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I need to generate a full-context git diff programmatically for a web ui.
A CLI for generating a full-context diff was covered in questions:
The prevalent answer is something like git diff -U99999
With a -U / --unified option with a ridiculously high threshold (e.g. 999,999), doing git diff -U999999:
Is there a -U option to show the whole file?
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No, commit objects in git don't contain diffs - instead, each commit object contains a hash of the tree, which recursively and completely defines the content of the source tree at that commit.
The git diff command helps you see, compare, and understand changes in your project. You can use it in many different situations, e.g. to look at current changes in your working copy, past changes in commits, or even to compare branches.
When viewing a combined diff, if the two files you're comparing have a line that's different from what they were merged into, you will see the ++ to represent: one line that was added does not appear in either file1 or file2.
If you just use a large number with -U, you could choose the large number to be the point at which your application can't handle displaying such a large file (diff).
it's a correctness issue if my file is larger than 1M lines
And to address this issue, you can check the output for more than one @@ ... @@ line to determine whether it's complete — this allows you to avoid silently giving a wrong number.
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Frankly, the best option is to use git difftool rather than vanilla git diff. To see which tools your version of git supports, enter
git difftool --tool-help
which, with my version (2.3.0), shows the following
$ git difftool --tool-help
'git difftool --tool=<tool>' may be set to one of the following:
araxis
gvimdiff
gvimdiff2
gvimdiff3
meld
vimdiff
vimdiff2
vimdiff3
The following tools are valid, but not currently available:
bc
bc3
codecompare
deltawalker
diffmerge
diffuse
ecmerge
emerge
kdiff3
kompare
opendiff
p4merge
tkdiff
xxdiff
I usually use meld, but that's just a personal preference. git difftool takes the same arguments as git diff plus a few to help with the process (I find -y useful to prevent the prompts when moving from one file to the next).
To check out the changes introduced by a specific commit, for example, you can use
git difftool -y -t meld 08f0f82^..08f0f82
obviously replacing 08f0f82 with the correct SHA-1.
My biggest complaint is that it launches the tool for each modified file in sequence (hence specifying the -y option).
If you only wanted to examine the changes to a particular file in that commit, you can just add the filename to the command line.
git difftool -y -t meld 08f0f82^..08f0f82 myfile.c
Obviously, this is for interactive use - not for scripting
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