Exploring History
Last updated on 2024-11-19 | Edit this page
Estimated time: 25 minutes
Overview
Questions
- How can I identify old versions of files?
- How do I review my changes?
- How can I recover old versions of files?
Objectives
- Explain what the HEAD of a repository is and how to use it.
- Identify and use Git commit numbers.
- Compare various versions of tracked files.
- Restore old versions of files.
As we saw in the previous episode, we can refer to commits by their
identifiers. You can refer to the most recent commit of the
working directory by using the identifier HEAD.
We’ve been adding one line at a time to
amr-data-dictionary.txt, so it’s easy to track our progress
by looking, so let’s do that using our HEADs. Before we
start, let’s make a change to amr-data-dictionary.txt,
adding yet another line.
Showing only the last few lines:
OUTPUT
* sex_male Binary - indicates whether the person from whom the specimen was taken was male or not. 1 (male) 0 (not male)
* region Character - a string indicating the England region of laboratory testing the specimen
* had_surgery_past_yr Binary - indicates whether person from whom sample was taken had undergone surgery in hospital in the past year before specimen taken. 1 (surgery within last year) 0 (No surgery within last year)
* ethnicity Character - indicates self-reported ethnicity group according to Office for National Statistics groupings
* imd Integer - indicates the Index of Multiple Deprivation for residence for person from whom specimen was taken. Range: 1 (least deprived) - 5 (most deprived)
* organism Character - indicates the species name for the organism detected
* coamox Binary - indicates specimen was resistant to Coamoxiclav
* gentam Binary - indicates specimen was resistant to Gentamicin
* ciprof Binary - indicates specimen was resistant to Ciprofloxacin
* name Character - a string giving the name of the person from whom the specimen was takenNow, let’s see what we get.
OUTPUT
diff --git a/amr-data-dictionary.txt b/amr-data-dictionary.txt
index c9a8214..d7d742c 100644
--- a/amr-data-dictionary.txt
+++ b/amr-data-dictionary.txt
@@ -15,3 +15,4 @@ These data represent the sort of data that might be obtained from the Second Gen
* coamox Binary - indicates specimen was resistant to Coamoxiclav
* gentam Binary - indicates specimen was resistant to Gentamicin
* ciprof Binary - indicates specimen was resistant to Ciprofloxacin
+* name Character - a string giving the name of the person from whom the specimen was takenwhich is the same as what you would get if you leave out
HEAD (try it). The real goodness in all this is when you
can refer to previous commits. We do that by adding ~1
(where “~” is “tilde”, pronounced [til-duh])
to refer to the commit one before HEAD.
If we want to see the differences between older commits we can use
git diff again, but with the notation HEAD~1,
HEAD~2, and so on, to refer to them:
OUTPUT
diff --git a/amr-data-dictionary.txt b/amr-data-dictionary.txt
index 55895cc..d7d742c 100644
--- a/amr-data-dictionary.txt
+++ b/amr-data-dictionary.txt
@@ -2,3 +2,17 @@ AMR data
100,000 rows of 12 variables
These data represent the sort of data that might be obtained from the Second Generation Surveillance System (SGSS)
+
+* id Integer - A unique identifier for each person
+* dob Character - a string giving the date of birth
+* spec_date Character - a string giving the date a specimen was taken
+* sex_male Binary - indicates whether the person from whom the specimen was taken was male or not. 1 (male) 0 (not male)
+* region Character - a string indicating the England region of laboratory testing the specimen
+* had_surgery_past_yr Binary - indicates whether person from whom sample was taken had undergone surgery in hospital in the past year before specimen taken. 1 (surgery within last year) 0 (No surgery within last year)
+* ethnicity Character - indicates self-reported ethnicity group according to Office for National Statistics groupings
+* imd Integer - indicates the Index of Multiple Deprivation for residence for person from whom specimen was taken. Range: 1 (least deprived) - 5 (most deprived)
+* organism Character - indicates the species name for the organism detected
+* coamox Binary - indicates specimen was resistant to Coamoxiclav
+* gentam Binary - indicates specimen was resistant to Gentamicin
+* ciprof Binary - indicates specimen was resistant to Ciprofloxacin
+* name Character - a string giving the name of the person from whom the specimen was takenWe could also use git show which shows us what changes
we made at an older commit as well as the commit message, rather than
the differences between a commit and our working directory that
we see by using git diff.
