Keeping track of your work

What is Git?

Git is a version control system. That means that it allows you to track changes made to files over time by taking snapshots. Furthermore it allows you to view differences between snapshots and revert back to previous snapshots. In fact Git can do a lot more than this, but this is all we need to know for this chapter. We will go into more detail on the collaborative aspects of Git in the Collaborating on projects chapter.

Git tracks snapshots of files in what is called a repository. You can have many Git repositories. On my computer I have a directory named projects that currently contains around fifty sub-directories representing the projects that I have been working on recently. Each one of these sub-directories is a separate Git repository.

Git can track some or all of the files in a repository. The files can be located in the top level directory or in sub-directories. However, Git cannot track any files that live in parent directories of the base Git repository, e.g. none of my Git repositories can track files located in the parent projects directory.

In a Git repository files can be in one of four states: untracked, staged, unmodified and modified. When a file is initially created it is in an untracked state, meaning that it is not yet under version control. To start tracking the file one adds it to Git, and the state of the file then changes to staged. This means that the file is staged to be included in the next snapshot. Multiple files can be in a staged state when one takes a snapshot. To take a snapshot one commits all the files that are in the so called “staging area”. The state of the files then changes from staged to unmodified, see figure Fig. 4.

Any subsequent editing of the files under version control would result in their state changing from unmodified to modified. When one is happy with the edits made one would then add the files to the staging area and their state would change from modified to staged. At that point one is ready to take another snapshot by committing the staged edits. The state of the files then, again, change from staged to unmodified. And the cycle continues (Fig. 4).

Fig. 4 Diagram illustrating the states a file can have in a Git repository. The commands git add and git commit are the key components to creating a snapshot. The vim command symbolises editing the file using the vim text editor, but any program altering the content of the file will result in the file being marked as modified by Git.

First time configuration of Git

Git may or may not be installed by default on your system. To find out if it is try running the command below.

$ git --version

If Git is installed you will see output along the lines of the below.

git version 2.6.3

If Git is not installed you will see a command not found message. In this case you will need to install it. If you do not know how to do this you can find general instructions on how to install software in Managing your system.

Although we won’t go into it in this chapter, Git is fundamentally a collaboration tool that helps people work on projects together. This means that we need to give Git some information about us for it to be able to keep track of who has done what, specifically our name and email address.

$ git config --global user.name "Tjelvar Olsson"
$ git config --global user.email "tjelvar@biologistsguide2computing.com"

We will look at the collaboration aspect of git in the Collaborating on projects chapter.

Initialise the project

The first thing to do is to initialise the project using the git init command. If run with no argument it will set up tracking of files in the current working directory. If given an argument, such as protein-count, git will create a new directory with this name and set up tracking of files within it.

git init protein-count
cd protein-count/

Use your editor of choice and create the markdown file README.md and add the content below to it.

# Protein count

Count the number of proteins of particular species
in a SwissProt FASTA file.

As mentioned, files in a Git repository, the project directory, can be in one of four states: untracked, unmodified, modified and staged. To view the state one can use the command git status.

$ git status

The command below produces the output below.

On branch master

Initial commit

Untracked files:
  (use "git add <file>..." to include in what will be committed)

        README.md

nothing added to commit but untracked files present (use "git add" to track)

This tells us that the README.md file is untracked, in other words it is not yet under version control in Git. However, we would like to track it, so we add it to the Git repository using the git add command.

$ git add README.md

Let’s see how this affected the status of the repository.

$ git status
On branch master

Initial commit

Changes to be committed:
  (use "git rm --cached <file>..." to unstage)

        new file:   README.md

This stages the README.md file to be committed. To commit the current snapshot of the project to the Git repository we use the git commit command.

$ git commit -m "Added readme file."
[master (root-commit) e1dc880] Added readme file.
 1 file changed, 12 insertions(+)
 create mode 100644 README.md

In the above the -m option allows us to specify a commit message on the command line. The commit message should describe the change that your are committing. It will be useful when you review the project at a later date. It also helps communicate your change to any collaborators working on the same project.

Again, let’s see how this affected the status of the repository.

$ git status
On branch master
nothing to commit, working directory clean

That’s all you need to know to get started with Git. Start by initialising a project using git init. Then use git add and git commit iteratively to stage and commit snapshots of your project to version control.

Create a script for downloading the SwissProt FASTA file

We will now convert the command we used to download the SwissProt FASTA file from First steps towards automation into a script. To add some organisation we will put this script in a directory named scripts. We will also create a directory named data for storing the downloaded file. By specifying more than one argument to the mkdir command one can create multiple directories.

