Is there a git-merge --dry-run option

Question

I m merging in a remote branch that may have a lot of conflicts   How can I tell if it will have conflicts or not   I don t see anything like a --dry-run on git-merge

User · Answer

Make a temporary copy of your working copy  then merge into that  and diff the two

User · Answer

Just diff your current branch against the remote branch  this will tell you what is going to change when you do a pull merge    see diff between current master and remote branch git diff master origin master

User · Answer

I just had to implement a method that automatically finds conflicts between a repository and its remote  This solution does the merge in memory so it won t touch the index  nor the working tree  I think this is the safest possible way you can solve this problem  Here s how it works    Fetch the remote to your repository  For example  git fetch origin master Run git merge-base  git merge-base FETCH HEAD master Run git merge-tree  git merge-tree mergebase master FETCH HEAD  mergebase is the hexadecimal id that merge-base printed in the previous step    Now suppose that you want to merge the remote master with your local master  but you can use any branches  git merge-tree will execute the merge in memory and print the result to the standard output  Grep for the pattern  lt  lt  or  gt  gt   Or you can print the output to a file and check that  If you find a line starting with  changed in both  then most probably there will be a conflict

User · Answer

My simple brute-force solution to this is    Create a  pre-master  branch  from master of course    Merge all the things you want to into this pre-master  Then you can see how the merging happened without touching master     Merge pre-master into master OR Merge all wannabe-released branches into master    Anyway  I would follow  orange80 s advice

User · Answer

This might be interesting  From the documentation      If you tried a merge which resulted in complex conflicts and want to   start over  you can recover with git merge --abort    But you could also do it the naive  but slow  way   rm -Rf  tmp repository cp -r repository  tmp  cd  tmp repository git merge        if successful  do the real merge        Note  It won t work just cloning to  tmp  you d need a copy  in order to be sure that uncommitted changes will not conflict

User · Answer

My solution is to merge backwards   Instead of merging your branch into the remote  target  branch  merge that branch into yours   git checkout my-branch git merge origin target-branch   You will see if there are any conflicts and can plan on how to solve them   After that you can either abort the merge via git merge --abort  or  if there weren t any conflicts and merge has happened  roll back to previous commit via git reset --hard HEAD 1

User · Answer

Undoing a merge with git is so easy you shouldn t even worry about the dry run     git pull  REMOTE  BRANCH   uh oh  that wasn t right   git reset --hard ORIG HEAD   all is right with the world   EDIT  As noted in the comments below  if you have changes in your working directory or staging area you ll probably want to stash them before doing the above  otherwise they will disappear following the git reset above

User · Answer

Not exactly like that  But you can use the --no-commit option  so it does not automatically commit the result after the merge  In this way you can inspect  and if desired  to undo the merge without messing with the commit tree

User · Answer

Git introduced a --ff-only option when merging   From  http   git-scm com docs git-merge  --ff-only  Refuse to merge and exit with a non-zero status unless the current HEAD is already up-to-date or the merge can be resolved as a fast-forward    Doing this will attempt to merge and fast-forward  and if it can t it aborts and prompts you that the fast-forward could not be performed  but leaves your working branch untouched  If it can fast-forward  then it will perform the merge on your working branch  This option is also available on git pull  Thus  you could do the following  git pull --ff-only origin branchA  See if you can pull down and merge branchA  git merge --ff-only branchA branchB  See if you can merge branchA into branchB

User · Answer

Undoing a merge with git is so easy you shouldn t even worry about the dry run     git pull  REMOTE  BRANCH   uh oh  that wasn t right   git reset --hard ORIG HEAD   all is right with the world   EDIT  As noted in the comments below  if you have changes in your working directory or staging area you ll probably want to stash them before doing the above  otherwise they will disappear following the git reset above

User · Answer

If you want to fast forward from B to A  then you must make sure that git log B  A shows you nothing  i e  A has nothing that B doesn t have  But even if B  A has something  you might still be able to merge without conflicts  so the above shows two things  that there will be a fast-forward  and thus you won t get a conflict

