Added whistle stop tour of command line

Author: mParkesUCL

lessons/solving_problems/command_line.ipynb

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+# Using the command line
+
+It is often necessary (or quicker) to run Python from the command line. This is a *very* quick guide to using a command line, it will get you started.
+
+The first step is to open your terminal. How you do this depends on the type of PC you are using.
+
+On Windows see if the following are installed:
+-   Terminal (this is a more modern implementation)
+-   Powershell (if you have conda installed run Anaconda Powershell Prompt)
+-   Cmd (this is considered a bit out of date now, if you have conda installed run Anaconda Prompt)
+
+On a Mac
+-   Terminal
+
+On Linux it will depend on your distro. Generally look for an app called one of the following:
+-   Terminal
+-   Command
+-   Prompt
+-   Shell
+
+Once opened we can do various things. Initially it should look something like this:
+
+![An image of a terminal with just the current directory showing](/lessons/solving_problems/images/term_1.png)
+
+Depending on the 'shell' that you have you might have more information, such as your user name. We can now enter commands into this prompt.
+
+Most useful will be 
+
+| Command | Effect |
+|---|---|
+| *cd* | Followed by a path will change directory |
+| *ls* | Lists the contents of the current directory |
+| *mkdir* | Followed by a name will create a directory of that name |
+
+When we type a path we can do so in two ways. Either absolute or relative. Absolute is a full path from the users *root* directory. This is normally the folder you start in when you open a terminal. In the example above we start in Users/User. Say we want to open the Documents folder that is in this User folder. We can type the following:
+
+```bash
+ cd Users/User/Documents
+```
+However, we can also  do a relative path.
+
+```bash
+cd ./Documents
+```
+Here the path is relative to which ever directory we are *currently* in. A single period means the current folder while two periods means go up a level.
+
+For example, when we are in the Users/User/Documents folder, if we type:
+
+```bash
+cd ../
+```
+Then we will move back into the Users/User folder. Or again starting in Users/User/Documents
+```bash
+cd ../../
+```
+this will move us up two levels in the Users folder
+
+To then run a Python script, easiest is to navigate to the folder where the script is and run the following (assuming that the script is call python_prog.py)
+
+```bash
+python ./python_prog.py
+```
+