OUTPUT
commit 0f988204ddcf33c060ecb849d640b3bd7aec71cc
Author: John Doe <john.doe@unknown.com>
Date: Wed Aug 14 14:54:11 2024 +0100
Start data dictionary
diff --git a/amr-data-dictionary.txt b/amr-data-dictionary.txt
new file mode 100644
index 0000000..55895cc
--- /dev/null
+++ b/amr-data-dictionary.txt
@@ -0,0 +1,4 @@
+AMR data
+100,000 rows of 12 variables
+
+These data represent the sort of data that might be obtained from the Second Generation Surveillance System (SGSS)In this way, we can build up a chain of commits. The most recent end
of the chain is referred to as HEAD; we can refer to
previous commits using the ~ notation, so
HEAD~1 means “the previous commit”, while
HEAD~123 goes back 123 commits from where we are now.
We can also refer to commits using those long strings of digits and
letters that both git log and git show
display. These are unique IDs for the changes, and “unique” really does
mean unique: every change to any set of files on any computer has a
unique 40-character identifier. Our first commit was given the ID
0f988204ddcf33c060ecb849d640b3bd7aec71cc, so let’s try
this:
OUTPUT
diff --git a/amr-data-dictionary.txt b/amr-data-dictionary.txt
index 55895cc..d7d742c 100644
--- a/amr-data-dictionary.txt
+++ b/amr-data-dictionary.txt
@@ -2,3 +2,17 @@ AMR data
100,000 rows of 12 variables
These data represent the sort of data that might be obtained from the Second Generation Surveillance System (SGSS)
+
+* id Integer - A unique identifier for each person
+* dob Character - a string giving the date of birth
+* spec_date Character - a string giving the date a specimen was taken
+* sex_male Binary - indicates whether the person from whom the specimen was taken was male or not. 1 (male) 0 (not male)
+* region Character - a string indicating the England region of laboratory testing the specimen
+* had_surgery_past_yr Binary - indicates whether person from whom sample was taken had undergone surgery in hospital in the past year before specimen taken. 1 (surgery within last year) 0 (No surgery within last year)
+* ethnicity Character - indicates self-reported ethnicity group according to Office for National Statistics groupings
+* imd Integer - indicates the Index of Multiple Deprivation for residence for person from whom specimen was taken. Range: 1 (least deprived) - 5 (most deprived)
+* organism Character - indicates the species name for the organism detected
+* coamox Binary - indicates specimen was resistant to Coamoxiclav
+* gentam Binary - indicates specimen was resistant to Gentamicin
+* ciprof Binary - indicates specimen was resistant to Ciprofloxacin
+* name Character - a string giving the name of the person from whom the specimen was takenThat’s the right answer, but typing out random 40-character strings is annoying, so Git lets us use just the first few characters (typically seven for normal size projects):
All right! So we can save changes to files and see what we’ve
changed. Now, how can we restore older versions of things? Let’s suppose
we change our mind about the last update to
amr-data-dictionary.txt (we realise the name
variable is not actually part of the data).
