21 shell Here Document/String

Here Document Usage

Here Document it can be understood as "embedded documents" "embedded documents" "Now documents", which data or commands to be processed and the code string together , the amount of data suitable for a command to be processed is small, unnecessarily in which case a separate file.

Generally speaking, when embedding multiple rows of data in a short script, Here Document is useful, but embedding a lot of data, or should keep your logic (your code) and your input (your data ) separation, preferably in different files.

usage	Explanation
command <<END document END	`command`Shell is a command, `<<END`a start flag, `END`the end flag `document`is input document (that is, line by line string). Use Akebayashi command processing document data portion until it encounters until the terminator END (END terminator is not read) Terminator END must be on a separate line, and must be the top grid write. Terminator may occur in the normal data stream, as long as it is not the top grid to write a separate line, it will not be as complete flag. Terminator may be defined by the user Single or double quotes surround the delimiter document command can replace a failed In `<<`and `END`increase between`-来消除制表符的影响` `Here Document 常用于向用户显示命令或者脚本的用法信息`
demand	script	result
cat command typically read from the file and the contents output to the display, With Here Document, cat command can read from the keyboard	cat <<TEST > test1 > Test > Spaces before TEST # terminator, do not take effect > TEST # top grid write a terminator, not output	test1 Test TEST `<`The second layer is a command prompt Marks the end of the body can also occur `TEST`, but as long as it is not a separate row, And not the top grid to write, I will not take effect.
Here Document use in a script file, content and document conversion in uppercase.	#!/bin/bash # Immediately document in a script file tr a-z A-Z<<END one two three AbCdefGH END	ONE TWO THREE ABCDEFGH tr command to convert said capital letters lowercase letters
Command substitution And then to the command by default, document appear in variable or command will be evaluated or run, Shell will first replace them later They may be surrounded delimiter single or double quotes makes replacing failed Shell	1. By default name=qpy cat <<END > ${name} is a girl > END 2. single / double quotes the command to replace a failed END 1) single quotes cat <<'END' > ${name} is a girl > END 2) double quotes cat <<"END" > ${name} is a girl > END	Output: qpy is a girl 输出：${name} is a girl 输出：${name} is a girl
Here Document when used in a script file, if the line in the document's first use of tabs, By default, the first row of tabs is also treated as part of the body, but can also in `<<`and `END`increasing between`-来消除制表符的影响` Tabs are mainly used for code alignment	1. By default #!/bin/bash cat <<END aaa bbb ccc END 2. `<<`and `END`increase between`-来消除制表符的影响` #!/bin/bash cat <<-END aaa bbb ccc END	Export aaa bbb ccc Export aaa bbb ccc
Here Document most commonly used functions or display usage information to a user command or script

Here String Usage

Here String is a variant of the Here Document

usage	Explanation
command <<< string	Shell command is the command, string is a general string. Here String可用于发送较短数据到进程中当字符串中有空格时，必须使用单引号或双引号将字符串包围起来使用双引号或不适用引号包围字符串时会解析字符串中的变量使用单引号包围字符串时不会解析字符串中的变量使用引号是，Here String 还可以接收多行字符串作为命令的输入与 Here Document 相比，Here String 在发送变量内容（而不是文件）到像 grep 或者 sed 这样的过滤程序时相当方便
需求	脚本	结果
将小写字符串转换为大写字符串	tr a-z A-Z <<< one	ONE
使用双引号，会解析字符串中的变量	tr a-z A-Z <<< "one two three" var=two tr a-z A-Z <<<"one $var there"	ONE TWO THREE ONE TWO THREE
使用单引号，不会解析变量	var=two tr a-z A-Z <<<'one $var there'	ONE $VAR THERE
使用引号，Here String 可以接收多行字符串作为命令的输入	tr a-z A-Z <<<"one two there > four five six > seven eight"	ONE TWO THERE FOUR FIVE SIX SEVEN EIGHT