I often want to determine if my current Git branch is up to date with some other branch. One specific scenario is when I want to check if my current feat/ branch is up to date with main (i.e. no new commits have been introduced in main that aren’t in feat/).
How to Check if a Branch is Up to Date
Assuming we want to check if feat/x is up to date with main (i.e. feat/x contains all of the commits from main), we would run
git merge-base --is-ancestor main feat/x && \
echo 'is up to date' || echo 'is NOT up to date'
which will output either “is up to date” or “is NOT up to date”.
How this Works
The command git merge-base --is-ancestor main feat/x checks if the commit at main is an ancestor of (i.e. appears somewhere in the history of) the commit at feat/x.
Note: This command does not create text output. Instead it returns an invisible exit code of 0 on success and 1 on failure.
We add && echo 'is up to date' || echo 'is NOT up to date' to the end of the command to create visual output.
When git merge-base... returns 0 (i.e. success) our “and” (&&) echoes “is up to date”.
When git merge-base... return 1 (i.e. failure) our “or” (||) echoes “is NOT up to date”.
Git Alias
This command is more than I want to type (or remember). I want an alias called git is-up-to-date-with, so I can do the following
git checkout feat/x
git is-up-to-date-with main
See salcode/salcode-git PR 28 where I add this alias to my Git configuration.
is-up-to-date-with = "!f() { \
: git branch ; \
[ -e "${1}" ] && echo "Missing branch name to compare" && exit 1; \
branch=$1; \
if [[ "$branch" == '-' ]]; \
then \
branch='@{-1}'; \
fi; \
git "merge-base --is-ancestor \
${branch} $(git rev-parse --abbrev-ref HEAD)" \
&& echo "Yes, this branch is up to date with $branch" && exit 0 \
|| echo "No, this branch is NOT up to date with $branch" && exit 1; \
}; f"
How this Alias Works
Tab Completion
This line tells Git to use branch names for tab completion (see git alias tab completion for functions for more details).
: git branch ;
Exit Early When No Branch is Given
When no branch name is given (i.e. someone runs git is-up-to-date-with) we display an error message and exit with an exit code of 1 (indicating there was an error).
[ -e "${1}" ] && echo "Missing branch name to compare" && exit 1;
Store the Branch Name in the Variable $branch
If the user ran git is-up-to-date-with abc123, we store abc123 into $branch.
branch=$1;
Handle – Shortcut
Some Git commands support using - as a shortcut for the previous branch (i.e. @{-1}). See Git Previous Branch for more information.
This code adds support for - in our command.
We check if $branch is - and if it is, we replace it with @{-1} (the Git identifier for previous branch that works anywhere).
if [[ "$branch" == '-' ]];
then
branch='@{-1}';
fi;
Is our Current Branch an Ancestor of $branch
We compare our $branch variable to our current branch, $(git rev-parse --abbrev-ref HEAD) (see Git Get Current Branch Name).
git "merge-base --is-ancestor ${branch} $(git rev-parse --abbrev-ref HEAD)"
and based on the exit code either
echo "Yes, this branch is up to date with $branch" && exit 0
or
echo "No, this branch is NOT up to date with $branch" && exit 1
Related
In Check If We Can Do a Git Fast-Forward Merge, we are checking the same thing but in the other order. Here we are checking if the current branch could be fast-forward merged into the given branch. While there we are checking if the given branch could be fast-forward merged into the current branch.
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