有些时候，你必须把某些文件放到Git工作目录中，但又不能提交它们，比如保存了数据库密码的配置文件啦，等等，每次git status都会显示Untracked files ...。
解决的方法就是在git的根目录下编写一个.gitinnore文件。
原则
GitHub已经为我们准备了各种配置的.gitignore文件，只需要组合一下就可以使用了。所有配置文件可以直接在线浏览：https://github.com/github/gitignore
忽略文件的原则是：
- 忽略操作系统自动生成的文件，比如缩略图等；
- 忽略编译生成的中间文件、可执行文件等，也就是如果一个文件是通过另一个文件自动生成的，那自动生成的文件就没必要放进版本库，比如Java编译产生的
.class文件； - 忽略你自己的带有敏感信息的配置文件，比如存放口令的配置文件。
文件语句的语法规则
空行， 不匹配任何文件，所以可以添加空行，用来提高文件的可读性。
#, 开头的行是注释。Put a backslash ("\") in front of the first hash for patterns that begin with a hash.
一行语句后面的空格，将被忽略，除非它们用反斜杠(“\”)括起来。
An optional prefix "!" which negates the pattern; any matching file excluded by a previous pattern will become included again. It is not possible to re-include a file if a parent directory of that file is excluded. Git doesn’t list excluded directories for performance reasons, so any patterns on contained files have no effect, no matter where they are defined. Put a backslash ("\") in front of the first "!" for patterns that begin with a literal "!", for example, "\!important!.txt"
If the pattern ends with a slash, it is removed for the purpose of the following description, but it would only find a match with a directory. In other words, foo/ will match a directory foo and paths underneath it, but will not match a regular file or a symbolic link foo (this is consistent with the way how pathspec works in general in Git).
If the pattern does not contain a slash /, Git treats it as a shell glob pattern and checks for a match against the pathname relative to the location of the .gitignore file (relative to the toplevel of the work tree if not from a .gitignore file).
Otherwise, Git treats the pattern as a shell glob: "*" matches anything except "/", "?" matches any one character except "/" and "[]" matches one character in a selected range. See fnmatch(3) and the FNM_PATHNAME flag for a more detailed description.
A leading slash matches the beginning of the pathname. For example, "/*.c" matches "cat-file.c" but not "mozilla-sha1/sha1.c".
Two consecutive asterisks ("**") in patterns matched against full pathname may have special meaning:
A leading "**" followed by a slash means match in all directories. For example, "**/foo" matches file or directory "foo" anywhere, the same as pattern "foo". "**/foo/bar" matches file or directory "bar" anywhere that is directly under directory "foo".
A trailing "/**" matches everything inside. For example, "abc/**" matches all files inside directory "abc", relative to the location of the .gitignore file, with infinite depth.
A slash followed by two consecutive asterisks then a slash matches zero or more directories. For example, "a/**/b" matches "a/b", "a/x/b", "a/x/y/b" and so on.
Other consecutive asterisks are considered regular asterisks and will match according to the previous rules.
示例
忽略Windows自动生成的垃圾文件：
# Windows: Thumbs.db ehthumbs.db Desktop.ini
然后，继续忽略Python编译产生的.pyc、.pyo、dist等文件或目录：
# Python: *.py[cod] *.so *.egg *.egg-info dist build
加上你自己定义的文件：
# My configurations:
db.ini
deploy_key_rsa
最后一步就是把.gitignore也提交到Git，就完成了！当然检验.gitignore的标准是git status命令是不是说working directory clean。、
如果你确实想添加该文件，可以用-f强制添加到Git：
$ git add -f App.class
或者你发现，可能是.gitignore写得有问题，需要找出来到底哪个规则写错了，可以用git check-ignore命令检查：
$ git check-ignore -v App.class
.gitignore:3:*.class App.class
Git会告诉我们，.gitignore的第3行规则忽略了该文件，于是我们就可以知道应该修订哪个规则。
参考文章
忽略特殊文件, 廖雪峰的官方网站
gitignore , 文件的官方定义