User · Answer

As a summary of existed answers  there are two way to check if there would be merge conflicts  git format-patch   git merge-base branch1 branch2   branch2 --stdout   git apply --3way --check -  Note  your current branch should be branch1 when you run above command  Another way   git merge --no-commit branch2   check the return code here git merge --abort

User · Answer

I know this is theoretically off-topic  but practically very on-topic for people landing here from a Google search  When in doubt  you can always use the Github interface to create a pull-request and check if it indicates a clean merge is possible

User · Answer

I just had to implement a method that automatically finds conflicts between a repository and its remote  This solution does the merge in memory so it won t touch the index  nor the working tree  I think this is the safest possible way you can solve this problem  Here s how it works    Fetch the remote to your repository  For example  git fetch origin master Run git merge-base  git merge-base FETCH HEAD master Run git merge-tree  git merge-tree mergebase master FETCH HEAD  mergebase is the hexadecimal id that merge-base printed in the previous step    Now suppose that you want to merge the remote master with your local master  but you can use any branches  git merge-tree will execute the merge in memory and print the result to the standard output  Grep for the pattern  lt  lt  or  gt  gt   Or you can print the output to a file and check that  If you find a line starting with  changed in both  then most probably there will be a conflict

User · Answer

As noted previously  pass in the --no-commit flag  but to avoid a fast-forward commit  also pass in --no-ff  like so     git merge --no-commit --no-ff  BRANCH   To examine the staged changes     git diff --cached   And you can undo the merge  even if it is a fast-forward merge     git merge --abort

User · Answer

You can do git merge --abort after seeing that there are conflicts

User · Answer

As a summary of existed answers  there are two way to check if there would be merge conflicts  git format-patch   git merge-base branch1 branch2   branch2 --stdout   git apply --3way --check -  Note  your current branch should be branch1 when you run above command  Another way   git merge --no-commit branch2   check the return code here git merge --abort

User · Answer

My solution is to merge backwards   Instead of merging your branch into the remote  target  branch  merge that branch into yours   git checkout my-branch git merge origin target-branch   You will see if there are any conflicts and can plan on how to solve them   After that you can either abort the merge via git merge --abort  or  if there weren t any conflicts and merge has happened  roll back to previous commit via git reset --hard HEAD 1

User · Answer

I use the request-pull git command to do so  It allows you to see every change that would happen when merging  but without doing anything on your local or remote repositories   For instance  imagine you want to merge a branch named  feature-x  into your master branch  git request-pull master origin feature-x   will show you a summary of what would happen  without doing anything    The following changes since commit fc01dde318      Layout updates  2015-06-25 11 00 47  0200  are available in the git repository at      http   fakeurl com myrepo git  feature-x for you to fetch changes up to 841d3b41ad  ---------------------------------------------------------------- john  2       Adding some layout     Refactoring ioserver js                8    --- package json               7      - server js                  4  -- layout ldkdsd js         277                                       4 files changed  289 insertions     7 deletions -  create mode 100644 layout ldkdsd js   If you add the -pparameter  you will also get the full patch text  exactly like if you were doing a git diff on every changed file

User · Answer

As noted previously  pass in the --no-commit flag  but to avoid a fast-forward commit  also pass in --no-ff  like so     git merge --no-commit --no-ff  BRANCH   To examine the staged changes     git diff --cached   And you can undo the merge  even if it is a fast-forward merge     git merge --abort

User · Answer

Undoing a merge with git is so easy you shouldn t even worry about the dry run     git pull  REMOTE  BRANCH   uh oh  that wasn t right   git reset --hard ORIG HEAD   all is right with the world   EDIT  As noted in the comments below  if you have changes in your working directory or staging area you ll probably want to stash them before doing the above  otherwise they will disappear following the git reset above

User · Answer

I know this is theoretically off-topic  but practically very on-topic for people landing here from a Google search  When in doubt  you can always use the Github interface to create a pull-request and check if it indicates a clean merge is possible

User · Answer

Make a temporary copy of your working copy  then merge into that  and diff the two

