download a very small
tab-delimited file that just tells us what data is available on the Ensembl bacteria server.
Before we can start our download, we need to know whether we’re using
To see which program you have, type:
$ which curl $ which wget
which is a BASH program that looks through everything you have
installed, and tells you what folder it is installed to. If it can’t
find the program you asked for, it returns nothing, i.e. gives you no
On Mac OSX, you’ll likely get the following output:
$ which curl
$ which wget
This output means that you have
curl installed, but not
Once you know whether you have
wget, use one of the
following commands to download the file:
$ cd $ wget ftp://ftp.ensemblgenomes.org/pub/release-37/bacteria/species_EnsemblBacteria.txt
$ cd $ curl -O ftp://ftp.ensemblgenomes.org/pub/release-37/bacteria/species_EnsemblBacteria.txt
Since we wanted to download the file rather than just view it, we used
any modifiers. With
curl however, we had to use the -O flag, which simultaneously tells
download the page instead of showing it to us and specifies that it should save the
file using the same name it had on the server: species_EnsemblBacteria.txt
It’s important to note that both
wget download to the computer that the
command line belongs to. So, if you are logged into AWS on the command line and execute
curl command above in the AWS terminal, the file will be downloaded to your AWS
machine, not your local one.
Moving files between your laptop and your instance
What if the data you need is on your local computer, but you need to get it into the cloud? There are also several ways to do this, but it’s always easier to start the transfer locally. This means if you’re typing into a terminal, the terminal should not be logged into your instance, it should be showing your local computer. If you’re using a transfer program, it needs to be installed on your local machine, not your instance.
Transferring Data Between your Local Machine and the Cloud
These directions are platform specific, so please follow the instructions for your system:
Please select the platform you wish to use for the exercises:
Uploading Data to your Virtual Machine with scp
scp stands for ‘secure copy protocol’, and is a widely used UNIX tool for moving files
between computers. The simplest way to use
scp is to run it in your local terminal,
and use it to copy a single file:
scp <file I want to move> <where I want to move it>
Note that you are always running
scp locally, but that doesn’t mean that
you can only move files from your local computer. In order to move a file from your local computer to an AWS instance, the command would look like this:
$ scp <local file> <AWS instance>
To move it back to your local computer, you re-order the
$ scp <AWS instance> <local file>
Uploading Data to your Virtual Machine with scp
Open the terminal and use the
scp command to upload a file (e.g. local_file.txt) to the dcuser home directory:
$ scp local_file.txt email@example.com:/home/dcuser/
Downloading Data from your Virtual Machine with scp
Let’s download a text file from our remote machine. You should have a file that contains bad reads called ~/shell_data/scripted_gdp_per_capita.csv.
Tip: If you are looking for another (or any really) text file in your home directory to use instead, try:
$ find ~ -name *.txt
Download the bad reads file in ~/shell_data/scripted_gdp_per_capita.csv to your home ~/Download directory using the following command (make sure you substitute firstname.lastname@example.org with your remote login credentials):
$ scp email@example.com:/home/dcuser/shell_data/untrimmed_fastq/scripted_gdp_per_capita.csv ~/Downloads
Remember that in both instances, the command is run from your local machine, we’ve just flipped the order of the to and from parts of the command.
Uploading Data to your Virtual Machine with PSCP
If you’re using a PC, we recommend you use the PSCP program. This program is from the same suite of tools as the PuTTY program we have been using to connect.
- If you haven’t done so, download pscp from http://the.earth.li/~sgtatham/putty/latest/x86/pscp.exe
- Make sure the PSCP program is somewhere you know on your computer. In this case, your Downloads folder is appropriate.
- Open the windows PowerShell; go to your start menu/search enter the term ‘cmd’; you will be able to start the shell (the shell should start from C:\Users\your-pc-username>).
- Change to the Downloads directory:
> cd Downloads
- Locate a file on your computer that you wish to upload (be sure you know the path). Then upload it to your remote machine (you will need to know your AMI instance address (which starts with ec2), and login credentials). You will be prompted to enter a password, and then your upload will begin. (make sure you substitute ‘your-pc-username’ for your actual pc username and ‘ec2-54-88-126-85.compute-1.amazonaws.com’ with your AMI instance address)
C:\User\your-pc-username\Downloads> pscp.exe local_file.txt firstname.lastname@example.org:/home/dcuser/
Downloading Data from your Virtual Machine with PSCP
- Follow the instructions in the Upload section to download (if needed) and access the PSCP program (steps 1-3)
- Download the text file using the following command (make sure you substitute ‘your-pc-username’ for your actual pc username and ‘ec2-54-88-126-85.compute-1.amazonaws.com’ with your AMI instance address)
C:\User\your-pc-username\Downloads> pscp.exe email@example.com:/home/dcuser/shell_data/untrimmed_fastq/scripted_gdp_per_capita.csv. C:\User\your-pc-username\Downloads
Scripts are a collection of commands executed together.
Transferring information to and from virtual and local computers.