$ mkdir scripts data

Using your favorite text editor enter the text below into the file scripts/get_data.bash.

#!/bin/bash

curl --location --output data/uniprot_sprot.fasta.gz http://bit.ly/1l6SAKb

The only difference between this script and the command we entered on the command line is the first line #!/bin/bash. This is a special construct, called the shebang, and is used to specify the shell to use when executing the content of the file.

However, in order to be able to execute the file, i.e. run it as a program, it needs to have “execute permissions”. One can view the current set of permissions of a file by using ls -l (mnemonics ls list, -l long).

$ ls -l scripts/get_data.bash
-rw-r--r--  1 olssont  1340193827  88 29 Nov 10:45 scripts/get_data.bash

Note the first ten characters, the first specifies the file type and the remaining nine relate to the permissions of the file, see Fig. 5. There are three modes that can be turned on or off: read (r), write (w) and execute (x). Furthermore, these can be specified for the owner (u), group (g) and all users (a or o). The nine characters above state that the owner has read and write permissions on the file rw-, whereas both the group and all other users only have permission to read the file r--.

Fig. 5 Figure illustrating how the file permissions string should be interpreted. In the above the user has read, write and execute permissions. The members of the group have read and exectue permissions. All other users only have execute permissions on the file. In this case the file type character - indicates that the file is a regular/executable file. Other file type characters include d and l which are used to represent directories and symbolic links respectively.

Let’s take a moment to expand on the concept of groups in Unix-like operating systems. A user can be part of several groups. However, a file can only belong to one group. For example a PhD student could be part of the groups famous-project-leader-group and awesome-institute-group. In this hypothetical scenario the default group for the PhD student is the famous-project-leader-group. Any files that the student generates would therefore be assigned the group famous-project-leader-group. If the student wanted to make a file more widely accessible throughout the institute they could change the file’s group to awesome-institute-group.

Let us give the file execute permissions. This is achieved using the chmod command, mnemonic “change file modes”. The chmod command can be invoked in a number of different ways. Here we use the symbolic mode to specify that the user and the group (ug) should be given execute permissions (+x) on the scripts/get_data.bash file.

$ chmod ug+x scripts/get_data.bash
$ ls -l scripts/get_data.bash
-rwxr-xr--  1 olssont  1340193827  88 29 Nov 10:45 scripts/get_data.bash

Let us test the script by running it.

$ ./scripts/get_data.bash
$ ls data/
uniprot_sprot.fasta.gz

The file was downloaded to the data directory, success!

This is a good time to add the script to version control.

$ git add scripts/get_data.bash
$ git commit -m "Added script for downloading SwissProt FASTA file."
[master f80731e] Added script for downloading SwissProt FASTA file.
 1 file changed, 3 insertions(+)
 create mode 100755 scripts/get_data.bash

Let us check the status of our project.

$ git status
On branch master
Untracked files:
  (use "git add <file>..." to include in what will be committed)

        data/

nothing added to commit but untracked files present (use "git add" to track)

Git is telling us that there are files in the data directory that are currently not being tracked. However, in this project the data directory will contain files downloaded from a canonical resource and as the download script is in version control we do not need or want to track the files in this directory.

It is possible to tell Git to ignore files. Using your text editor of choice create the file .gitignore and add the content below to it.

data/*

In Bash the * symbol represents a wild card pattern that can match any string. The * symbol can be used in the same fashion in the .gitignore file. As such the line we added to our .gitignore file tells Git to ignore all files in the data directory.

$ git status
On branch master
Untracked files:
  (use "git add <file>..." to include in what will be committed)

        .gitignore

nothing added to commit but untracked files present (use "git add" to track)

Git now ignores the content of the data directory and tells us that the .gitignore file is untracked. Let us add this file.

$ git add .gitignore
$ git commit -m "Added gitignore file."
$ git status
On branch master
nothing to commit, working directory clean

Improve script for downloading SwissProt FASTA file

However, the current setup has got an issue in terms of reproducibility. Depending on when the SwissProt FASTA file was downloaded one may obtain different results. It would therefore be useful to include the date of access in the file name. This can be achieved using the date command.

$ date
Thu 26 Nov 2015 09:20:32 GMT

The date command can be configured to create custom output formats using the + symbol followed by a string template specifying the desired format. In the below %Y, %m and %d will be replaced by the year, month and day respectively.

