The Nitty Gitty

Git is not complicated, but it is complex (and confusing). In this talk, Daniel attempts to de-mystify git. He explains the basics of git and its building blocks: commits, blobs, trees and refs. He then goes on to discuss the branch model, and how you can use git to rewrite history.

Presented online at Megabites Tech Lunch n Learn, May 2020.

https://www.meetup.com/MEGABITES-Tech-Lunch-n-Learns/events/270359119/

Transcript

1. Daniel van Berzon @ocastahq ocasta.com @dvberzon vanberzon.com
3. Developers treat git like a car • ! Complicated, intricate and dangerous • " Learn enough to get from A to B • ⚠ Problem? Call for help • $ Rarely lift the hood
4. Git is not a car
5. Git is not a car ... it's a skateboard • Many complicated, intricate and dangerous things you can do with Git • But Git itself is pretty simple
6. Git is ... Complicated ? Confusing Complex 145 commands Naming
7. Check out ...a book from a library • Check out a file • File is locked • Only you can change it • Check in file when finished
8. checkout • Move onto a branch • Start a new branch • Temporarily move on to a commit • Remove local changes to a file > git checkout mybranch > git checkout -b mybranch > git checkout 5bd33a3b > git checkout index.html
9. Demystify Building blocks Branch model History Basics
10. Basics
11. ? • Version control system (VCS) • Keep a history of your project • Restore previous versions • Multi-task • Collaborate • Document evolution of project
12. Snapshots
13. Snapshots
14. Snapshots
15. Snapshots
16. Basics Git repository Working directory Index index.html css styles.css index.html css styles.css
17. Basics Git repository Working directory Index index.html css styles.css index.html css styles.css
18. Basics Git repository Working directory Index index.html css styles.css index.html css styles.css index.html > git add styles.css
19. Basics Git repository Working directory Index index.html css styles.css index.html css styles.css index.html > git add styles.css index.html css styles.css > git commit
20. Building blocks
21. Blob Tree Ref Commit Object database
22. Blob Tree Ref Commit Object database
23. %Blob Binary Long Object mr-blobby.jpg 1111111111011000111111111110000000000000000100 0001001010010001100100100101000110000000000000 0001000000010000000000000000000000010000000000 0000010000000000000000111111111101101100000000 1000010000000000000001010000001100000100000010 0000000110000001110000100000000101000001010000 1000000001100000010100000101000001010000011000 0001110000010100000101000001110000010100000111 0000010100000101000001110000010100000101000001 0100000111000010000000011100000111000001110000 0111000001110000011100001010000100000000101100 0001110000100000001110000010010000011100000111 0000110100010101000011010000111000010001000100 0100010011000100110001001100000111000010110001 0110000110000001011000010010000110000001000000 0100100001001100010010000000010000010100000101 0000010100001000000001110000100000001111000010 0100001001000011110001001000001101000011010000 1101000100100001001000010010000100100001001000 0100100001001000010010000100100001001000010010 0001001000010010000100100001001000010010000100 1000010010000100100001001000010010000100100001 0010000100100001001000010010000100100001001000 0100100001001000010010000100100001001000010010 0001001000010010000100100001001000010010000100 1000010010000100100001001000010010000100100001 0010000100100001001000010010000100101111111111 0000000000000000010001000010000000001011010000 0000010100000000000000110000000100100010000000
24. %Blob Binary Long Object mr-blobby.jpg index.html 1111000010000101100100011011110110001101110100 0111100101110000011001010010000001101000011101 0001101101011011000011111000001010000010100011 1100011010000111010001101101011011000010000001 1011000110000101101110011001110011110100100010 0110010101101110001000100011111000001010001111 0001101000011001010110000101100100001111100000 1010001000000010000000111100011011010110010101 1101000110000100100000011000110110100001100001 0111001001110011011001010111010000111101001000 1001110101011101000110011000101101001110000010 0010001111100000101000100000001000000011110001 1101000110100101110100011011000110010100111110 0100000101110011011110010110111001100011001000 0001000111011010010111010000100000010101000110 0001011011000110101100111100001011110111010001 1010010111010001101100011001010011111000001010 0010000000100000001111000110110101100101011101 0001100001001000000110111001100001011011010110 0101001111010010001001100100011001010111001101 1000110111001001101001011100000111010001101001 0110111101101110001000100010000001100011011011 1101101110011101000110010101101110011101000011 1101001000100100000101110011011110010110111001 1000110010000001100111011010010111010000100000 0111010001100001011011000110101100100010001111 1000001010001000000010000000111100011011010110 0101011101000110000100100000011011100110000101 1011010110010100111101001000100110000101110101
