如何在Bash Shell脚本中使用Heredoc重定向功能(此处为文档)将数据写入文件？
这里的文档不过是I/O重定向，它告诉bash shell从当前源读取输入，直到看到仅包含定界符的行。

这对于为ftp，cat，echo，ssh和许多其他有用的Linux/Unix命令提供命令很有用。
此功能也应适用于bash或者Bourne/Korn/POSIX shell。

Heredoc语法

语法为：

command <<EOF
cmd1
cmd2 arg1
EOF

或者允许使用`EOF以自然的方式缩进shell脚本中的此处文档

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

或者将其重定向并覆盖到名为my_output_file.txt的文件中：

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

或者将其重定向并追加到名为my_output_file.txt的文件中：

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

例子

以下脚本将所需的内容写入名为/tmp/output.txt的文件：

#!/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..."

您可以使用cat命令查看/tmp/output.txt：

$ cat /tmp/output.txt

输出示例：

Status of backup as on Thu Nov 16 17:00:21 IST 2016
  Backing up files /home/Hyman and /etc/

禁用路径名/参数/变量扩展，命令替换，算术扩展

脚本中解释了$HOME之类的变量和$(date)之类的命令。
要禁用它，请对EOF使用单引号，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..."

您可以使用cat命令查看/tmp/output.txt：

$ cat /tmp/output.txt

输出示例：

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

关于使用tee命令的注意事项

语法为：

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/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..."

关于在此处使用内存的说明文档

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..."