here 文档 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。

redirect output of here document to a text file

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

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

$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 译者:Flowsnow 校对:wxy

本文由 LCTT 原创编译,Linux中国 荣誉推出

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