Add episode on environment variables

_config.yml

@@ -81,11 +81,12 @@ episode_order:
   - 11-connecting
   - 12-cluster
   - 13-scheduler
-  - 14-modules
-  - 15-transferring-files
-  - 16-parallel
-  - 17-resources
-  - 18-responsibility
+  - 14-environment-variables
+  - 15-modules
+  - 16-transferring-files
+  - 17-parallel
+  - 18-resources
+  - 19-responsibility
 
 #------------------------------------------------------------
 # Values for this lesson

_episodes/13-scheduler.md

@@ -235,8 +235,6 @@ later episode of this lesson.
 > {: .solution}
 {: .challenge}
 
-{% include {{ site.snippets }}/scheduler/print-sched-variables.snip %}
-
 Resource requests are typically binding. If you exceed them, your job will be
 killed. Let's use wall time as an example. We will request 1 minute of
 wall time, and attempt to run a job for two minutes.

_episodes/14-environment-variables.md

@@ -0,0 +1,258 @@
+---
+title: Environment Variables
+teaching: 10
+exercises: 5
+questions:
+- "How are variables set and accessed in the Unix shell?"
+- "How can I use variables to change how a program runs?"
+objectives:
+- "Understand how variables are implemented in the shell"
+- "Read the value of an existing variable"
+- "Create new variables and change their values"
+- "Change the behaviour of a program using an environment variable"
+- "Explain how the shell uses the `PATH` variable to search for executables"
+keypoints:
+- "Shell variables are by default treated as strings"
+- "Variables are assigned using \"`=`\" and recalled using the variable's name prefixed by \"`$`\""
+- "Use \"`export`\" to make an variable available to other programs"
+- "The `PATH` variable defines the shell's search path"
+---
+
+> ## Episode provenance
+>
+> This episode has been remixed from the
+> [Shell Extras episode on Shell Variables](https://github.com/carpentries-incubator/shell-extras/blob/gh-pages/_episodes/08-environment-variables.md)
+> and the [HPC Shell episode on scripts](https://github.com/hpc-carpentry/hpc-shell/blob/gh-pages/_episodes/05-scripts.md)
+{: .callout}
+
+The shell is just a program, and like other programs, it has variables.
+Those variables control its execution,
+so by changing their values
+you can change how the shell behaves (and with a little more effort how other
+programs behave).
+
+Variables
+are a great way of saving information under a name you can access later. In
+programming languages like Python and R, variables can store pretty much
+anything you can think of. In the shell, they usually just store text. The best
+way to understand how they work is to see them in action.
+
+Let's start by running the command `set` and looking at some of the variables
+in a typical shell session:
+
+~~~
+$ set
+~~~
+{: .language-bash}
+
+~~~
+COMPUTERNAME=TURING
+HOME=/home/vlad
+HOSTNAME=TURING
+HOSTTYPE=i686
+NUMBER_OF_PROCESSORS=4
+PATH=/Users/vlad/bin:/usr/local/git/bin:/usr/bin:/bin:/usr/sbin:/sbin:/usr/local/bin
+PWD=/home/vlad
+UID=1000
+USERNAME=vlad
+...
+~~~
+{: .output}
+
+As you can see, there are quite a few — in fact,
+four or five times more than what's shown here.
+And yes, using `set` to *show* things might seem a little strange,
+even for Unix, but if you don't give it any arguments,
+it might as well show you things you *could* set.
+
+Every variable has a name.
+All shell variables' values are strings,
+even those (like `UID`) that look like numbers.
+It's up to programs to convert these strings to other types when necessary.
+For example, if a program wanted to find out how many processors the computer
+had, it would convert the value of the `NUMBER_OF_PROCESSORS` variable from a
+string to an integer.
+
+## Showing the Value of a Variable
+
+Let's show the value of the variable `HOME`:
+
+~~~
+$ echo HOME
+~~~
+{: .language-bash}
+
+~~~
+HOME
+~~~
+{: .output}
+
+That just prints "HOME", which isn't what we wanted
+(though it is what we actually asked for).
+Let's try this instead:
+
+~~~
+$ echo $HOME
+~~~
+{: .language-bash}
+
+~~~
+/home/vlad
+~~~
+{: .output}
+
+The dollar sign tells the shell that we want the *value* of the variable
+rather than its name.
+This works just like wildcards:
+the shell does the replacement *before* running the program we've asked for.
+Thanks to this expansion, what we actually run is `echo /home/vlad`,
+which displays the right thing.
+
+## Creating and Changing Variables
+
+Creating a variable is easy — we just assign a value to a name using "="
+(we just have to remember that the syntax requires that there are _no_ spaces