User · Answer

As noted previously  pass in the --no-commit flag  but to avoid a fast-forward commit  also pass in --no-ff  like so     git merge --no-commit --no-ff  BRANCH   To examine the staged changes     git diff --cached   And you can undo the merge  even if it is a fast-forward merge     git merge --abort

User · Answer

My simple brute-force solution to this is    Create a  pre-master  branch  from master of course    Merge all the things you want to into this pre-master  Then you can see how the merging happened without touching master     Merge pre-master into master OR Merge all wannabe-released branches into master    Anyway  I would follow  orange80 s advice

User · Answer

I made an alias for doing this and works like a charm  I do this    git config --global alias mergetest   f    git merge --no-commit --no-ff   1   git merge --abort  echo  Merge aborted     f     Now I just call  git mergetest  lt branchname gt    To find out if there are any conflicts

User · Answer

Undoing a merge with git is so easy you shouldn t even worry about the dry run     git pull  REMOTE  BRANCH   uh oh  that wasn t right   git reset --hard ORIG HEAD   all is right with the world   EDIT  As noted in the comments below  if you have changes in your working directory or staging area you ll probably want to stash them before doing the above  otherwise they will disappear following the git reset above

User · Answer

If you want to fast forward from B to A  then you must make sure that git log B  A shows you nothing  i e  A has nothing that B doesn t have  But even if B  A has something  you might still be able to merge without conflicts  so the above shows two things  that there will be a fast-forward  and thus you won t get a conflict

User · Answer

I use git log to see what has changed on a feature branch from master branch  git log does this branch  contain this branch changes   e g  - to see what commits are in a feature branch that has not been merged to master   git log master  feature branch

User · Answer

This might be interesting  From the documentation      If you tried a merge which resulted in complex conflicts and want to   start over  you can recover with git merge --abort    But you could also do it the naive  but slow  way   rm -Rf  tmp repository cp -r repository  tmp  cd  tmp repository git merge        if successful  do the real merge        Note  It won t work just cloning to  tmp  you d need a copy  in order to be sure that uncommitted changes will not conflict

User · Answer

Git introduced a --ff-only option when merging   From  http   git-scm com docs git-merge  --ff-only  Refuse to merge and exit with a non-zero status unless the current HEAD is already up-to-date or the merge can be resolved as a fast-forward    Doing this will attempt to merge and fast-forward  and if it can t it aborts and prompts you that the fast-forward could not be performed  but leaves your working branch untouched  If it can fast-forward  then it will perform the merge on your working branch  This option is also available on git pull  Thus  you could do the following  git pull --ff-only origin branchA  See if you can pull down and merge branchA  git merge --ff-only branchA branchB  See if you can merge branchA into branchB

User · Answer

I made an alias for doing this and works like a charm  I do this    git config --global alias mergetest   f    git merge --no-commit --no-ff   1   git merge --abort  echo  Merge aborted     f     Now I just call  git mergetest  lt branchname gt    To find out if there are any conflicts

User · Answer

As noted previously  pass in the --no-commit flag  but to avoid a fast-forward commit  also pass in --no-ff  like so     git merge --no-commit --no-ff  BRANCH   To examine the staged changes     git diff --cached   And you can undo the merge  even if it is a fast-forward merge     git merge --abort

User · Answer

I m surprised nobody has suggested using patches yet   Say you d like to test a merge from your branch into master  I m assuming you have master checked out      git diff master your branch  gt  your branch patch   git apply --check your branch patch   rm your branch patch   That should do the trick   If you get errors like  error  patch failed  test txt 1 error  test txt  patch does not apply   that means that the patch wasn t successful and a merge would produce conflicts  No output means the patch is clean and you d be able to easily merge the branch    Note that this will not actually change your working tree  aside from creating the patch file of course  but you can safely delete that afterwards   From the git-apply documentation   --check     Instead of applying the patch  see if the patch is applicable to the     current working tree and or the index file and detects errors  Turns     off  apply       Note to anyone who is smarter more experienced with git than me  please do let me know if I m wrong here and this method does show different behaviour than a regular merge  It seems strange that in the 8  years that this question has existed noone would suggest this seemingly obvious solution

