To stage a file is simply to prepare it finely for a commit. Git, with its index allows you to commit only certain parts of the changes you’ve done since the last commit. Say you’re working on two features – one is finished, and one still needs some work done. You’d like to make a commit and go home (5 o’clock, finally!) but wouldn’t like to commit the parts of the second feature, which is not done yet. You stage the parts you know belong to the first feature, and commit. Now your commit is your project with the first feature done, while the second is still in work-in-progress in your working directory.
Since everyone so far has answered it the “formal” way, let me do this with alternatives to enhance learning with the power of metaphors.
So the staging area is like:
- a cache of files that you want to commit
- not a series of tubes but actually a dump truck, ready to move the work you load it with, in to the repository
- a magical place where selected files will be turned into stone with your wizardry and can be magically transported to the repository at your whim
- the yellow brick road for the files to go happily to the repository (or fall off if you want to revert)
- the fictional place at the sea port where files are received a pair of cement shoes and then thrown into the repository sea
- the receptions desk at the library, you put the files there for the librarian to prepare for filing into the library
- a box where you put things in before shoving it under your bed, where your bed is a repository of boxes you’ve previously have shoved in
- the loading bay of files before it goes into the repository warehouse with the power loader
- the filter of an electric drip coffee maker, if the files are like the coffee powder, then the committed files are the brewed coffee
- the Scrooge McDuck’s office next to the vault, the files are like the coins before they go into the vault of his massive Money Bin
- the pet store, once you bring a pet home you’re committed
Staging is a step before the commit process in git. That is, a commit in git is performed in two steps: staging and actual commit.
As long as a changeset is in the staging area, git allows you to edit it as you like (replace staged files with other versions of staged files, remove changes from staging, etc.).
Broken metaphor time:
Consider a scenario where you call the movers to get your stuff from your old appartment to your new appartment. Before you do that, you will go through your stuff, decide what you take with you and what you throw away, pack it in bags and leave it in the main hallway. The movers simply come, get the (already packed) bags from the hallway and transport them.
In this example, everything until the movers get your stuff, is staging: you decide what goes where, how to pack it and so on (e.g. you may decide that half your stuff will be thrown away before the movers even get there – that’s part of staging).
From a technical point of view, staging also supports transactional commits, by splitting all operations into what can fail (staging) and what cannot fail (commit):
The commit in git is implemented transactionally, after the staging is sucessfull. Several steps in the staging can fail (for example, you need to commit, but your HDD is 99.9999% full, and git has no space to perform a commit). This will fail in staging (your repository will not be corrupted by a partial commit) and the staging process doesn’t affect your commit history (it doesn’t corrupt your repository in case of an error).