+around the `=`!):
+
+~~~
+$ SECRET_IDENTITY=Dracula
+$ echo $SECRET_IDENTITY
+~~~
+{: .language-bash}
+
+~~~
+Dracula
+~~~
+{: .output}
+
+To change the value, just assign a new one:
+
+~~~
+$ SECRET_IDENTITY=Camilla
+$ echo $SECRET_IDENTITY
+~~~
+{: .language-bash}
+
+~~~
+Camilla
+~~~
+{: .output}
+
+## Environment variables
+
+When  we ran the `set` command we saw there were a lot of variables whose names
+were in upper case. That's because, by convention, variables that are also
+available to use by _other_ programs are given upper-case names. Such variables
+are called _environment variables_ as they are shell variables that are defined
+for the current shell and are inherited by any child shells or processes.
+
+To create an environment variable you need to `export` a shell variable. For
+example, to make our `SECRET_IDENTITY` available to other programs that we call
+from our shell we can do:
+
+~~~
+$ SECRET_IDENTITY=Camilla
+$ export SECRET_IDENTITY
+~~~
+{: .language-bash}
+
+You can also create and export the variable in a single step:
+
+~~~
+$ export SECRET_IDENTITY=Camilla
+~~~
+{: .language-bash}
+
+> ## Using environment variables to change program behaviour
+>
+> Set a shell variable `TIME_STYLE` to have a value of `iso` and check this
+> value using the `echo` command.
+>
+> Now, run the command `ls` with the option `-l` (which gives a long format).
+>
+> `export` the variable and rerun the `ls -l` command. Do you notice any
+> difference?
+>
+> > ## Solution
+> >
+> > The `TIME_STYLE` variable is not _seen_ by `ls` until is exported, at which
+> > point it is used by `ls` to decide what date format to use when presenting
+> > the timestamp of files.
+> >
+> {: .solution}
+{: .challenge}
+
+You can see the  complete set of environment variables in your current shell
+session with the command `env` (which returns a subset of what the command
+`set` gave us). **The complete set of environment variables is called
+your _runtime environment_ and can affect the behaviour of the programs you
+run**.
+
+{% include {{ site.snippets }}/scheduler/print-sched-variables.snip %}
+
+To remove a variable or environment variable you can use the `unset` command,
+for example:
+
+~~~
+$ unset SECRET_IDENTITY
+~~~
+{: .language-bash}
+
+## The `PATH` Environment Variable
+
+Similarly, some environment variables (like `PATH`) store lists of values.
+In this case, the convention is to use a colon ':' as a separator.
+If a program wants the individual elements of such a list,
+it's the program's responsibility to split the variable's string value into
+pieces.
+
+Let's have a closer look at that `PATH` variable.
+Its value defines the shell's search path for executables,
+i.e., the list of directories that the shell looks in for runnable programs
+when you type in a program name without specifying what directory it is in.
+
+For example, when we type a command like `analyze`,
+the shell needs to decide whether to run `./analyze` or `/bin/analyze`.
+The rule it uses is simple:
+the shell checks each directory in the `PATH` variable in turn,
+looking for a program with the requested name in that directory.
+As soon as it finds a match, it stops searching and runs the program.
+
+To show how this works,
+here are the components of `PATH` listed one per line:
+
+~~~
+/Users/vlad/bin
+/usr/local/git/bin
+/usr/bin
+/bin
+/usr/sbin
+/sbin
+/usr/local/bin
+~~~
+{: .output}
+
+On our computer,
+there are actually three programs called `analyze`
+in three different directories:
+`/bin/analyze`,
+`/usr/local/bin/analyze`,
+and `/users/vlad/analyze`.
+Since the shell searches the directories in the order they're listed in `PATH`,
+it finds `/bin/analyze` first and runs that.
+Notice that it will *never* find the program `/users/vlad/analyze`
+unless we type in the full path to the program,
+since the directory `/users/vlad` isn't in `PATH`.
+
+This means that I can have executables in lots of different places as long as
+I remember that I need to to update my `PATH` so that my shell can find them.
+
+What if I want to run two different versions of the same program?
+Since they share the same name, if I add them both to my `PATH` the first one
+found will always win.
+In the next episode we'll learn how to use helper tools to help us manage our
+runtime environment to make that possible without us needing to do a lot of
+bookkeeping on what the value of `PATH` (and other important environment
+variables) is or should be.
+
+{% include links.md %}

