PUB:20171116 How to use a here documents to write data to a file in bash script.md

@Flowsnow https://linux.cn/article-10224-1.html

published/20171116 How to use a here documents to write data to a file in bash script.md

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+Bash 脚本中如何使用 here 文档将数据写入文件
+======
+
+<ruby>here 文档<rt>here document</rt></ruby> （LCTT 译注：here 文档又称作 heredoc ）不是什么特殊的东西，只是一种 I/O 重定向方式，它告诉 bash shell 从当前源读取输入，直到读取到只有分隔符的行。
+
+![redirect output of here document to a text file][1]
+
+这对于向 ftp、cat、echo、ssh 和许多其他有用的 Linux/Unix 命令提供指令很有用。 此功能适用于 bash 也适用于 Bourne、Korn、POSIX 这三种 shell。
+
+### here 文档语法
+
+语法是：
+
+```
+command <<EOF
+cmd1
+cmd2 arg1
+EOF
+```
+
+或者允许 shell 脚本中的 here 文档使用 `EOF<<-` 以自然的方式缩进：
+
+```
+command <<-EOF
+  msg1
+  msg2 
+  $var on line 
+EOF
+```
+
+或者
+
+```
+command <<'EOF'
+ cmd1
+ cmd2 arg1
+ $var won't expand as parameter substitution turned off
+ by single quoting
+EOF
+```
+
+或者 **重定向并将其覆盖** 到名为 `my_output_file.txt` 的文件中：
+
+```
+command <<EOF > my_output_file.txt
+ mesg1
+ msg2
+ msg3
+ $var on $foo
+EOF
+```
+
+或**重定向并将其追加**到名为 `my_output_file.txt` 的文件中：
+
+```
+command <<EOF >> my_output_file.txt
+ mesg1
+ msg2
+ msg3
+ $var on $foo
+EOF
+```
+
+### 示例
+
+以下脚本将所需内容写入名为 `/tmp/output.txt` 的文件中：
+
+```
+#!/bin/bash
+OUT=/tmp/output.txt
+
+echo "Starting my script..."
+echo "Doing something..."
+
+cat <<EOF >$OUT
+  Status of backup as on $(date)
+  Backing up files $HOME and /etc/
+EOF
+
+echo "Starting backup using rsync..."
+```
+
+你可以使用[cat命令][3]查看/tmp/output.txt文件：
+
+```
+$ cat /tmp/output.txt
+```
+
+示例输出：
+
+```
+ Status of backup as on Thu Nov 16 17:00:21 IST 2017
+ Backing up files /home/vivek and /etc/
+```
+
+### 禁用路径名/参数/变量扩展、命令替换、算术扩展
+
+像 `$HOME` 这类变量和像 `$(date)` 这类命令在脚本中会被解释为替换。 要禁用它，请使用带有 `'EOF'` 这样带有单引号的形式，如下所示：
+
+```
+#!/bin/bash
+OUT=/tmp/output.txt
+
+echo "Starting my script..."
+echo "Doing something..."
+# No parameter and variable expansion, command substitution, arithmetic expansion, or pathname expansion is performed on word.  
+# If any part of word is quoted, the delimiter  is  the  result  of  quote removal  on word, and the lines in the here-document 
+# are not expanded. So EOF is quoted as follows
+cat <<'EOF' >$OUT
+  Status of backup as on $(date)
+  Backing up files $HOME and /etc/
+EOF
+
+echo "Starting backup using rsync..."
+```
+
+你可以使用 [cat 命令][3]查看 `/tmp/output.txt` 文件：
+
+```
+$ cat /tmp/output.txt
+```
+
+示例输出：
+
+```
+ Status of backup as on $(date)
+ Backing up files $HOME and /etc/
+
+```
+
+### 关于 tee 命令的使用
+
+语法是：
+
+```
+tee /tmp/filename <<EOF >/dev/null
+line 1
+line 2
+line 3
+$(cmd)
+$var on $foo
+EOF
+```
+
+或者通过在单引号中引用 `EOF` 来禁用变量替换和命令替换：
+
+```
+tee /tmp/filename <<'EOF' >/dev/null
+line 1
+line 2
+line 3
+$(cmd)
+$var on $foo
+EOF
+```
+
+这是我更新的脚本：
+
+```
+#!/bin/bash
+OUT=/tmp/output.txt
+
+echo "Starting my script..."
+echo "Doing something..."
+
+tee $OUT <<EOF >/dev/null
+  Status of backup as on $(date)
+  Backing up files $HOME and /etc/
+EOF
+
+echo "Starting backup using rsync..."
+```
+
+### 关于内存 here 文档的使用
+
+这是我更新的脚本：
+
+```
+#!/bin/bash
+OUT=/tmp/output.txt
+
+## in memory here docs 
+## thanks https://twitter.com/freebsdfrau
+exec 9<<EOF
+  Status of backup as on $(date)
+  Backing up files $HOME and /etc/
+EOF
+
+## continue
+echo "Starting my script..."
+echo "Doing something..."
+
+## do it
+cat <&9 >$OUT
+
+echo "Starting backup using rsync..."
+```
+
+--------------------------------------------------------------------------------
+
+via: https://www.cyberciti.biz/faq/using-heredoc-rediection-in-bash-shell-script-to-write-to-file/
+
+作者：[Vivek Gite][a]
+译者：[Flowsnow](https://github.com/Flowsnow)
+校对：[wxy](https://github.com/wxy)
+
+本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
+
+[a]: https://www.cyberciti.biz
+[1]: https://www.cyberciti.biz/media/new/faq/2017/11/redirect-output-of-here-document-to-a-text-file.jpg
+[2]: https://bash.cyberciti.biz/guide/Here_documents
+[3]: https//www.cyberciti.biz/faq/linux-unix-appleosx-bsd-cat-command-examples/