Introduction to research computing on the Palmetto cluster

Configuration and .bashrc

Learning objectives

  • Understand how to define and export variables
  • Understand the concept of environment variables
  • Understand where the shell looks for things
  • Edit the .bashrc to configure the shell and modify the environment

Let’s create a directory called thesis and create a file there:

$ cd
$ mkdir thesis
$ touch ~/thesis/draft.txt

To access this directory thesis from anywhere in the filesystem, we have to use the absolute path to it:

$ cd /home/username/thesis

If we (or our programs) are going to be accessing the directory a lot, this can be inconvenient. Fortunately, the shell let’s us define variables that can make it easier. Let’s create a variable that refers to this directory:

$ THESIS_DIR=/home/username/thesis

Note that the equal to sign (=) cannot have spaces to its left or right. Now, we can simply use the variable to access the directory:

$ ls

Of course, variables don’t have to refer to just paths, they can refer to anything:

$ NUM=4
$ echo $NUM

Let’s create a simple bash script that tries to print this value of NUM:

# script

echo $NUM
$ bash

Oops. It doesn’t look like the script knows about this variable yet. To make the variable accessible to the script, we’ll have to export the variable:

export NUM=4
$ bash

When you export a variable, it gets added to the shell’s environment. Variables in the environment are accessible to other programs. You can have a look at all the current environment variables using the env command:

$ env
SSH_CLIENT= 63708 22

If you look in the list, you will find the NUM variable we exported earlier too.

Environment variables are a convenient way to configure programs. For example, the bash shell uses the environment variable PS1 to decide what the prompt should look like. Let’s change the variable to see this in action:

$ export PS1='>> '

The prompt is now changed to >> (to return back to your previous prompt, you have to log out and log back in).

Although PS1 is not an environment variable that you may want to change, it’s a good example of how programs use environment variables to configure their behaviour.

The PATH environment variable

An important environment variable is PATH. Let’s have a look at it:

$ echo $PATH

The output is a list of locations, separated by the : character. These are locations on the filesystem that the shell will look for executables. When you type in, for example, the whatsfree command - it looks in each of the above locations for an executable called whatsfree. If it finds it in any of those locations, it executes it, and if not, it gives you an error. To confirm that the whatsfree executable is actually in one of the above locations, you can use the which command:

which whatsfree

You see that whatsfree is in the directory /usr/bin, which is included in the PATH.

Installing software

As an example of installing software, we will install the Hello package. We start by downloading and unpacking the software:

$ wget
--2016-06-17 05:29:15--
Resolving, 2001:4830:134:3::b
Connecting to||:80... connected.
HTTP request sent, awaiting response... 200 OK
Length: 725946 (709K) [application/x-gzip]
Saving to: 'hello-2.10.tar.gz'

hello-2.10.tar.gz                       100%[=============================================================================>] 708.93K  1.11MB/s    in 0.6s

2016-06-17 05:29:16 (1.11 MB/s) - 'hello-2.10.tar.gz' saved [725946/725946]

Once downloaded, we’ll have to unpack it. This .tar.gz file is a popular format on unix systems, and needs to be unpacked using the tar command:

$ tar -xvf hello-2.10.tar.gz
x hello-2.10/
x hello-2.10/COPYING
x hello-2.10/tests/
x hello-2.10/tests/greeting-1
x hello-2.10/tests/traditional-1
x hello-2.10/tests/greeting-2
x hello-2.10/tests/hello-1
x hello-2.10/tests/last-1
x hello-2.10/
x hello-2.10/
x hello-2.10/
x hello-2.10/README
x hello-2.10/INSTALL
x hello-2.10/NEWS
x hello-2.10/GNUmakefile

To begin installing the package, let’s cd into the newly created hello-2.10 directory, and have look at the INSTALL file:

$ cd hello-2.10
$ less INSTALL