_includes/snippets_library/Birmingham_Baskerville_slurm/_config_options.yml

@@ -62,8 +62,9 @@ episode_order:
   - 11-connecting
   - 12-cluster
   - 13-scheduler
-  - 14-modules
-  - 15-transferring-files
-  - 16-parallel
-  - 17-resources
-  - 18-responsibility
+  - 14-environment-variables
+  - 15-modules
+  - 16-transferring-files
+  - 17-parallel
+  - 18-resources
+  - 19-responsibility

_includes/snippets_library/ComputeCanada_Graham_slurm/_config_options.yml

@@ -62,8 +62,9 @@ episode_order:
   - 11-connecting
   - 12-cluster
   - 13-scheduler
-  - 14-modules
-  - 15-transferring-files
-  - 16-parallel
-  - 17-resources
-  - 18-responsibility
+  - 14-environment-variables
+  - 15-modules
+  - 16-transferring-files
+  - 17-parallel
+  - 18-resources
+  - 19-responsibility

_includes/snippets_library/EPCC_Cirrus_slurm/_config_options.yml

@@ -68,8 +68,9 @@ episode_order:
   - 11-connecting
   - 12-cluster
   - 13-scheduler
-  - 14-modules
-  - 15-transferring-files
-  - 16-parallel
-  - 17-resources
-  - 18-responsibility
+  - 14-environment-variables
+  - 15-modules
+  - 16-transferring-files
+  - 17-parallel
+  - 18-resources
+  - 19-responsibility

_includes/snippets_library/HPCC_MagicCastle_slurm/_config_options.yml

@@ -58,8 +58,9 @@ episode_order:
   - 11-connecting
   - 12-cluster
   - 13-scheduler
-  - 14-modules
-  - 15-transferring-files
-  - 16-parallel
-  - 17-resources
-  - 18-responsibility
+  - 14-environment-variables
+  - 15-modules
+  - 16-transferring-files
+  - 17-parallel
+  - 18-resources
+  - 19-responsibility

_includes/snippets_library/Magic_Castle_EESSI_slurm/_config_options.yml

@@ -71,14 +71,15 @@ sched:
   comment: "#SBATCH"
   hist: "sacct -u yourUsername"
   hist_filter: ""
-  
+
 episode_order:
   - 10-hpc-intro
   - 11-connecting
   - 12-cluster
   - 13-scheduler
-  - 14-modules
-  - 15-transferring-files
-  - 16-parallel
-  - 17-resources
-  - 18-responsibility
+  - 14-environment-variables
+  - 15-modules
+  - 16-transferring-files
+  - 17-parallel
+  - 18-resources
+  - 19-responsibility

_includes/snippets_library/NIST_CTCMS_slurm/_config_options.yml

@@ -63,7 +63,8 @@ episode_order:
   - 11-connecting
   - 12-cluster
   - 13-scheduler
-  - 15-transferring-files
-  - 16-parallel
-  - 17-resources
-  - 18-responsibility
+  - 14-environment-variables
+  - 16-transferring-files
+  - 17-parallel
+  - 18-resources
+  - 19-responsibility

_includes/snippets_library/Norway_SIGMA2_SAGA_slurm/_config_options.yml

@@ -62,8 +62,9 @@ episode_order:
   - 11-connecting
   - 12-cluster
   - 13-scheduler
-  - 14-modules
-  - 15-transferring-files
-  - 16-parallel
-  - 17-resources
-  - 18-responsibility
+  - 14-environment-variables
+  - 15-modules
+  - 16-transferring-files
+  - 17-parallel
+  - 18-resources
+  - 19-responsibility