Question

以下命令的作用是什么？

cat >file.c << EOF
C line
EOF

这会创建一个名为“ file.c”的空C文件，然后下面的行将是其中的内容吗？

Answer 1

此shell构造的技术术语是此处文档（“此处”与“脚本中的数据在此处”相同）。如果只有cat >file.c，则将读取标准输入，直到满足文件结束条件（在Unix上通常为CTRL-D）。结果将被写入file.c。

<< EOF部分现在指示Shell将stdin上的以下行传递到cat（而不是将stdin连接到终端）。外壳程序将一行上的EOF本身识别为结束标记。详细信息在您的Shell手册中，例如对于bash，它是此段：

Here Documents
   This type of redirection instructs the shell to read input from  the  current  source
   until  a  line  containing  only  word (with no trailing blanks) is seen.  All of the
   lines read up to that point are then used as the standard input for a command.

   The format of here-documents is:

          <<[-]word
                  here-document
          delimiter

   No parameter expansion,  command  substitution,  arithmetic  expansion,  or  pathname
   expansion  is performed on word.  If any characters in word are quoted, the delimiter
   is the result of quote removal on word, and the lines in the  here-document  are  not
   expanded.   If  word  is  unquoted,  all  lines of the here-document are subjected to
   parameter expansion, command substitution, and arithmetic expansion.  In  the  latter
   case,  the  character sequence \<newline> is ignored, and \ must be used to quote the
   characters \, $, and ‘.

   If the redirection operator is <<-, then all leading tab characters are stripped from
   input  lines  and  the  line containing delimiter.  This allows here-documents within
   shell scripts to be indented in a natural fashion.

这个带<<操作符的cat命令有什么作用？

1 个答案: