| layout | title | subtitle | minutes |
|---|---|---|---|
page |
Version Control with Git |
Exploring History |
25 |
- Identify and use Git commit numbers.
- Compare various versions of tracked files.
- Restore old versions of files.
If we want to see what we changed at different steps, we can use git diff
again, but with the notation HEAD~1, HEAD~2, and so on, to refer to old
commits:
$ git diff HEAD~1 mars.txt
diff --git a/mars.txt b/mars.txt
index 315bf3a..b36abfd 100644
--- a/mars.txt
+++ b/mars.txt
@@ -1,2 +1,3 @@
Cold and dry, but everything is my favorite color
The two moons may be a problem for Wolfman
+But the Mummy will appreciate the lack of humidity
$ git diff HEAD~2 mars.txt
diff --git a/mars.txt b/mars.txt
index df0654a..b36abfd 100644
--- a/mars.txt
+++ b/mars.txt
@@ -1 +1,3 @@
Cold and dry, but everything is my favorite color
+The two moons may be a problem for Wolfman
+But the Mummy will appreciate the lack of humidity
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 (pronounced "head minus one")
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 git log displays.
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
f22b25e3233b4645dabd0d81e651fe074bd8e73b,
so let's try this:
$ git diff f22b25e3233b4645dabd0d81e651fe074bd8e73b mars.txt
diff --git a/mars.txt b/mars.txt
index df0654a..b36abfd 100644
--- a/mars.txt
+++ b/mars.txt
@@ -1 +1,3 @@
Cold and dry, but everything is my favorite color
+The two moons may be a problem for Wolfman
+But the Mummy will appreciate the lack of humidity
That's the right answer, but typing out random 40-character strings is annoying, so Git lets us use just the first few characters:
$ git diff f22b25e mars.txt
diff --git a/mars.txt b/mars.txt
index df0654a..b36abfd 100644
--- a/mars.txt
+++ b/mars.txt
@@ -1 +1,3 @@
Cold and dry, but everything is my favorite color
+The two moons may be a problem for Wolfman
+But the Mummy will appreciate the lack of humidity
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 accidentally overwrite our file:
$ nano mars.txt
$ cat mars.txt
We will need to manufacture our own oxygen
git status now tells us that the file has been changed,
but those changes haven't been staged:
$ git status
# On branch master
# Changes not staged for commit:
# (use "git add <file>..." to update what will be committed)
# (use "git checkout -- <file>..." to discard changes in working directory)
#
# modified: mars.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 checkout:
$ git checkout HEAD mars.txt
$ cat mars.txt
Cold and dry, but everything is my favorite color
The two moons may be a problem for Wolfman
But the Mummy will appreciate the lack of humidity
As you might guess from its name,
git checkout checks out (i.e., restores) an old version of a file.
In this case,
we're telling Git that we want to recover 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:
$ git checkout f22b25e mars.txt
It'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 get rid of.
In the example below, we want to retrieve the state from before the most
recent commit (HEAD~1), which is commit f22b25e:
So, to put it all together:
If you read the output of
git statuscarefully, you'll see that it includes this hint:(use "git checkout -- <file>..." to discard changes in working directory)As it says,
git checkoutwithout a version identifier restores files to the state saved inHEAD. The double dash--is needed to separate the names of the files being recovered from the command itself: without it, Git would try to use the name of the file as the commit identifier.
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.
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 checkout HEAD ~~~
~~~ $ git checkout HEAD data_cruncher.py ~~~
~~~ $ git checkout HEAD~1 data_cruncher.py ~~~
~~~ $ git checkout <unique ID of last commit> data_cruncher.py ~~~
- Both 2 & 4
What is the output of cat venus.txt at the end of this set of commands?
$ cd planets $ nano venus.txt #input the following text: Venus is beautiful and full of love $ git add venus.txt $ nano venus.txt #add the following text: Venus is too hot to be suitable as a base $ git commit -m "comments on Venus as an unsuitable base" $ git checkout HEAD venus.txt $ cat venus.txt #this will print the contents of venus.txt to the screen
~~~ {.output} Venus is too hot to be suitable as a base ~~~
~~~ {.output} Venus is beautiful and full of love ~~~
~~~ {.output} Venus is beautiful and full of love Venus is too hot to be suitable as a base ~~~
~~~ {.output} Error because you have changed venus.txt without committing the changes ~~~