使用回声； > 和 >> 有什么区别

Question

I came across this on a git site:

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

So in the first instance of echo, we are writing the string to the file .gitignore in the log dir. In the second instance, are we writing tmp to the file .gitignore (in current dir). Why do we need to use >> versus > ?

Answer 1

When echoing something to a file, >> appends to the file and > overwrites the file.

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

From the example you posted, a log directory is created and then *.log is put into log/.gitignore so that no log files are committed to git. Since > was used, if a .gitignore file had existed, it would be overwritten with only *.log .

The log directory itself is then added to your local git stage.

On the next line, >> is added so that tmp is appended to the end of the .gitignore file instead of overwriting it. It is then added to the staging area.

Answer 2

> is a redirection operator. < > >| << >> <& >& <<- <> < > >| << >> <& >& <<- <> are all redirection operators in the shell command interpreter.

In your examples, essentially > overwrites and >> appends.

See man sh , (from your terminal you can access the manual via man sh ).

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.