Is there a command I can invoke which will count the lines changed by a specific author in a Git repository? I know that there must be ways to count the number of commits as Github does this for their Impact graph.

This gives some statistics about the author, modify as required.

Using Gawk:

git log --author="_Your_Name_Here_" --pretty=tformat: --numstat \
| gawk '{ add += $1; subs += $2; loc += $1 - $2 } END { printf "added lines: %s removed lines: %s total lines: %s\n", add, subs, loc }' -

Using Awk on Mac OSX:

git log --author="_Your_Name_Here_" --pretty=tformat: --numstat | awk '{ add += $1; subs += $2; loc += $1 - $2 } END { printf "added lines: %s, removed lines: %s, total lines: %s\n", add, subs, loc }' -

Using count-lines git-alias:

Simply create count-lines alias (once per system), like:

git config --global alias.count-lines "! git log --author=\"\$1\" --pretty=tformat: --numstat | awk '{ add += \$1; subs += \$2; loc += \$1 - \$2 } END { printf \"added lines: %s, removed lines: %s, total lines: %s\n\", add, subs, loc }' #"

And use each time later, like:

git count-lines [email protected]

For Windows, works after adding Git-Bash to PATH (environment-variable).
For Linux, maybe replace awk part with gawk.
For MacOS, works without any change.

Using existing script ^{(Update 2017)}

There is a new package on github that looks slick and uses bash as dependencies (tested on linux). It's more suitable for direct usage rather than scripts.

It's git-quick-stats (github link).

Copy git-quick-stats to a folder and add the folder to path.

mkdir ~/source
cd ~/source
git clone [email protected]:arzzen/git-quick-stats.git
mkdir ~/bin
ln -s ~/source/git-quick-stats/git-quick-stats ~/bin/git-quick-stats
chmod +x ~/bin/git-quick-stats
export PATH=${PATH}:~/bin

Usage:

git-quick-stats