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213 lines
4.3 KiB
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213 lines
4.3 KiB
Markdown
Bash 脚本中如何使用 here 文档将数据写入文件
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======
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<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。
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![redirect output of here document to a text file][1]
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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。
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### here 文档语法
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语法是：
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```
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command <<EOF
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cmd1
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cmd2 arg1
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EOF
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```
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或者允许 shell 脚本中的 here 文档使用 `EOF<<-` 以自然的方式缩进：
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```
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command <<-EOF
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msg1
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msg2
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$var on line
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EOF
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```
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或者
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```
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command <<'EOF'
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cmd1
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cmd2 arg1
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$var won't expand as parameter substitution turned off
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by single quoting
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EOF
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```
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或者 **重定向并将其覆盖** 到名为 `my_output_file.txt` 的文件中：
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```
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command <<EOF > my_output_file.txt
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mesg1
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msg2
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msg3
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$var on $foo
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EOF
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```
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或**重定向并将其追加**到名为 `my_output_file.txt` 的文件中：
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```
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command <<EOF >> my_output_file.txt
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mesg1
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msg2
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msg3
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$var on $foo
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EOF
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```
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### 示例
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以下脚本将所需内容写入名为 `/tmp/output.txt` 的文件中：
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```
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#!/bin/bash
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OUT=/tmp/output.txt
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echo "Starting my script..."
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echo "Doing something..."
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cat <<EOF >$OUT
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Status of backup as on $(date)
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Backing up files $HOME and /etc/
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EOF
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echo "Starting backup using rsync..."
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```
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你可以使用[cat命令][3]查看/tmp/output.txt文件：
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```
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$ cat /tmp/output.txt
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```
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示例输出：
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```
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Status of backup as on Thu Nov 16 17:00:21 IST 2017
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Backing up files /home/vivek and /etc/
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```
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### 禁用路径名/参数/变量扩展、命令替换、算术扩展
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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：
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```
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#!/bin/bash
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OUT=/tmp/output.txt
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echo "Starting my script..."
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echo "Doing something..."
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# No parameter and variable expansion, command substitution, arithmetic expansion, or pathname expansion is performed on word.
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# If any part of word is quoted, the delimiter is the result of quote removal on word, and the lines in the here-document
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# are not expanded. So EOF is quoted as follows
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cat <<'EOF' >$OUT
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Status of backup as on $(date)
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Backing up files $HOME and /etc/
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EOF
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echo "Starting backup using rsync..."
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```
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你可以使用 [cat 命令][3]查看 `/tmp/output.txt` 文件：
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```
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$ cat /tmp/output.txt
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```
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示例输出：
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```
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Status of backup as on $(date)
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Backing up files $HOME and /etc/
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```
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### 关于 tee 命令的使用
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语法是：
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```
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tee /tmp/filename <<EOF >/dev/null
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line 1
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line 2
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line 3
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$(cmd)
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$var on $foo
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EOF
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```
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或者通过在单引号中引用 `EOF` 来禁用变量替换和命令替换：
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```
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tee /tmp/filename <<'EOF' >/dev/null
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line 1
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line 2
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line 3
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$(cmd)
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$var on $foo
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EOF
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```
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这是我更新的脚本：
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```
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#!/bin/bash
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OUT=/tmp/output.txt
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echo "Starting my script..."
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echo "Doing something..."
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tee $OUT <<EOF >/dev/null
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Status of backup as on $(date)
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Backing up files $HOME and /etc/
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EOF
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echo "Starting backup using rsync..."
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```
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### 关于内存 here 文档的使用
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这是我更新的脚本：
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```
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#!/bin/bash
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OUT=/tmp/output.txt
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## in memory here docs
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## thanks https://twitter.com/freebsdfrau
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exec 9<<EOF
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Status of backup as on $(date)
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Backing up files $HOME and /etc/
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EOF
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## continue
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echo "Starting my script..."
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echo "Doing something..."
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## do it
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cat <&9 >$OUT
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echo "Starting backup using rsync..."
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```
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--------------------------------------------------------------------------------
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via: https://www.cyberciti.biz/faq/using-heredoc-rediection-in-bash-shell-script-to-write-to-file/
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作者：[Vivek Gite][a]
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译者：[Flowsnow](https://github.com/Flowsnow)
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校对：[wxy](https://github.com/wxy)
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本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
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[a]: https://www.cyberciti.biz
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[1]: https://www.cyberciti.biz/media/new/faq/2017/11/redirect-output-of-here-document-to-a-text-file.jpg
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[2]: https://bash.cyberciti.biz/guide/Here_documents
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[3]: https//www.cyberciti.biz/faq/linux-unix-appleosx-bsd-cat-command-examples/
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