here文档，即here document，它有几个名称如：heredoc、hereis、here字符串或here脚本等，它可以在命令行shell和程序语言中使用。
通常多行或带缩进的字符可以用到here文档，一些程序语言可以在字符串里执行变量替换和命令替换。
语法：
here文档最通用的语法是 <<紧跟一个标识字符串，通常为大写字母，如EOF，然后下一行为字符串的内容，最后单独一行的EOF作为结尾，表示标识字符串的关闭。
标识符不一定是EOF，也可以是STR、EOT、INDENT等字符串，但结尾行的标识符要和开始的对应。
在*nix里，here文档通常用来给命令提供输入内容。
<<EOF
line one
line two
line three
EOF示例：
- 变量和命令默认会被替换
$ cat <<EOF
working in $PWD
EOF执行会输出
working in /root/- 禁止替换变量
可以用单引号'或双引号"来包裹标识符
$ cat <<"EOF"
working in $PWD
EOF或
$ cat <<'EOF'
working in $PWD
EOF以下会输出
working in $PWD- 忽略tab
<<-EOF
line one
line two
line three
EOF实用技巧：
-
在
shell脚本将多行文本覆盖（>）或追加（>>）文件覆盖文件内容
cat <<'EOF' >/etc/yum.repos.d/Centos-7-aliyun.repo
# CentOS-Base.repo
#
# The mirror system uses the connecting IP address of the client and the
# update status of each mirror to pick mirrors that are updated to and
# geographically close to the client. You should use this for CentOS updates
# unless you are manually picking other mirrors.
#
# If the mirrorlist= does not work for you, as a fall back you can try the
# remarked out baseurl= line instead.
#
#
[base]
name=CentOS-$releasever - Base - mirrors.aliyun.com
failovermethod=priority
baseurl=http://mirrors.aliyun.com/centos/$releasever/os/$basearch/
http://mirrors.aliyuncs.com/centos/$releasever/os/$basearch/
http://mirrors.cloud.aliyuncs.com/centos/$releasever/os/$basearch/
gpgcheck=1
gpgkey=http://mirrors.aliyun.com/centos/RPM-GPG-KEY-CentOS-7
#released updates
[updates]
name=CentOS-$releasever - Updates - mirrors.aliyun.com
failovermethod=priority
baseurl=http://mirrors.aliyun.com/centos/$releasever/updates/$basearch/
http://mirrors.aliyuncs.com/centos/$releasever/updates/$basearch/
http://mirrors.cloud.aliyuncs.com/centos/$releasever/updates/$basearch/
gpgcheck=1
gpgkey=http://mirrors.aliyun.com/centos/RPM-GPG-KEY-CentOS-7
#additional packages that may be useful
[extras]
name=CentOS-$releasever - Extras - mirrors.aliyun.com
failovermethod=priority
baseurl=http://mirrors.aliyun.com/centos/$releasever/extras/$basearch/
http://mirrors.aliyuncs.com/centos/$releasever/extras/$basearch/
http://mirrors.cloud.aliyuncs.com/centos/$releasever/extras/$basearch/
gpgcheck=1
gpgkey=http://mirrors.aliyun.com/centos/RPM-GPG-KEY-CentOS-7
#additional packages that extend functionality of existing packages
[centosplus]
name=CentOS-$releasever - Plus - mirrors.aliyun.com
failovermethod=priority
baseurl=http://mirrors.aliyun.com/centos/$releasever/centosplus/$basearch/
http://mirrors.aliyuncs.com/centos/$releasever/centosplus/$basearch/
http://mirrors.cloud.aliyuncs.com/centos/$releasever/centosplus/$basearch/
gpgcheck=1
enabled=0
gpgkey=http://mirrors.aliyun.com/centos/RPM-GPG-KEY-CentOS-7
#contrib - packages by Centos Users
[contrib]
name=CentOS-$releasever - Contrib - mirrors.aliyun.com
failovermethod=priority
baseurl=http://mirrors.aliyun.com/centos/$releasever/contrib/$basearch/
http://mirrors.aliyuncs.com/centos/$releasever/contrib/$basearch/
http://mirrors.cloud.aliyuncs.com/centos/$releasever/contrib/$basearch/
gpgcheck=1
enabled=0
gpgkey=http://mirrors.aliyun.com/centos/RPM-GPG-KEY-CentOS-7
EOF以上示例是将阿里云的centos-7-aliyun.repo内容覆盖到/etc/yum.repos.d/Centos-7-aliyun.repo文件，该文件之前的内容将会被覆盖，由于文本中存在$releasever等变量，为了防止heredoc转换这些变量的内容，EOF需要用引号包围变成'EOF'。
追加文件内容
cat <<'EOF' >>/etc/vimrc
set ts=4
set expandtab
EOF以上示例是将vim的两个指令追加到/etc/vimrc文件中，指令内容会在文件的最底部追加两行，不会覆盖原文件。
参考：
https://ruby-china.org/topics/25983