使用echo；>和>>有什么区别
使用echo；>和>>有什么区别
提问于 2016-08-19 01:21:04
我在一个git网站上发现了这个：
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文件.gitignore。在第二个实例中，我们是否将临时文件写入文件.gitignore (在当前目录中)。为什么我们需要使用>>而不是>？
回答 2
Stack Overflow用户
回答已采纳
发布于 2016-08-19 02:09:10
在向文件回显某些内容时，>>会附加到文件，而>会覆盖该文件。
$ echo foobar > test
$ cat test
foobar
$ echo baz >> test
$ cat test
foobar
baz
$ echo foobar > test
$ cat test
foobar在您发布的示例中，创建了一个日志目录，然后将*.log放入log/.gitignore，这样就不会将日志文件提交到git。由于使用了>，因此如果.gitignore文件已经存在，则只会用*.log覆盖该文件。
然后将日志目录本身添加到本地git工作台。
在下一行中，添加了>>，以便将tmp附加到.gitignore文件的末尾，而不是覆盖它。然后将其添加到临时区域。
Stack Overflow用户
发布于 2016-08-19 02:29:30
>是一个重定向操作符。< > >| << >> <& >& <<- <>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.页面原文内容由Stack Overflow提供。腾讯云小微IT领域专用引擎提供翻译支持
原文链接：
https://stackoverflow.com/questions/39024073
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