Reference

Version control is like an unlimited ‘undo’.

Version control also allows many people to work in parallel.

Use git config with the --global option to configure a user name, email address, editor, and other preferences once per machine.

git init initializes a repository.

Git stores all of its repository data in the .git directory.

git status shows the status of a repository.

Files can be stored in a project’s working directory (which users see), the staging area (where the next commit is being built up) and the local repository (where commits are permanently recorded).

git add puts files in the staging area.

git commit saves the staged content as a new commit in the local repository.

Write commit messages that accurately describe your changes.

git log lists the commits made to the local repository.

git diff displays differences between commits.

git checkout recovers old versions of files.

git remote add origin links a local repository to a remote one and names it ‘origin’.

git push copies changes from a local repository to a remote repository.

git pull copies changes from a remote repository to a local repository.

Branches are versions of a repository that can contain different commits.

Pull requests on GitHub can be used to merge different branches together.

git clone copies a remote repository to create a local repository with a remote called origin automatically set up.

Conflicts occur when different commits change the same lines of the same file.

The version control system does not allow changes to overwrite each other, but highlights conflicts so that they can be resolved.

git checkout -b creates a new branch and checks it out at the same time.

git push -u links a local branch with an ‘upstream’ branch on a remote repository.

git pull can pull changes from one branch into another locally.

The .gitignore file tells Git what files to ignore.

Start the python interpreter by typing python in the shell.

Variables are named memory locations, they are used to access data.

A list is an ordered collection of items of any type.

Values in the list can be accessed using their index in square brackets e.g. my_list[ix]

Lists can be manipulated in place using attribute functions e.g. my_list.reverse()

Ranges of values in a list can be obtained via slicing e.g. mylist[start:stop]

We can use the ‘for in’ syntax to loop over collections or generators.

The python function open lets us read r or write w to files by creating a file handler.

We can use string operations such as line.split(',') to process data in files.

We can use logical operations to change the behavior of our code when it meets certain conditions.

Using if, elif, and else we can check conditions and add a branch that runs if none of the conditions are met.

We can combine conditions using and and or to make more complicated logical statements.

A function is created using the def keyword.

Functions take variables that are specified in the function definition and use the return keyword to specify their output.

We can use a module to keep our functions separate to the main body of our code to improve code readability.

Python has built in error names that give a hint to the type of problem you are looking for.

We can use the traceback to find which bit of the code threw an error.

Python uses the sys library to acess command line arguments. sys.argv is a list of command line arguments.

Python program outputs can be used in a pipeline, however, due to the way python works we need to use the signal library to make sure it handles piping output correctly.

Python has many libraries that add to the core language to improve functionality in specific use cases.

Numpy is a numerical python library that makes working with vectors, matricies, or large data tables easier.

Numpy can be used to load datasets directly from CSV files bypassing Pythons built in file systems.

We can use matplotlib to create and manipulate a wide variety of plots in Python.

Once a plot has been made we can use matplotlib’s function savefig to output it in formats appropriate for publication.

Pep8 provides a guide for styling your python code.

The shell lets you define repeatable workflows.

The shell is available on systems where graphical interfaces are not.

The file system is responsible for managing information on the disk.

Information is stored in files, which are stored in directories (folders).

Directories can also store other directories, which then form a directory tree.

cd [path] changes the current working directory.

ls [path] prints a listing of a specific file or directory; ls on its own lists the current working directory.

pwd prints the user’s current working directory.

/ on its own is the root directory of the whole file system.

Most commands take options (flags) that begin with a -.

A relative path specifies a location starting from the current location.

An absolute path specifies a location from the root of the file system.

Directory names in a path are separated with / on Unix, but \ on Windows.

.. means ‘the directory above the current one’; . on its own means ‘the current directory’.

Command line text editors let you edit files in the terminal.

You can open up files with either command-line or graphical text editors.

nano [path] creates a new text file at the location [path], or edits an existing one.

cat [path] prints the contents of a file.

rmdir [path] deletes an (empty) directory.

rm [path] deletes a file, rm -r [path] deletes a directory (and contents!).

mv [old_path] [new_path] moves a file or directory from [old_path] to [new_path].

mv can be used to rename files, e.g. mv a.txt b.txt.

Using . in mv can move a file without renaming it, e.g. mv a/file.txt b/..

cp [original_path] [copy_path] creates a copy of a file at a new location.

wc counts lines, words, and characters in its inputs.

cat displays the contents of its inputs.

sort sorts its inputs.

head displays the first 10 lines of its input.

tail displays the last 10 lines of its input.

command > [file] redirects a command’s output to a file (overwriting any existing content).

command >> [file] appends a command’s output to a file.

[first] | [second] is a pipeline: the output of the first command is used as the input to the second.

The best way to use the shell is to use pipes to combine simple single-purpose programs (filters).

Save commands in files (usually called shell scripts) for re-use.

bash [filename] runs the commands saved in a file.

$@ refers to all of a shell script’s command-line arguments.

$1, $2, etc., refer to the first command-line argument, the second command-line argument, etc.

Place variables in quotes if the values might have spaces in them.

Letting users decide what files to process is more flexible and more consistent with built-in Unix commands.

A for loop repeats commands once for every thing in a list.

Every for loop needs a variable to refer to the thing it is currently operating on.

Use $name to expand a variable (i.e., get its value). ${name} can also be used.

Do not use spaces, quotes, or wildcard characters such as ‘*’ or ‘?’ in filenames, as it complicates variable expansion.

Give files consistent names that are easy to match with wildcard patterns to make it easy to select them for looping.

Use the up-arrow key to scroll up through previous commands to edit and repeat them.

Use Ctrl+R to search through the previously entered commands.

Use history to display recent commands, and ![number] to repeat a command by number.

find finds files with specific properties that match patterns.

grep selects lines in files that match patterns.

--help is an option supported by many bash commands, and programs that can be run from within Bash, to display more information on how to use these commands or programs.

man [command] displays the manual page for a given command.

$([command]) inserts a command’s output in place.

date prints the current date in a specified format.

Scripts can save the output of a command to a variable using $(command)

basename removes directories from a path to a file, leaving only the name

cut lets you select specific columns from files, with -d',' letting you select the column separator, and -f letting you select the columns you want.