git status now tells us that the file has been changed,
but those changes haven’t been staged:
OUTPUT
On branch main
Changes not staged for commit:
(use "git add <file>..." to update what will be committed)
(use "git restore <file>..." to discard changes in working directory)
modified: amr-data-dictionary.txt
no changes added to commit (use "git add" and/or "git commit -a")We can put things back the way they were by using
git restore:
OUTPUT
* spec_date Character - a string giving the date a specimen was taken
* sex_male Binary - indicates whether the person from whom the specimen was taken was male or not. 1 (male) 0 (not male)
* region Character - a string indicating the England region of laboratory testing the specimen
* had_surgery_past_yr Binary - indicates whether person from whom sample was taken had undergone surgery in hospital in the past year before specimen taken. 1 (surgery within last year) 0 (No surgery within last year)
* ethnicity Character - indicates self-reported ethnicity group according to Office for National Statistics groupings
* imd Integer - indicates the Index of Multiple Deprivation for residence for person from whom specimen was taken. Range: 1 (least deprived) - 5 (most deprived)
* organism Character - indicates the species name for the organism detected
* coamox Binary - indicates specimen was resistant to Coamoxiclav
* gentam Binary - indicates specimen was resistant to Gentamicin
* ciprof Binary - indicates specimen was resistant to CiprofloxacinAs you might guess from its name, git restore restores
an old version of a file. By default, it recovers the version of the
file recorded in HEAD, which is the last saved commit. If
we want to go back even further, we can use a commit identifier instead,
using -s option:
OUTPUT
AMR data
100,000 rows of 12 variables
These data represent the sort of data that might be obtained from the Second Generation Surveillance System (SGSS)OUTPUT
On branch main
Changes not staged for commit:
(use "git add <file>..." to update what will be committed)
(use "git restore <file>..." to discard changes in working directory)
modified: amr-data-dictionary.txt
no changes added to commit (use "git add" and/or "git commit -a")
Notice that the changes are currently in the staging area. Again, we
can put things back the way they were by using
git restore:
OUTPUT
AMR data
100,000 rows of 12 variables
These data represent the sort of data that might be obtained from the Second Generation Surveillance System (SGSS)
* id Integer - A unique identifier for each person
* dob Character - a string giving the date of birth
* spec_date Character - a string giving the date a specimen was taken
* sex_male Binary - indicates whether the person from whom the specimen was taken was male or not. 1 (male) 0 (not male)
* region Character - a string indicating the England region of laboratory testing the specimen
* had_surgery_past_yr Binary - indicates whether person from whom sample was taken had undergone surgery in hospital in the past year before specimen taken. 1 (surgery within last year) 0 (No surgery within last year)
* ethnicity Character - indicates self-reported ethnicity group according to Office for National Statistics groupings
* imd Integer - indicates the Index of Multiple Deprivation for residence for person from whom specimen was taken. Range: 1 (least deprived) - 5 (most deprived)
* organism Character - indicates the species name for the organism detected
* coamox Binary - indicates specimen was resistant to Coamoxiclav
* gentam Binary - indicates specimen was resistant to Gentamicin
* ciprof Binary - indicates specimen was resistant to CiprofloxacinIt’s important to remember that we must use the commit number that
identifies the state of the repository before the change we’re
trying to undo. A common mistake is to use the number of the commit in
which we made the change we’re trying to discard. In the example below,
we want to retrieve the state from before the most recent commit
(HEAD~1), which is commit f22b25e. We use the
. to mean all files:
So, to put it all together, here’s how Git works in cartoon form:
The fact that files can be reverted one by one tends to change the way people organize their work. If everything is in one large document, it’s hard (but not impossible) to undo changes to the introduction without also undoing changes made later to the conclusion. If the introduction and conclusion are stored in separate files, on the other hand, moving backward and forward in time becomes much easier.
Recovering Older Versions of a File
Jennifer has made changes to the Python script that she has been working on for weeks, and the modifications she made this morning “broke” the script and it no longer runs. She has spent ~ 1hr trying to fix it, with no luck…
Luckily, she has been keeping track of her project’s versions using
Git! Which commands below will let her recover the last committed
version of her Python script called data_cruncher.py?
$ git restore$ git restore data_cruncher.py$ git restore -s HEAD~1 data_cruncher.py$ git restore -s <unique ID of last commit> data_cruncher.pyBoth 2 and 4
The answer is (5)-Both 2 and 4.
The restore command restores files from the repository,
overwriting the files in your working directory. Answers 2 and 4 both
restore the latest version in the repository of the
file data_cruncher.py. Answer 2 uses HEAD to
indicate the latest, whereas answer 4 uses the unique ID of the
last commit, which is what HEAD means.
Answer 3 gets the version of data_cruncher.py from the
commit before HEAD, which is NOT what we
wanted.
Answer 1 results in an error. You need to specify a file to restore.
If you want to restore all files you should use
git restore .
Reverting a Commit
Jennifer is collaborating with colleagues on her Python script. She
realizes her last commit to the project’s repository contained an error,
and wants to undo it. Jennifer wants to undo correctly so everyone in
the project’s repository gets the correct change. The command
git revert [erroneous commit ID] will create a new commit
that reverses the erroneous commit.