25. 88d4d9db7773a672280c5dfb085689041f0e9a70 14b564ced10ef9ef0e741718d60ac1966d6913c0 SHA hash %Blob
26. • Represents a folder in the project file system • Associated with an SHA hash • Has an entry for each file and subfolder • Each entry has: & Tree - Name (index.html) - Type (blob or tree) - SHA hash
27. & Tree blob a0423896973644771497bdc03eb99d5281615b51 index.html tree aaa96ced2d9a1c8e72c56b253a0e2fe78393feb7 css aaa96ced2d9a1c8e72c56b253a0e2fe78393feb7 blob 790cce91278c3ad02f160faa6e1e61ca147a5c3a styles.css & ad5a762c172647d211a0881b8d9e98d38edc1de9 & index.html css styles.css
28. • Commit is a snapshot of file system • Commit points project root tree • Meta-data: - SHA hash - Author, committer and date - A commit message - Parent commit ☄ Commit
29. • Store a blob for the new version of index.hml • Store a tree for the project root • Store a commit pointing to the new tree ☄ Commit Index.html css styles.css tree be5b6a45346870a56564ed2652bc51c5f48e1b59 parent 343e4048d816fa663b56ff5021f2679018381b8d author Daniel van Berzon <dvberzon@gmail.com> 1564416292 +0200 committer Daniel van Berzon <dvberzon@gmail.com> 1564416292 +0200 Updating index.html with new banner
30. ( Ref • Points to a commit • Has a meaningful name • Two types: ) Tag * Branch Fixed to the commit Updates to latest commit
31. ) Tag a a b c d e f ( Ref ) v2.0
32. ) Tag a a b c d * Branch a a b c d e f ( Ref ) v2.0 * my-feature
33. ) Tag a a b c d * Branch a a b c d e f e f ( Ref ) v2.0 * my-feature
34. Thats it for data! % Blob & Tree ☄ Commit ( Ref > commands
35. Branch Model
36. • Points to commit • Each commit has a parent • Branch represents a linear history * Branch * my-feature
37. • Branches are lightweight and disposable • Default branch is master • Create a branch • Move onto a branch • (Create and move) * Branch > git checkout mybranch > git branch mybranch > git checkout -b mybranch
38. • Current position • Normally points to a branch • Checked out onto that branch • Possible to point directly to a commit + HEAD "detached HEAD"
39. * Branch > git init > git commit a * master +
40. * Branch > git init > git commit > git commit > git commit b a c * master +
41. * Branch > git init > git commit > git commit > git commit > git branch mybranch b a c * mybranch * master +
42. * Branch > git init > git commit > git commit > git commit > git branch mybranch > git checkout mybranch b a c * mybranch * master +
43. * Branch > git init > git commit > git commit > git commit > git branch mybranch > git checkout mybranch > git commit > git commit b a c d e * mybranch * master +
44. * Branch > git init > git commit > git commit > git commit > git branch mybranch > git checkout mybranch > git commit > git commit > git checkout master b a c d e * mybranch * master +
45. * Branch > git init > git commit > git commit > git commit > git branch mybranch > git checkout mybranch > git commit > git commit > git checkout master > git commit > git commit b a c d e f g * mybranch * master +
46. * Branch b a c f g * mybranch d e b a c * master
47. * Branch - why? • Multi-task
48. * Branch - why? • Multi-task • Collaborate
49. * Branch - why? • Multi-task • Collaborate • Experiment
50. * Branch - why? • Multi-task • Collaborate • Experiment How can you share work between branches?
51. • Incorporate changes from one branch into another • Bad name! • Merge: two things → one thing • Merge branch A into B → still two branches • Should be called Import , Merge
52. , Merge Simplest case... d e b a c * destination * source b a c Source is "ahead" of destination
53. , Merge * destination d e b a c * source b a c > git checkout destination > git merge source d e Destination has changed - source is unchanged
54. , Merge Fast-Forward merge d e b a c * source * destination
55. , Merge Fast-Forward merge d e b a c * source * destination
56. , Merge More complex case... source and destination have "diverged" * destination d e b a c * source b a c f g