$ date +'%Y-%m-%d'
2015-11-26

To get the output of the date command into the file name one can use Bash’s concept of command substitution. Command substitution makes it possible to evaluate the result of a command within a string. To see this in action we can use the echo command, which simply prints out the string that it is given.

$ echo "Today it is $(date +'%d')th"
Today it is 26th

It is time to introduce the concept of a variable. A variable is basically a means of storing a piece of information using a descriptive name. In bash one can assign a variable using the equals character (=). Below we create a variable named PRIBNOV_BOX and assign it the value TATAAT.

$ PRIBNOV_BOX="TATAAT"

The value of the variable can then be accessed by prefixing the variable name with the dollar character ($).

$ echo $PRIBNOV_BOX
TATAAT

We now have all the information we need to improve the script. Edit the script/get_data.bash file to look like the below.

#!/bin/bash

FNAME="data/uniprot_sprot.$(date +'%Y-%m-%d').fasta.gz"
curl --location --output $FNAME http://bit.ly/1l6SAKb

Let’s try running the script.

$ ./scripts/get_data.bash

Now we can check that the script has produced an appropriately named file.

$ ls data/
uniprot_sprot.2015-11-26.fasta.gz uniprot_sprot.fasta.gz

We have added a piece of functionality and have tested that it works as expected. This is a good time to commit our changes to Git. However, before we do that let us examine the changes to the project since the last commit using the git diff command.

$ git diff
diff --git a/scripts/get_data.bash b/scripts/get_data.bash
index d8e9bda..338d82c 100755
--- a/scripts/get_data.bash
+++ b/scripts/get_data.bash
@@ -1,3 +1,4 @@
 #!/bin/bash

-curl --location --output data/uniprot_sprot.fasta.gz http://bit.ly/1l6SAKb
+FNAME="data/uniprot_sprot.$(date +'%Y-%m-%d').fasta.gz"
+curl --location --output $FNAME http://bit.ly/1l6SAKb

The command above tells us that one line has been removed, the one prefixed by a minus sign, and that two lines have been added, the ones prefixed by a plus sign. In fact we have modified one line and added one, but the effect is the same.

Let us now add and commit the changes to Git. We need to do this as working with Git is a cyclical process. You make changes by editing the files, you add the changes that you want to snapshot to the staging area, then you commit the staged changes. At this point the cycle starts all over again Fig. 4.

$ git add scripts/get_data.bash
$ git commit -m "Updated download script to include date in file name."
[master 7512894] Updated download script to include date in file name.
 1 file changed, 2 insertions(+), 1 deletion(-)

By adding the date of download to the file name reproducibility is improved and it means that we can download the file on different dates and ensure that no data is overwritten.

However, it is still possible to accidentally delete or modify the data file. To overcome this, and further improve reproducibility, it is good practise to give the data file read-only permissions. This means that the file cannot be modified or deleted, only read. To do this we will make use of the chmod command. In this instance we will make use of an absolute mode. Absolute modes encode the permissions using the numbers 1, 2 and 4 that represent execute, write and read modes respectively. These numbers can be combined to create any permission, for example 7 represents read, write and execute permissions and 5 represents read and execute permissions.

To set the permissions for the owner, group and all other users one simply uses three such numbers. For example to give the owner read and write permissions and the group and all other users read-only permissions one would use the absolute mode 644.

In this instance we want to set the file to read-only for the owner, group and all other users so we will use the absolute mode 444.

#!/bin/bash

FNAME="data/uniprot_sprot.$(date +'%Y-%m-%d').fasta.gz"
curl --location --output $FNAME http://bit.ly/1l6SAKb
chmod 444 $FNAME

If you run the script now you will see that it changes the permissions of the downloaded file. If you run the script again, on the same day, you will notice that the it complains that it has not got permissions to write to the file. This is expected as the curl command is wanting to overwrite the existing read-only file.

Let’s add these changes to the staging area.

$ git add scripts/get_data.bash

It is good practise to try to make the commit message no more than 50 characters long. Sometimes this is not enough. In these cases you can create a multi line commit message using a text editor (likely to be vim by default) by omitting the -m flag.

Let’s try this now.

$ git commit

This should open a text editor with the text below.

# Please enter the commit message for your changes. Lines starting
# with '#' will be ignored, and an empty message aborts the commit.
# On branch master
# Changes to be committed:
#       modified:   scripts/get_data.bash

Use your text editor to edit this message to the below.

Set permissions of data file to read only

The intention of this change is to prevent accidental deletion or
modification of the raw data file.