The command git revert is different from
git restore -s [commit ID] . because
git restore returns the files not yet committed within the
local repository to a previous state, whereas git revert
reverses changes committed to the local and project repositories.
Below are the right steps and explanations for Jennifer to use
git revert, what is the missing command?
________ # Look at the git history of the project to find the commit IDCopy the ID (the first few characters of the ID, e.g. 0b1d055).
git revert [commit ID]Type in the new commit message.
Save and close
The command git log lists project history with commit
IDs.
The command git show HEAD shows changes made at the
latest commit, and lists the commit ID; however, Jennifer should
double-check it is the correct commit, and no one else has committed
changes to the repository.
Understanding Workflow and History
What is the output of the last command in
BASH
$ cd planets
$ echo "Venus is beautiful and full of love" > venus.txt
$ git add venus.txt
$ echo "Venus is too hot to be suitable as a base" >> venus.txt
$ git commit -m "Comment on Venus as an unsuitable base"
$ git restore venus.txt
$ cat venus.txt #this will print the contents of venus.txt to the screenOUTPUT
Venus is too hot to be suitable as a baseOUTPUT
Venus is beautiful and full of loveOUTPUT
Venus is beautiful and full of love Venus is too hot to be suitable as a baseOUTPUT
Error because you have changed venus.txt without committing the changes
The answer is 2.
The command git add venus.txt places the current version
of venus.txt into the staging area. The changes to the file
from the second echo command are only applied to the
working copy, not the version in the staging area.
So, when
git commit -m "Comment on Venus as an unsuitable base" is
executed, the version of venus.txt committed to the
repository is the one from the staging area and has only one line.
At this time, the working copy still has the second line (and
git status will show that the file is modified). However,
git restore venus.txt replaces the working copy with the
most recently committed version of venus.txt.
So, cat venus.txt will output
OUTPUT
Venus is beautiful and full of love.Checking Understanding of
git diff
Consider this command:
git diff HEAD~9 amr-data-dictionary.txt. What do you
predict this command will do if you execute it? What happens when you do
execute it? Why?
Try another command,
git diff [ID] amr-data-dictionary.txt, where [ID] is
replaced with the unique identifier for your most recent commit. What do
you think will happen, and what does happen?
Getting Rid of Staged Changes
git restore can be used to restore a previous commit
when unstaged changes have been made, but will it also work for changes
that have been staged but not committed? Make a change to
amr-data-dictionary.txt, add that change using
git add, then use git restore to see if you
can remove your change.
After adding a change, git restore can not be used
directly. Let’s look at the output of git status:
OUTPUT
On branch main
Changes to be committed:
(use "git restore --staged <file>..." to unstage)
modified: amr-data-dictionary.txt
Note that if you don’t have the same output you may either have forgotten to change the file, or you have added it and committed it.
Using the command git restore amr-data-dictionary.txt
now does not give an error, but it does not restore the file either. Git
helpfully tells us that we need to use git restore --staged
first to unstage the file:
Now, git status gives us:
OUTPUT
On branch main
Changes not staged for commit:
(use "git add <file>..." to update what will be committed)
(use "git git restore <file>..." to discard changes in working directory)
modified: amr-data-dictionary.txt
no changes added to commit (use "git add" and/or "git commit -a")This means we can now use git restore to restore the
file to the previous commit:
OUTPUT
On branch main
nothing to commit, working tree cleanExplore and Summarize Histories
Exploring history is an important part of Git, and often it is a challenge to find the right commit ID, especially if the commit is from several months ago.
Imagine the data-dictionary project has more than 50
files. You would like to find a commit that modifies some specific text
in amr-data-dictionary.txt. When you type
git log, a very long list appeared. How can you narrow down
the search?
Recall that the git diff command allows us to explore
one specific file, e.g., git diff amr-data-dictionary.txt.
We can apply a similar idea here.
Unfortunately some of these commit messages are very ambiguous, e.g.,
update files. How can you search through these files?
Both git diff and git log are very useful
and they summarize a different part of the history for you. Is it
possible to combine both? Let’s try the following:
You should get a long list of output, and you should be able to see both commit messages and the difference between each commit.
Question: What does the following command do?
Key Points
-
git diffdisplays differences between commits. -
git restorerecovers old versions of files.