How do I copy a version of a single file from one Git branch to another?

Run this from the branch where you want the file to end up:

git checkout otherbranch myfile.txt

General formulas:

git checkout <commit_hash> <relative_path_to_file_or_dir>
git checkout <remote_name>/<branch_name> <file_or_dir>

Some notes (from comments):

• Using the commit hash, you can pull files from any commit
• This works for files and directories
• Overwrites the file myfile.txt and mydir
• Wildcards don't work, but relative paths do
• Multiple paths can be specified

An alternative:

git show commit_id:path/to/file > path/to/file

I would use git restore (available since Git 2.23):

git restore --source otherbranch path/to/myfile.txt

Why is better than other options?

• by default git restore modify files only in working directory.

git checkout otherbranch -- path/to/myfile.txt copy file to working directory (your files on disk) but also to staging area. It's similar effect as if you would copy this file manually and executed git add on it. git restore by default change only working directory.

To get the same result as for git checkout otherbranch -- path/to/myfile.txt you can write git restore --source otherbranch --staged --worktree path/to/myfile.txt

• by default git restore deletes files from working directory when they are missing in other branch

git restore can be used to restore the whole folder with git restore --source otherbranch path/to/dir. You can do similar operation with git checkout but git restore by default will delete files that are missing on otherbranch. To get git checkout behaviour use --overlay option.

For example, if there are fewer files on otherbranch than in the current working directory (and these files are tracked) without --overlay option git restore will delete them. But this is good default behaviour, you most likely want the state of directory to be "the same like in otherbranch", not "the same like in otherbranch but with additional files from my current branch".

To get the same result as for git checkout otherbranch -- path/to/dir you can write git restore --source otherbranch --staged --worktree --overlay path/to/dir

• git restore doesn't use shell redirection to create file (Powershell only problem)

git show otherbranch:path/to/myfile.txt > path/to/myfile.txt uses standard shell redirection. If you use PowerShell then there might be problem with text encoding or you could get broken file if it's binary. With git restore changing files is done all by the git executable.

Use the checkout command:

  git diff --stat "$branch"
  git checkout --merge "$branch" "$file"
  git diff --stat "$branch"

Please note that in the accepted answer, the first option stages the entire file from the other branch (like git add ... had been performed), and that the second option just results in copying the file, but doesn't stage the changes (as if you had just edited the file manually and had outstanding differences).

Git copy file from another branch without staging it

Changes staged (e.g. git add filename):

$ git checkout directory/somefile.php feature-B

$ git status
On branch feature-A
Your branch is up-to-date with 'origin/feature-A'.
Changes to be committed:
  (use "git reset HEAD <file>..." to unstage)

        modified:   directory/somefile.php

Changes outstanding (not staged or committed):

$ git show feature-B:directory/somefile.php > directory/somefile.php

$ git status
On branch feature-A
Your branch is up-to-date with 'origin/feature-A'.
Changes not staged for commit:
  (use "git add <file>..." to update what will be committed)
  (use "git checkout -- <file>..." to discard changes in working directory)

        modified:   directory/somefile.php

no changes added to commit (use "git add" and/or "git commit -a")

Sorry, no one mentioned that before restoring a file you really want to preview local changes in relation to that branch, so:

git diff <other-branch-name> -- <filename>

Then when you're accepting a lost (overwritting), you can follow by:

git restore --source <other-branch-name> <filename>
or
git checkout <other-branch-name> <filename>