User · Answer

I m assuming you just want to find out how much trouble you re getting yourself into prior to actually attempting the merge   and resetting to the last commit after a failed merge is relatively easy so I wouldn t be surprised if that is the intended approach   That said  if you really don t want to touch your existing files in the working tree - you could create a patch and test it against the target branch  This also has the benefit of showing exactly what changes were made to which files - just open up the patch file in a text editor   git checkout -b mycrazybranch  change some stuff     git add   git commit -m  changed some stuff  git format-patch master --stdout  gt  crazy patch git checkout master git apply crazy patch --check  all good  cleanup     rm crazy patch   As you can see  this will create a patch file  you can then test it with --check and see if there are any errors  then remove the patch file

User · Answer

You can do git merge --abort after seeing that there are conflicts

User · Answer

I use the request-pull git command to do so  It allows you to see every change that would happen when merging  but without doing anything on your local or remote repositories   For instance  imagine you want to merge a branch named  feature-x  into your master branch  git request-pull master origin feature-x   will show you a summary of what would happen  without doing anything    The following changes since commit fc01dde318      Layout updates  2015-06-25 11 00 47  0200  are available in the git repository at      http   fakeurl com myrepo git  feature-x for you to fetch changes up to 841d3b41ad  ---------------------------------------------------------------- john  2       Adding some layout     Refactoring ioserver js                8    --- package json               7      - server js                  4  -- layout ldkdsd js         277                                       4 files changed  289 insertions     7 deletions -  create mode 100644 layout ldkdsd js   If you add the -pparameter  you will also get the full patch text  exactly like if you were doing a git diff on every changed file

User · Answer

I m assuming you just want to find out how much trouble you re getting yourself into prior to actually attempting the merge   and resetting to the last commit after a failed merge is relatively easy so I wouldn t be surprised if that is the intended approach   That said  if you really don t want to touch your existing files in the working tree - you could create a patch and test it against the target branch  This also has the benefit of showing exactly what changes were made to which files - just open up the patch file in a text editor   git checkout -b mycrazybranch  change some stuff     git add   git commit -m  changed some stuff  git format-patch master --stdout  gt  crazy patch git checkout master git apply crazy patch --check  all good  cleanup     rm crazy patch   As you can see  this will create a patch file  you can then test it with --check and see if there are any errors  then remove the patch file

User · Answer

Not exactly like that  But you can use the --no-commit option  so it does not automatically commit the result after the merge  In this way you can inspect  and if desired  to undo the merge without messing with the commit tree

User · Answer

I m surprised nobody has suggested using patches yet   Say you d like to test a merge from your branch into master  I m assuming you have master checked out      git diff master your branch  gt  your branch patch   git apply --check your branch patch   rm your branch patch   That should do the trick   If you get errors like  error  patch failed  test txt 1 error  test txt  patch does not apply   that means that the patch wasn t successful and a merge would produce conflicts  No output means the patch is clean and you d be able to easily merge the branch    Note that this will not actually change your working tree  aside from creating the patch file of course  but you can safely delete that afterwards   From the git-apply documentation   --check     Instead of applying the patch  see if the patch is applicable to the     current working tree and or the index file and detects errors  Turns     off  apply       Note to anyone who is smarter more experienced with git than me  please do let me know if I m wrong here and this method does show different behaviour than a regular merge  It seems strange that in the 8  years that this question has existed noone would suggest this seemingly obvious solution

User · Answer

I use git log to see what has changed on a feature branch from master branch  git log does this branch  contain this branch changes   e g  - to see what commits are in a feature branch that has not been merged to master   git log master  feature branch

User · Answer

Just diff your current branch against the remote branch  this will tell you what is going to change when you do a pull merge    see diff between current master and remote branch git diff master origin master

[git] Is there a git-merge --dry-run option?

Examples related to git

Examples related to git-merge