why git branch and $(git branch) outputs different result
It's not the command substitution, it's the quoting. Here is the result of git branch on a repo:
$ git branch 5-job-test-fails* master revisions with_cool_backgroundNotice the askerisk.
When you write echo $(git branch), since the argument is unquoted, the asterisk will expand to the files in the current directory:
$ echo $(git branch)5-job-test-fails app bin cable config config.ru db erd.pdf Gemfile Gemfile.lock Guardfile lib log public Rakefile README.rdoc test tmp vendor master revisions with_cool_backgroundTo overcome this, quote the argument:
$ echo "$(git branch)" 5-job-test-fails* master revisions with_cool_background