heredoc

方便在sh脚本里将数据写入文件

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#!/bin/bash
OUT=/tmp/output.txt
echo "Starting my script..."
echo "Doing something..."
cat <$OUT
Status of backup as on $(date)
Backing up files $HOME and /etc/
EOF
echo "Starting backup using rsync..."

作者:

Vivek Gite

译者: LCTT [Suncle Chen(https://linux.cn/lctt/Flowsnow)

转载自:https://linux.cn/article-10224-1.html

here document (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。

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:

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command <

或者允许 shell 脚本中的 here 文档使用 EOF<<-< code> 以自然的方式缩进:

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command <<-eof msg1 msg2 $var on line eof< span>

或者

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command <<'eof' cmd1 cmd2 arg1 $var won't expand as parameter substitution turned off by single quotingeof< span>

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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command < my_output_file.txt mesg1 msg2 msg3 $var on $fooEOF

或重定向并将其追加到名为 my_output_file.txt 的文件中:

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command <> my_output_file.txt mesg1 msg2 msg3 $var on $fooEOF

示例

以下脚本将所需内容写入名为 /tmp/output.txt 的文件中:

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#!/bin/bashOUT=/tmp/output.txtecho "Starting my script..."echo "Doing something..."cat <$OUT  Status of backup as on $(date)  Backing up files $HOME and /etc/EOFecho "Starting backup using rsync..."

你可以使用cat命令查看/tmp/output.txt文件:

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$ cat /tmp/output.txt

示例输出:

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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

$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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#!/bin/bashOUT=/tmp/output.txtecho "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 followscat <<'eof'>$OUT  Status of backup as on $(date)  Backing up files $HOME and /etc/EOFecho "Starting backup using rsync..."

你可以使用 cat 命令查看 /tmp/output.txt 文件:

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$ cat /tmp/output.txt

示例输出:

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Status of backup as on $(date) Backing up files $HOME and /etc/

关于 tee 命令的使用

语法是:

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tee /tmp/filename </dev/nullline 1line 2line 3$(cmd)$var on $fooEOF

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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tee /tmp/filename <<'eof'>/dev/nullline 1line 2line 3$(cmd)$var on $fooEOF

这是我更新的脚本:

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#!/bin/bashOUT=/tmp/output.txtecho "Starting my script..."echo "Doing something..."tee $OUT </dev/null  Status of backup as on $(date)  Backing up files $HOME and /etc/EOFecho "Starting backup using rsync..."

关于内存 here 文档的使用

这是我更新的脚本:

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#!/bin/bashOUT=/tmp/output.txt## in memory here docs ## thanks https://twitter.com/freebsdfrauexec 9<$OUTecho "Starting backup using rsync..."