hash-object

• Create blob object
• .git maintains a separate database with its own files
• Every object in git is stored with a hash filename in the .git folder
• Hash-object returns the hexadecimal SHA1 hash of the object
• The hash location is .git/objects/hash-folder/hash-file, where:
• Hash folder: first 2 letters of the hash
• Hash file: the remaining characters of the hash

• Different values generate different hashes, therefore git can keep track of what files are identical and what files are not

• Example blob object

blob 11\0Hello, Git

# Generate object hash
git hash-object "value"
git hash-object "/path/to/file"
echo "value" | git hash-object --stdin

# Write object to git's database
git hash-object "value" -w
git hash-object "/path/to/file" -w
echo "value" | git hash-object --stdin -w

# Create object with value
echo "Hello, Git" | git hash-object --stdin -w

# Create object with filename
git hash-object ./index.js -w

Hash function

• Take any-length input and convert it to a fixed length hash
• Hash function always generate same hash for same input
• Different input generate completely different hashes
• It's a one-way function. You cannot take the input value from the hash

• The input can be anything from a short string to a gigabyte file

• Some hash functions

• MD5 (128bit)

• SHA1 (160bit - 40 hex)
• SHA256 (256bit)
• SHA384 (384bit)
• SHA512 (512bit)

SHA1

# Generate SHA1 hash
echo "Hello" | shasum

• Number of different hashes from SHA1 hash function
• 2^160 = 16^40 = 1.461501637×10^48
• Chance of producing same exact hash for different files in SHA1 (Dice theory)
• Probability of each SHA1 hash: 1/(2^160)
• Probability of same exact hash: (1/(2^160))*(1/_(2^160)) = 1/(2^320)
• Chance of producing any exact hash for different files in SHA1 (Hash collision probability)
• Probability of same any hash: for 2 files: 2.84*10^49
• The probability increases with more files. E.g., 3 files -> 2.05*10^48