57. , Merge * source b a c f g d e Merge commit * destination
58. , Merge * source b a c f g d e index.html Merge commit styles.css styles.css M styles.css index.html Merge commit * destination > git checkout destination > git merge source
59. , Merge * destination * source b a c f g d e M • Merge commit has two parents • Contains diffs from commits in source branch • Source branch remains unchanged
60. History
61. So far we have been adding commits to history Git allows you to change history?
62. Why?
63. Two ways to look at git history Story Record
64. ... div.panel { background-color: #eee; display: flex; flex: 1 0 0; min-width: 250px; } ... styles.css - Story telling . in IE11
65. ... div.panel { background-color: #eee; display: flex; - flex: 1 0 0; + flex: 1 0 0%; min-width: 250px; } ... styles.css Document the change? - Story telling
66. ... div.panel { background-color: #eee; display: flex; - flex: 1 0 0; + /* IE11 requires % unit */ + flex: 1 0 0%; min-width: 250px; } ... styles.css - Story telling
67. ... div.panel { background-color: #eee; display: flex; - flex: 1 0 0; + flex: 1 0 0%; min-width: 250px; } ... styles.css ✉ Commit message commit 762258f4a4e64cdf156960d12fea785a3f11fdcf Author: Daniel van Berzon <dvberzon@gmail.com> Date: Mon Jul 29 18:54:33 2019 +0200 Added unit to flex basis This was causing a bug in IE11 where `flex` command is ignored if the basis part doesn't have a unit. More details here: https://github.com/philipwalton/flexbugs#flexbug-4
68. - Story telling • A commit message can be used to document a change • Works best if each commit contains one logical change - Atomic commit • A logical sequence of atomic commits can tell a story
70. Different histories
71. 0 Record
72. 0 Record
73. 0 Record
74. - Story
75. - Story
76. - Story
77. • Git history that tells a coherent story has advantages: - Documents reason for changes - Easier to do code reviews - Search history to help fix bugs • Process of coding is messy • Use git commands to refactor history and tell the story - Story telling
78. ✏ git commit --amend • Same process as commit • Instead of adding a new commit, adds changes to the current commit. • Can also edit the commit message b a c styles.css d index.html Ammend d
79. ✏ git commit --amend • Same process as commit • Instead of adding a new commit, adds changes to the current commit. • Can also edit the commit message b a c styles.css d d' index.html d' has new SHA
80. ↩ git reset • Opposite process to commit • Removes all commits up to and including the target • Diffs in the removed commits become local changes b a d Reset to b styles.css c index.html
81. ↩ git reset • Opposite process to commit • Removes all commits up to and including the target • Diffs in the removed commits become local changes b a Reset to b styles.css index.html
82. ⚾ git rebase • Change starting point (base) of a sequence of commits • Rewind commits to the starting point • Reapply commits from the new starting point b a c d e f g * branch1 * master Current base New base
83. ⚾ git rebase • Change starting point (base) of a sequence of commits • Rewind commits to the starting point • Reapply commits from the new starting point b a c d e f g * branch1 * master > git checkout branch1 > git rebase master Current base New base
84. ⚾ git rebase • Change starting point (base) of a sequence of commits • Rewind commits to the starting point • Reapply commits from the new starting point b a c d e * master f' g' * branch1 f' and g' have new SHAs
85. * master * feature1 b a c f g ⚾ why rebase? • Useful for keeping a clean history shape d e
86. * master * feature1 b a c f g ⚾ why rebase? • Useful for keeping a clean history shape d e M h M
87. * master * feature1 * master * feature1 b a c f g ⚾ why rebase? • Useful for keeping a clean history shape d e M h M b a c d e f g
88. * master * feature1 * master * feature1 b a c f g ⚾ why rebase? • Useful for keeping a clean history shape d e M h M b a c d e g' M h f'
89. ⚾ why rebase? Merge Rebase
90. That's all we have time for
91. 4 In Memoriam Remotes Conflicts Workflows Commands > git cherry-pick > git stash > git revert > git reflog > git blame (annotate) > git log -S (pickaxe)
92. 5 Useful links git-scm.com/book/en/v2 • Pro git • Atlassian Git Tutorials • Git CLI tutorial • Oh shit git! www.atlassian.com/git ohshitgit.com/ > git help tutorial
93. Thank you Daniel van Berzon @ocastahq ocasta.com @dvberzon vanberzon.com