# Please enter the commit message for your changes. Lines starting
# with '#' will be ignored, and an empty message aborts the commit.
# On branch master
# Changes to be committed:
#       modified:   scripts/get_data.bash

When you save the file you should see the output below in the terminal.

$ git commit
[master ad2a4c5] Set permissions of data file to read only
 1 file changed, 1 insertion(+)

$ chmod u+w file_i_want_to_edit.txt

Create script for counting the number of proteins in a genome

Now that we have a script for downloading the SwissProt FASTA file let us convert what we learnt in First steps towards automation into a script for counting the number of proteins for a particular species.

Add the lines below to the file scripts/protein_count.bash.

#!/bin/bash

gunzip -c data/uniprot_sprot.fasta.gz | grep 'OS=Homo sapiens' \
| cut -d '|' -f 2 | wc -l

Make the file executable and test the script.

$ chmod +x scripts/protein_count.bash
$ ./scripts/protein_count.bash
   20194

At the moment the path to the data file and the species are hard coded into the script. It would be nice if we could turn these two parameters into command line arguments. We can do this using the special variables $1 and $2 that represent the first and second command line arguments, respectively.

#!/bin/bash

DATA_FILE_PATH=$1
SPECIES=$2
echo "Input file: $DATA_FILE_PATH"
echo "Species: $SPECIES"

gunzip -c $DATA_FILE_PATH | grep "OS=$SPECIES" \
| cut -d '|' -f 2 | wc -l

$ SPECIES=H.sapiens
$ echo "Species: $SPECIES"
Species: H.sapiens
$ echo 'Species: $SPECIES'
Species: $SPECIES

This is a good point to test if things are working as expected.

$ ./scripts/protein_count.bash data/uniprot_sprot.2015-11-26.fasta.gz "Homo sapiens"
Input file: data/uniprot_sprot.2015-11-26.fasta.gz
Species: Homo sapiens
   20194

Success! Let us add and commit the script to Git.

$ git add scripts/protein_count.bash
$ git commit -m "Added script for counting the numbers of proteins."
[master b9de9bc] Added script for counting the numbers of proteins.
 1 file changed, 9 insertions(+)
 create mode 100755 scripts/protein_count.bash

More useful git commands

We’ve covered a lot of ground in this chapter. Can you remember everything that we did and the motivation behind each individual step? If not, that is okay, we can use Git to remind us using the git log command.

$ git log --oneline
b9de9bc Added script for counting the numbers of proteins.
a672257 Added command to set permissions of data file to read only.
7512894 Updated download script to include date in file name.
6c6f65b Added gitignore file.
f80731e Added script for downloading SwissProt FASTA file.
e1dc880 Added readme file.

Note that the comments above give a decent description of what was done. However, it would have been useful to include more information about the motive behind some changes. If one does not make use of the -m argument when using git commit one can use the default text editor to write a more comprehensive commit message. For example, a more informative commit message for commit a672257 could have looked something along the lines of:

Added command to set permissions of data file to read only.

The intention of this change is to prevent accidental deletion or
modification of the raw data file.

Another useful feature of Git is that it allows us to inspect the changes between commits using the git diff command. For example to understand what changed in commit a672257 we can compare it to the previous commit 7512894.

$ git diff 7512894 a672257
diff --git a/scripts/get_data.bash b/scripts/get_data.bash
index 338d82c..0bbc17b 100755
--- a/scripts/get_data.bash
+++ b/scripts/get_data.bash
@@ -2,3 +2,4 @@

 FNAME="data/uniprot_sprot.$(date +'%Y-%m-%d').fasta.gz"
 curl --location --output $FNAME http://bit.ly/1l6SAKb
+chmod 444 $FNAME

In the above we can see that we added the line chmod 444 $FNAME to the scripts/get_data.bash file.

Key concepts

• When working with files it is often desirable to be able to track changes
• When programming it is particularly useful to be able to save working states of the code
• This gives one the opportunity to roll back to a previously working state if things go wrong
• Git is a powerful version control system
• To get started with Git one only needs to get familiar with a handful of commands
• Use git init to initialise a Git repository
• Use git add file-to-add to start tracking a file in Git
• Use git commit -m "your summary message here" to record a snapshot in Git
• The overhead of using Git whilst programming is minimal
• The benefits of using Git are great
• Start using Git in your day-to-day work right now!
• On Unix-like systems files have write, read and execute permissions that can be turned on and off
• By making a file executable it can be run as an independent program
• By giving raw data files read only permissions one can ensure that they are not accidentally modified or deleted