使用回声； > 和 >> 有什么区别

Question

我在 git 站点上遇到了这个：

mkdir log
echo '*.log' > log/.gitignore
git add log
echo tmp >> .gitignore
git add .gitignore
git commit -m "ignored log files and tmp dir"

因此，在 echo 的第一个实例中，我们将字符串写入日志目录中的 .gitignore 文件。 在第二种情况下，我们是否将 tmp 写入文件 .gitignore（在当前目录中）。 为什么我们需要使用 >> 与 > ？

Answer 1

将某些内容回显到文件时， >>附加到文件中，而>覆盖文件。

$ echo foobar > test
$ cat test
foobar
$ echo baz >> test
$ cat test
foobar
baz
$ echo foobar > test
$ cat test
foobar

从您发布的示例中，创建了一个日志目录，然后将*.log放入log/.gitignore以便没有日志文件提交给 git。 由于>被使用，如果一个 .gitignore 文件已经存在，它将被仅覆盖*.log 。

然后将日志目录本身添加到本地 git 阶段。

在下一行，添加>>以便将tmp附加到 .gitignore 文件的末尾而不是覆盖它。 然后将其添加到暂存区。

Answer 2

>是重定向操作符。 < > >| << >> <& >& <<- <> < > >| << >> <& >& <<- <>都是 shell 命令解释器中的重定向操作符。

在您的示例中，本质上是>覆盖和>>附加。

请参阅man sh ，（从您的终端，您可以通过man sh访问手册）。

Redirections
     Redirections are used to change where a command reads its input or sends its output.  In
     general, redirections open, close, or duplicate an existing reference to a file.  The over‐
     all format used for redirection is:

           [n] redir-op file

     where redir-op is one of the redirection operators mentioned previously.  Following is a
     list of the possible redirections.  The [n] is an optional number, as in '3' (not '[3]'),
     that refers to a file descriptor.

           [n]> file   Redirect standard output (or n) to file.

           [n]>| file  Same, but override the -C option.

           [n]>> file  Append standard output (or n) to file.

           [n]< file   Redirect standard input (or n) from file.

           [n1]<&n2    Duplicate standard input (or n1) from file descriptor n2.

           [n]<&-      Close standard input (or n).

           [n1]>&n2    Duplicate standard output (or n1) to n2.

           [n]>&-      Close standard output (or n).

           [n]<> file  Open file for reading and writing on standard input (or n).

     The following redirection is often called a "here-document".

           [n]<< delimiter
                 here-doc-text ...
           delimiter

     All the text on successive lines up to the delimiter is saved away and made available to the
     command on standard input, or file descriptor n if it is specified.  If the delimiter as
     specified on the initial line is quoted, then the here-doc-text is treated literally, other‐
     wise the text is subjected to parameter expansion, command substitution, and arithmetic
     expansion (as described in the section on "Expansions").  If the operator is "<<-" instead
     of "<<", then leading tabs in the here-doc-text are stripped.